## Language reference

This documentation describes the PC-BASIC language, which aims to faithfully emulate GW-BASIC 3.23, IBM Advanced BASIC, IBM Cartridge BASIC and Tandy 1000 GW-BASIC.

Differences with the original languages do arise, and where this is the case they are documented.

Please note that Microsoft's official documentation for the original languages is rather hit-and-miss; it leaves several features undocumented and incorrectly describes others. To avoid making the same errors, the present documentation was written from scratch with reference to the actual behaviour. The errors in this document are therefore all my own. Please contact me if you encounter them.

### Metasyntax

In descriptions of BASIC syntax, the following conventions apply. Exact rendering of the markup may vary depending on the means used to display this documentation.

`bold`
  Type exactly as shown.
`italic`
  Replace with appropriate metavariable.
`[a]`
  Entities within square brackets are optional.
`{ a | b }`
  Disjunct alternatives of which one must be chosen.
`[ a | b ]`
  Optional disjunct alternatives.
`a ...`
  Preceding entity can be repeated.

### Definitions

A program line consists of a line number followed by a compound statement. Program lines are terminated by a `CR` or or by the end of the file (optionally through an `EOF` character). Anything on a program line after a `NUL` character is ignored.

A line number is a whole number in the range `[0—65535]`. Note that the line numbers `65530—65535` cannot be entered from the console or a text program file, but can be part of a tokenised program file.

A compound statement consists of statements separated by colons: `statement [: statement] ...`

An expression takes one of the following forms: `unary_operator {literal | variable | array_element | function}` `expression binary_operator expression` `(expression)` whose elements are described the sections Literals, Variables, Operators and Functions.

An array element takes the form `array {[|(} numeric_expression [, numeric_expression ] ... {)|]}`

### Literals

##### String literals

String literals are of the following form: `"[characters]{NUL|CR|EOF|"}` where `characters` is a string of characters. Any character from the current code page can be used, with the following exceptions, all of which terminate the string literal (aside from other effects they may have):

- `NUL` (`CHR$(&h00)`)
- `CR` (`CHR$(&h0D)`)
- `EOF` (`CHR$(&h1A)`)
- `"` (`CHR$(&h22)`)

Strings are also legally terminated by the end of the file in the absence of an `EOF` character.

Apart from these, string literals should not contain any of the characters in the ASCII range `&h0D`—`&h1F`, which lead to unpredictable results. There is no escaping mechanism. To include one of the above characters in a string, use string concatenation and the `CHR$` function.

##### Numeric literals

Numeric literals have one of the following forms: `[+|-] [0|1|2|3|4|5|6|7|8|9]... [.][0|1|2|3|4|5|6|7|8|9]... [{E|e|D|d}[+|-][0|1|2|3|4|5|6|7|8|9]...] |#|!|%]` `&{H|h}[0|1|2|3|4|5|6|7|8|9|A|B|C|D|E|F|a|b|c|d|e|f]...` `&[O|o] [0|1|2|3|4|5|6|7]...`

Hexadecimal literals must not contain spaces, but decimal and octal literals may. The `o` character in octal literals is optional: they can be specified equally as `&o777` or `&777`.

Hexadecimal and octal literals denote integers and do not include a sign. They must range between [`&h0`—`&hFFFF`], of which the range [`&h8000`—`&hFFFF`] is interpreted as a two's complement negative integer; for example, `&hFFFF = -1`. Signs can appear left of the `&` but these form an expression and are not part of the literal itself.

Floating-point literals must be specified in decimal notation. The decimal separator is the point. A base-10 exponent may be specified after `E` in single-precision floats, or after `D` in double-precision floats. Trailing `%` is ignored and does not indicate an integer literal. Trailing `!` or `#` mark the literal as single- or double-precision, respectively.

Examples of valid numeric literals are `-1` `42` `42!` `42#` `1.3523523` `.235435` `-.3` `3.` `.` `.e` `.D` `1.1e+7` `1.1d+7` `1e2` `1e-2` `&7` `&hffff` `&O20` `&h` `&` `65537%` `1.1%`

Note that expressions such as `&o-77` are legal; these are however not negative octals but rather the expression `&o` (empty octal; zero) less `77` (decimal 77).

### Variables

Variable names must start with a letter; all characters of the variable name (except the sigil) must be letters `A—Z`, figures `0—9`, or a dot `.` Only the first 40 characters in the name are significant. A variable name must not be identical to a reserved word or a reserved word plus sigil. Therefore, for example, you cannot name a variable `TO!` but you can name it `AS!`. Variable names may contain any reserved word. Variable names may also start with a reserved word, with the exception of `USR` and `FN`. Thus, `FNORD%` and `USRNME$` are not legal variable names while `FRECKLE%` and `LUSR$` are.

For each name, four different variables may exist corresponding to the four types. That is, you can have `A$`, `A%`, `A!` and `A#` as different variables. Which one of those is also known as `A` depends on the settings in `DEFINT`/`DEFDBL`/`DEFSNG`/`DEFSTR`. By default, `A` equals the single-precision `A!`.

Furthermore, the arrays `A$()`, `A%()`, `A!()`, `A#()` are separate from the scalar variables of the same name.

#### Types and sigils

PC-BASIC recognises four variable types, distinguished by their sigil or type character, the last character of the variable's full name:

sigil | type | size | range | precision
--- | --- | --- | --- | ---
`$` | string | 3 bytes plus allocated string space | 0—255 characters |
`%` | integer | 2 bytes | -32768—32767 | exact
`!` | single-precision float | 4 bytes | ±2.938726·10-39—±1.701412·1038 | ~6 significant figures
`#` | double-precision float | 8 bytes | ±2.938735877055719·10-39—±1.701411834604692·1038 | ~16 significant figures

Note that double-precision floats can hold more decimals than single-precision floats, but not larger or smaller numbers.

While all integers are signed, some statements will interpret negative integers as their two's complement.

#### Arrays

Arrays are indexed with round or square brackets; even mixing brackets is allowed. The following are all legal array elements: `A[0]`, `A(0)`, `A(0]`, `A[0)`. Multidimensional arrays are specified by separating the indices with commas: `A(0, 0)`, `A[0, 0, 0]`, etc.

By default, arrays are indexed from `0`. This can be changed to `1` using `OPTION BASE 1`.

Arrays can be allocated by specifying the largest allowed index using `DIM`. If all indices of the array are `10` or less, they need not be explicitly allocated. The first access of the array (read or write) will automatically allocate it with a maximum index of `10` and the same number of indices as in the first access. To re-allocate an array, the old array must first be deleted with `CLEAR` or `ERASE`.

Multi-dimensional arrays are stored in column-major order, such that `A%(2,0)` immediately follows `A%(1,0)`.

#### Conversions

PC-BASIC will implicitly convert between the three numerical data types. When a value of one type is assigned to a variable, array element or parameter of another type, it is converted according to the following rules:

- Single- and double-precision floats are converted to integer by rounding to the nearest whole number. Halves are rounded away from zero. If the resulting whole number is outside the allowed range for integers, `Overflow` is raised.
- Double-precision floats are converted to single-precision floats by Gaussian rounding of the mantissa, where the new least significant bit of the mantissa is rounded up if the clipped-off binary fraction is greater than one-half; halves are rounded to even.
- Integers are converted to their exact representation as single- or double-precision floats.
- Single-precision floats are converted to their exact representation as double-precision floats.
- There is no implicit conversion between strings and any of the numeric types. Attempting to assign a string value to a numeric variable, array element or parameter (or vice versa) will raise `Type mismatch`.

### Operators

#### Order of precedence

The order of precedence of operators is as follows, from tightly bound (high precedence) to loosely bound (low precedence):

1. `^`
2. - `*`
   - `/`
3. `\`
4. `MOD`
5. - `+`
   - `-` (unary and binary)
6. - `=`
   - `<>`
   - `><`
   - `<`
   - `>`
   - `<=`
   - `=<`
   - `>=`
   - `=>`
7. `NOT` (unary)
8. `AND`
9. `OR`
10. `XOR`
11. `EQV`
12. `IMP`

Expressions within parentheses `()` are evaluated first. All binary operators are left-associative: operators of equal precedence are evaluated left to right.

##### Examples

- Exponentiation is more tightly bound than negation: `-1^2 = -(1^2) = -1` but `(-1)^2 = 1`.
- Exponentiation is left-associative: `2^3^4 = (2^3)^4 = 4096`.

##### Errors

- If any operator other than `+`, `-` or `NOT` is used without a left operand, `Syntax error` is raised.
- At the end of a statement, if any operator is used without a right operand, `Missing operand` is raised. If this occurs elsewhere inside a statement, such as within brackets, `Syntax error` is raised.

#### Mathematical operators

Mathematical operators operate on numeric expressions only. Note however that `+` can take the role of the string concatenation operator if both operands are strings.

Code | Operation | Result
--- | --- | ---
`x ^ y` | Exponentiation | `x` raised to the power of `y`
`x * y` | Multiplication | Product of `x` and `y`
`x / y` | Division | Quotient of `x` and `y`
`x \ y` | Truncated division | Integer quotient of `x` and `y`
`x MOD y` | Modulo | Integer remainder of `x` by `y` (with the sign of `x`)
`x + y` | Addition | Sum of `x` and `y`
`x - y` | Subtraction | Difference of `x` and `y`
`+ y` | Unary Plus | Value of `y`
`- y` | Negation | Negative value of `y`

##### Notes

- Where necessary, the result of the operation will be upgraded to a data type able to hold the result. For example, dividing integers 3 by 2 will yield a single-precision 1.5.
- However, the exponentiation operator `^` will give at most a single-precision result unless the `double` option is used.
- The expression `0^0` will return `1` and not raise an error, even though, mathematically, raising zero to the zeroeth power is undefined.

##### Errors

- If either operand is a string, `Type mismatch` will be raised. The exception is `+` which will only raise `Type mismatch` if either but not both operands are strings.
- If `y=0`, `x / y`, `x MOD y` and `x \ y` will raise `Division by zero`.
- If `x=0` and `y<0`, `x^y` will raise `Division by zero`.
- If the result of any operation is too large to fit in a floating-point data type, `Overflow` is raised.
- If operands or result of `\` or `MOD` are not in `[-32768–32767]`, `Overflow` is raised.
- If `x<0` and `y` is a fractional number, `x ^ y` will raise `Illegal function call`.

#### Relational operators

Relational operators can operate on numeric as well as string operands; however, if one operand is string and the other numeric, `Type mismatch` is raised.

Relational operators return either `0` (for false) or `-1` for true.

Code | Operation | Result
--- | --- | ---
`=` | Equal | True if `a` equals `b`, false otherwise.
`<>` `><` | Not equal | False if `a` equals `b`, true otherwise.
`<` | Less than | True if `a` is less than `b`, false otherwise.
`>` | Greater than | True if `a` is greater than `b`, false otherwise.
`<=` `=<` | Less than or equal | False if `a` is greater than `b`, true otherwise.
`>=` `=>` | Greater than or equal | False if `a` is less than `b`, true otherwise.

When operating on numeric operands, both operands are compared as floating-point numbers according to the usual ordering of numbers. The equals operator tests for equality to within machine precision for the highest-precision of the two operator types.

When comparing strings, the ordering is as follows.

- Two strings are equal only if they are of the same length and every character code of the first string agrees with the corresponding character code of the second. This includes any whitespace or unprintable characters.
- Each character position of the strings is compared starting with the leftmost character. When a pair of different characters is encountered, the string with the character of lesser code point is less than the string with the character of greater code point.
- If the strings are of different length, but equal up to the length of the shorter string, then the shorter string is less than the longer string.

#### Bitwise operators

PC-BASIC has no Boolean type and does not implement Boolean operators. It does, however, implement bitwise operators.

Bitwise operators operate on numeric expressions only. Floating-point operands are rounded to integers before being used.

Code | Operation | Result
--- | --- | ---
`NOT y` | Complement | `-y-1`
`x AND y` | Bitwise conjunction | The bitwise AND of `x` and `y`
`x OR y` | Bitwise disjunction | The bitwise OR of `x` and `y`
`x XOR y` | Bitwise exclusive or | The bitwise XOR of `x` and `y`
`x EQV y` | Bitwise equivalence | `NOT(x XOR y)`
`x IMP y` | Bitwise implication | `NOT(x) OR y`

These operators can be used as Boolean operators only if `-1` is used to represent true while `0` represents false. Note that PC-BASIC represents negative integers using the two's complement, so `NOT 0 = -1`. The Boolean interpretation of bitwise operators is given in the table below.

Code | Operation | Result
--- | --- | ---
`NOT y` | Logical negation | True if `y` is false and vice versa
`x AND y` | Conjunction | Only true if both `x` and `y` are true
`x OR y` | Disjunction | Only false if both `x` and `y` are false
`x XOR y` | Exclusive or | True if the truth values of `x` and `y` differ
`x EQV y` | Equivalence | True if the truth values of `x` and `y` are the same
`x IMP y` | Implication | True if `x` is false or `y` is true

Be aware that when used on integers other than `0` and `-1`, bitwise operators can not be interpreted as Boolean operators. For example, `2 AND 1` returns `0`.

##### Errors

- If either operand is a string, `Type mismatch` will be raised.
- If the operands or result are not in `[-32768–32767]`, `Overflow` is raised.

#### String operators

The string concatenation operator is `+`. It has a binary as well as a unary form. The unary minus may also be used on strings, but has no effect.

Code | Operation | Result
--- | --- | ---
`x + y` | Concatenation | The string formed by `x` followed by `y`
`+ y` | Unary Plus | Value of `y`
`- y` | Unary Minus | Value of `y`

##### Errors

- If either (but not both) operands to a concatenation are numeric, `Type mismatch` will be raised.
- If `LEN(x) + LEN(y) > 255`, `x + y` will raise `String too long`.

### Functions

Functions can only be used as part of an expression within a statement; they may take input values between parentheses and produce a return value. For example, in `PRINT ABS(-1)` the `ABS` function is used in an expression within a `PRINT` statement; in `Y = SQR(X) + 2` the `SQR` function is used in an expression within a `LET` statement.

Some reference works also use terms such as system variable for functions that do not take an input, presumably since in the GW-BASIC syntax such functions have no parentheses, in contrast to the languages in the C family (and indeed some modern BASICs). However, this is simply the GW-BASIC syntax for functions without inputs. For example, one can do `DEF FNA=1: PRINT FNA` in which no parentheses are allowed.

#### `ABS`

`y = ABS(x)`

Returns the absolute value of `x` if `x` is a number and the value of `x` if `x` is a string.

##### Parameters

- `x` is an expression.

#### `ASC`

`val = ASC(char)`

Returns the code point (ASCII value) for the first character of `char`.

##### Parameters

- `char` is an expression with a string value.

##### Errors

- `char` has a numeric value: `Type mismatch`.
- `char` equals `""`: `Illegal function call`.

#### `ATN`

`y = ATN(x)`

Returns the inverse tangent of `x`.

##### Parameters

- `x` is a numeric expression that gives the angle in radians.

##### Notes

- Unless PC-BASIC is run with the `double` option, this function returns a single-precision value.
- `ATN(x)` differs in the least significant digit from GW-BASIC.

##### Errors

- `x` has a string value: `Type mismatch`.

#### `CDBL`

`y = CDBL(x)`

Converts the numeric expression `x` to a double-precision value.

##### Errors

- `x` has a string value: `Type mismatch`.

#### `CHR$`

`char = CHR$(x)`

Returns the character with code point `x`.

##### Parameters

- `x` is a numeric expression in the range `[0—255]`.

##### Errors

- `x` has a string value: `Type mismatch`.
- `x` is not in `[-32768—32767]`: `Overflow`.
- `x` is not in `[0—255]`: `Illegal function call`.

#### `CINT`

`y = CINT(x)`

Converts the numeric expression `x` to a signed integer. Halves are rounded away from zero, so that e.g. `CINT(2.5) = 3` and `CINT(-2.5) = -3`.

##### Errors

- `x` has a string value: `Type mismatch`.
- `x` is not in `[-32768—32767]`: `Overflow`.

#### `COS`

`cosine = COS(angle)`

Returns the cosine of `angle`. Unless PC-BASIC is run with the `double` option, this function returns a single-precision value.

##### Parameters

- `angle` is a numeric expression that gives the angle in radians.

##### Notes

- The return value usually differs from the value returned by GW-BASIC in the least significant figure.

##### Errors

- `angle` has a string value: `Type mismatch`.

#### `CSNG`

`y = CSNG(x)`

Converts the numeric expression `x` to a single-precision value by Gaussian rounding.

##### Errors

- `x` has a string value: `Type mismatch`.

#### `CSRLIN`

`y = CSRLIN`

Returns the screen row of the cursor on the active page. The return value is in the range `[1—25]`.

##### Notes

- This function takes no arguments.

#### `CVI`

`y = CVI(s)`

Converts a two-byte string to a signed integer.

##### Parameters

- `s` is a string expression that represents an integer using little-endian two's complement encoding. Only the first two bytes are used.

##### Errors

- `s` has a numeric value: `Type mismatch`.

#### `CVS`

`y = CVS(s)`

Converts a four-byte string to a single-precision floating-point number.

##### Parameters

- `s` is a string expression that represents a single-precision number in Microsoft Binary Format. Only the first four bytes are used.

##### Errors

- `s` has a numeric value: `Type mismatch`.

#### `CVD`

`y = CVD(s)`

Converts an eight-byte string to a double-precision floating-point number.

##### Parameters

- `s` is a string expression that represents a double-precision number in Microsoft Binary Format. Only the first eight bytes are used.

##### Errors

- `s` has a numeric value: `Type mismatch`.

#### `DATE$` (function)

`s = DATE$`

Returns the system date as a string in the format `"mm-dd-yyyy"`.

##### Notes

- This function takes no arguments.

#### `ENVIRON$`

`value = ENVIRON[ ]$(x)`

Returns an environment variable.

##### Parameters

`x` is an expression.

- If `x` has a string value, returns the value for the environment variable `x` or the empty string if no variable with the name `x` is set in the environment table.
- If `x` has a numeric value, it must be in `[1—255]`. Returns the `x`th entry in the environment table.

##### Errors

- `x` is the empty string: `Illegal function call`.
- `x` is a number not in `[-32768—32767]`: `Overflow`.
- `x` is a number not in `[1—255]`: `Illegal function call`.

#### `EOF`

`is_at_end = EOF(file_num)`

Returns -1 if file with number `file_num` has reached end-of-file; 0 otherwise. The file must be open in `INPUT` or `RANDOM` mode. `EOF(0)` returns 0.

##### Notes

- If `file_num` is open to `KYBD:`, performs a blocking read and returns -1 if `CTRL`+`Z` is entered, 0 otherwise. The character entered is then echoed to the console.

##### Errors

- `file_num` has a string value: `Type mismatch`.
- `file_num` is a number not in `[-32768—32767]`: `Overflow`.
- `file_num` is a number not in `[0—255]`: `Illegal function call`.
- `file_num` is not 0 or the number of an open file: `Bad file number`.
- The file with number `file_num` is in OUTPUT or APPEND mode: `Bad file mode`.

#### `ERDEV`

`zero = ERDEV`

Returns 0.

##### Notes

- In GW-BASIC, returns the value of a device error.
- This function is not implemented in PC-BASIC.
- This function takes no arguments.

#### `ERDEV$`

`empty = ERDEV[ ]$`

Returns the empty string.

##### Notes

- In GW-BASIC, returns the device name of a device error.
- This function is not implemented in PC-BASIC.
- This function takes no arguments.

#### `ERL`

`error_line = ERL`

Returns the line number where the last error was raised.

##### Notes

- If the error was raised by a direct statement, returns 65535.
- If no error has been raised, returns 0.
- This function takes no arguments.

#### `ERR`

`error_code = ERR`

Returns the number of the last error.

##### Notes

- If no error has been raised, returns 0.
- If the last error was a `Syntax error` raised by a direct statement, returns 0.
- This function takes no arguments.

#### `EXP`

`y = EXP(x)`

Returns the exponential of `x`, i.e. `e` to the power `x`.

##### Parameters

- `x` is a number- valued expression.

##### Notes

- Unless PC-BASIC is run with the `double` option, this function returns a single-precision value.
- The return value sometimes differs in the least significant digit from GW-BASIC. For large values of `x`, the difference may be 3 digits.

##### Errors

- `x` has a string value: `Type mismatch`.
- `x` is larger than the natural logarithm of the maximum single-precision value: `Overflow`.

#### `EXTERR`

`zero = EXTERR(x)`

Returns 0.

##### Parameters

- `x` is a numeric expression in `[0—3]`.

##### Notes

- In GW-BASIC, this function returns extended error information from MS-DOS.
- This function is not implemented in PC-BASIC.

##### Errors

- `x` has a string value: `Type mismatch`.
- `x` is not in `[-32768—32767]`: `Overflow`.
- `x` is not in `[0—3]`: `Illegal function call`.

#### `FIX`

`whole = FIX(number)`

Returns `number` truncated towards zero.

##### Parameters

- `number` is a numeric expression.

##### Notes

- `FIX` truncates towards zero: it removes the fractional part. By contrast, `INT` truncates towards negative infinity.

##### Errors

- `number` is a string expression: `Type mismatch`.

#### `FN`

`result = FN[ ]name [(arg_0 [, arg_1] ...)]`

Evaluates the user-defined function previously defined with `DEF FN name`. Spaces between `FN` and `name` are optional.

##### Parameters

- `name` is the name of a previously defined function.
- `arg_0, arg_1, ...` are expressions, given as parameters to the function.

##### Errors

- No function named `name` is defined: `Undefined user function`.
- The number of parameters differs from the function definition: `Syntax error`.
- The type of one or more parameters differs from the function definition: `Type mismatch`.
- The return type is incompatible with the function name's sigil: `Type mismatch`.
- The function being called is recursive or mutually recursive: `Out of memory`.

#### `FRE`

`free_mem = FRE(x)`

Returns the available BASIC memory.

##### Parameters

`x` is an expression.

- If `x` has a numeric value, it is ignored.
- If `x` has a string value, garbage collection is performed before returning available memory.

#### `HEX$`

`hex_repr = HEX$(x)`

Returns a string with the hexadecimal representation of `x`.

##### Parameters

- `x` is a numeric expression in `[-32768—65535]`. Values for negative `x` are shown as two's-complement.

##### Errors

- `x` is not in `[-32768—65535]`: `Overflow`.
- `x` has a string value: `Type mismatch`.

#### `INKEY$`

`key = INKEY$`

Returns one key-press from the keyboard buffer. If the keyboard buffer is empty, returns the empty string. Otherwise, the return value is a one- or two- character string holding the e-ASCII code of the pressed key.

##### Notes

- This function takes no arguments.
- When a function key `F1`–`F10` is pressed, `INKEY$` will return the letters of the associated macro — unless this macro has been set to empty with the `KEY` statement, in which case it returns the e-ASCII code for the function key.

#### `INP`

`code = INP(port)`

Returns the value of an emulated machine port.

##### Parameters

`port` is a numeric expression in `[0—65535]`.

`port` | Effect
--- | ---
`&h60` | Returns the keyboard scancode for the current key pressed or the last key released. The scancodes returned by `INP(&h60)` are those listed in the keyboard scancodes table. If a key is currently down, the return value is its scancode. If no key is down, the return value is the scancode of the last key released, incremented by 128.
`&h201` | Returns the value of the game port (joystick port). This value is constructed as follows:
Bit | Meaning
--- | ---
0 | joystick 2 x-axis
1 | joystick 1 y-axis
2 | joystick 1 x-axis
3 | joystick 2 y-axis
4 | joystick 2 button 1
5 | joystick 1 button 2
6 | joystick 1 button 1
7 | joystick 2 button 2
The button bits are 0 when the button is fired, 1 otherwise. The axis values are normally 0 but are set to 1 by `OUT &h201, x` and then fall back to 0 after a delay. The longer the delay, the higher the axis value.
other values | Returns zero.

##### Notes

- Only a limited number of machine ports are emulated in PC-BASIC.

##### Errors

- `port` is not in `[-32768—65535]`: `Overflow`.
- `port` has a string value: `Type mismatch`.

#### `INPUT$`

`chars = INPUT[ ]$ (num_chars [, [#] file_num])`

Returns a string of `num_chars` characters from the keyboard or, if `file_num` is provided, from a text file.

##### Parameters

- `num_chars` is a numeric expression in `[1—255]`.
- `file_num` is a numeric expression that returns the number of a text file opened in `INPUT` mode. The `#` is optional and has no effect.

##### Notes

- This is a blocking read. It will wait for characters if there are none in the buffer.
- All control characters except `Ctrl`+`Break`, `Ctrl`+`Scroll Lock` and `Pause` are passed to the string by `INPUT$`. `Ctrl`+`Break` and `Ctrl`+`Scroll Lock` break execution whereas `Pause` halts until another key is pressed (and not read).
- When reading from the keyboard directly or through `KYBD:`, arrow keys, `Del`, `Home`, `End`, `Pg Up`, `Pg Dn` are passed as `NUL` characters. Function keys are ignored if they are event-trapped, otherwise function-key macro replacement is active as normal.

##### Errors

- `num_chars` is not in `[-32768—32767]`: `Overflow`.
- `num_chars` is not in `[1—255]`: `Illegal function call`.
- `file_num` is not an open file: `Bad file number`.
- `file_num` is less than zero: `Illegal function call`.
- `file_num` is greater than `32767`: `Overflow`.
- `file_num` is not open for `INPUT`: `Bad file mode`.
- `num_chars` or `file_num` are strings: `Type mismatch`.
- `file_num` is open to a `COM` port and this is the first `INPUT`, `LINE INPUT` or `INPUT$` call on that port since the buffer has filled up completely (i.e. `LOF(file_num)` has become zero): `Communication buffer overflow`.

#### `INSTR`

`position = INSTR([start,] parent, child)`

Returns the location of the first occurrence of the substring `child` in `parent`.

##### Parameters

- `parent` and `child` are string expressions.
- `start` is a numeric expression in `[1—255]`, specifying the starting position from where to look; if not specified, the search starts at character 1.

##### Notes

- If `child` is not a substring of `parent` occurring at or before `start`, `INSTR` returns 0.

##### Errors

- `start` has a string value or `parent` or `child` have numeric values: `Type mismatch`.
- `start` is not in `[-32768—32767]`: `Overflow`.
- `start` is not in `[1—255]`: `Illegal function call`.

#### `INT`

`whole = INT(number)`

Returns `number` truncated towards negative infinity.

##### Parameters

- `number` is an expression.

##### Notes

- `FIX` truncates towards zero: it removes the fractional part. By contrast, `INT` truncates towards negative infinity.
- If `number` is a string expression, `INT` returns its value unchanged.

#### `IOCTL$`

`result = IOCTL[ ]$ ([#] file_num)`

Raises `Illegal function call`.

##### Notes

- In GW-BASIC, `IOCTL$` reads the reply to `IOCTL` from a device.
- This function is not implemented in PC-BASIC.

##### Errors

- `file_num` has a string value: `Type mismatch`.
- `file_num` is not in `[-32768—32767]`: `Overflow`.
- `file_num` is not an open file: `Bad file number`.
- Otherwise: `Illegal function call`

#### `LEFT$`

`child = LEFT$(parent, num_chars)`

Returns the leftmost `num_chars` characters of `parent`.

##### Parameters

- `parent` is a string expression.
- `num_chars` is a numeric expression in `[0—255]`.

##### Notes

- If `num_chars` is zero or `parent` is empty, `LEFT$` returns an empty string.
- If `num_chars` is greater than the length of `parent`, returns `parent`.

##### Errors

- `parent` has a numeric value or `num_chars` has a string value: `Type mismatch`.
- `num_chars` is not in `[-32768—32767]`: `Overflow`.
- `num_chars` is not in `[0—255]`: `Illegal function call`.

#### `LEN`

`length = LEN(string)`

Returns the number of characters in `string`.

##### Parameters

- `string` is a string expression.

##### Errors

- `string` has a number value: `Type mismatch`.

#### `LOC`

`location = LOC(file_num)`

Returns the current location in the file opened under number `file_num`.

- If the file is opened for `INPUT`, `OUTPUT` or `APPEND`, `LOC` returns the number of 128-byte blocks read or written since opening the file.
- If the file is opened for `RANDOM`, `LOC` returns the record number last read or written.
- If the file is opened to a `COM` device, `LOC` returns the number of characters in the input buffer, with a maximum of 255.
- If the file is opened to `KYBD:`, `LOC` returns 0.

##### Parameters

- `file_num` is a numeric expression in the range `[0—255]`.

##### Notes

- `file_num` must not be preceded by a `#`.
- In `OUTPUT` or `APPEND` mode, before any writes `LOC` returns 0. After the 128th character is written, `LOC` returns 1.
- In `INPUT` mode, before any reads `LOC` returns 1. After the 129th character is read, `LOC` returns 2.
- If `text-encoding` is set, characters may be encoded by sequences of more than one byte. `LOC` will return the number of bytes rather than the number of encoded characters.

##### Errors

- `file_num` has a string value: `Type mismatch`.
- `file_num` is not in `[-32768—32767]`: `Overflow`.
- `file_num` is not in `[0—255]`: `Illegal function call`.
- `file_num` is not an open file: `Bad file number`.
- `file_num` is open to a `LPT` device: `Bad file mode`.

#### `LOF`

`length = LOF(file_num)`

Returns the number of bytes in the file open under `file_num`.

##### Parameters

- `file_num` is a numeric expression in the range `[0—255]`.

##### Notes

- If `file_num` is open to a `COM:` device, `LOF` returns the number of bytes free in the input buffer.

##### Errors

- `file_num` has a string value: `Type mismatch`.
- `file_num` is not in `[-32768—32767]`: `Overflow`.
- `file_num` is not in `[0—255]`: `Illegal function call`.
- `file_num` is not an open file: `Bad file number`.
- `file_num` is open to a `LPT` device: `Bad file mode`.

##### Notes

- If `text-encoding` is set, characters may be encoded by sequences of more than one byte. `LOF` will return the number of bytes rather than the number of encoded characters.

#### `LOG`

`y = LOG(x)`

Returns the natural logarithm of `x`.

##### Parameters

- `x` is a numeric expression greater than zero.

##### Notes

- Unless PC-BASIC is run with the `double` option, this function returns a single-precision value.
- `LOG(x)` can differ from GW-BASIC by 1 in the least significant digit.

##### Errors

- `x` has a string value: `Type mismatch`.
- `x` is zero or negative: `Illegal function call`.

#### `LPOS`

`position = LPOS(printer_number)`

Returns the column position for a printer.

##### Parameters

- `printer_number` is a numeric expression in `[0—3]`. If it is 0 or 1, the position for `LPT1:` is returned. If it is 2, `LPT2:`; 3, `LPT3:`.

##### Notes

- When entering direct mode, `LPT1:` (but not other printers) is flushed and its position is reset to 1.

##### Errors

- `printer_number` has a string value: `Type mismatch`.
- `printer_number` is not in `[-32768—32767]`: `Overflow`.
- `printer_number` is not in `[0—3]`: `Illegal function call`.

#### `MID$` (function)

`substring = MID$(string, position [, length])`

Returns a substring of `string` starting at `position`, counting from 1. The substring has length `length` if specified. If `length` is not specified, the substring extends to the end of the string.

##### Parameters

- `string` is a string expression.
- `position` is a numeric expression between 1 and the string length, inclusive.
- `length` is a numeric expression in `[0—255]`.

##### Errors

- `string` has a number value or `position` or `length` have string values: `Type mismatch`.
- `position` or `length` are not in `[-32768—32767]`: `Overflow`.
- `position` is not in `[1—255]`: `Illegal function call`.
- `length` is not in `[0—255]`: `Illegal function call`.

#### `MKD$`

`bytes = MKD$(double)`

Returns the internal 8-byte Microsoft Binary Format representation of a double- precision number.

##### Errors

- `double` has a string value: `Type mismatch`.

#### `MKI$`

`bytes = MKI$(int)`

Returns the internal 2-byte little-endian representation of an integer.

##### Errors

- `int` has a string value: `Type mismatch`.
- `int` is not in `[-32768—32767]`: `Overflow`.

#### `MKS$`

`bytes = MKS$(single)`

Returns the internal 8-byte Microsoft Binary Format representation of a single- precision number.

##### Errors

- `single` has a string value: `Type mismatch`.

#### `OCT$`

`octal = OCT$(x)`

Returns a string with the octal representation of `x`.

##### Parameters

- `x` is a numeric expression in `[-32768—65535]`. Values for negative `x` are shown as two's-complement.

##### Errors

- `x` has a string value: `Type mismatch`.
- `x` is not in `[-32768—65535]`: `Overflow`.

#### `PEEK`

`value = PEEK(address)`

Returns the value of the memory at `segment * 16 + address` where `segment` is the current segment set with `DEF SEG`.

##### Parameters

- `address` is a numeric expression in `[-32768—65535]`. Negative values are interpreted as their two's complement.

##### Notes

- The memory is only partly emulated in PC-BASIC. See Memory model for supported addresses. Outside emulated areas, `PEEK` returns 0.
- Values for particular memory address can be preset on the command line using the `peek` option. This can be used for compatibility with old programs. These values will override video or data segment values, if they are in those locations.

##### Errors

- `address` has a string value: `Type mismatch`.
- `address` is not in `[-32768—65535]`: `Overflow`.

#### `PEN` (function)

`x = PEN(mode)`

Reads the light pen. What this function returns depends on `mode`:

`mode` | Return value
--- | ---
0 | Boolean; whether the light pen has been down since last poll.
1 | x coordinate of last pen down position
2 | y coordinate of last pen down position
3 | Boolean; whether the pen is currently down
4 | x coordinate of current pen position
5 | y coordinate of current pen position
6 | character row coordinate of last pen down position
7 | character column coordinate of last pen down position
8 | character row coordinate of current pen position
9 | character column coordinate of current pen position

##### Parameters

- `mode` is a numeric expression in `[0—9]`.

##### Notes

- In PC-BASIC, for pen down read mouse button pressed. For pen position read mouse pointer position.

##### Errors

- `mode` has a string value: `Type mismatch`.
- `mode` is not in `[-32768—32767]`: `Overflow`.
- `mode` is not in `[0—9]`: `Illegal function call`.

#### `PLAY` (function)

`length = PLAY(voice)`

Returns the number of notes in the background music queue. The return value is in `[0—32]`.

##### Parameters

- `voice` is a numeric expression in `[0—255]`. If `syntax={pcjr|tandy}`, indicates for which tone voice channel the number of notes is to be returned. If `voice` is not in `[0—2]`, the queue for voice 0 is returned. For other choices of `syntax`, the `voice` value has no effect.

##### Notes

- There are at most 32 notes in the music queue. However, unless the articulation is set to legato, there are short gaps between each note; these are counted as separate notes in the queue. Effectively, the queue length is thus 16 for the default and staccato articulations and 32 for legato.

##### Errors

- `voice` has a string value: `Type mismatch`.
- `voice` is not in `[0—255]`: `Illegal function call`.
- `voice` is not in `[-32768—32767]`: `Overflow`.

#### `PMAP`

`transformed_coord = PMAP(original_coord, fn)`

Maps between viewport and logical (`WINDOW`) coordinates. If no `VIEW` has been set, the viewport coordinates are physical coordinates.

Depending on the value of `fn`, `PMAP` transforms from logical to viewport coordinates or vice versa:

`fn` | Return value
--- | ---
0 | return viewport x given logical x
1 | return viewport y given logical y
2 | return logical x given viewport x
3 | return logical y given viewport y

##### Parameters

- `fn` is a numeric expression in `[0—3]`.

##### Notes

- Initially, in text mode, `PMAP` returns 0.
- In GW-BASIC, `PMAP` behaves anomalously on `SCREEN` changes, where it sometimes returns results as if the last `WINDOW` setting had persisted. This behaviour is not implemented in PC-BASIC.

##### Errors

- Any of the parameters has a string value: `Type mismatch`.
- A physical coordinate is not in `[-32768—32767]`: `Overflow`.
- `fn` is not in `[-32768—32767]`: `Overflow`.
- `fn` is not in `[0—3]`: `Illegal function call`.

#### `POINT` (current coordinate)

`coord = POINT(fn)`

Returns a currently active coordinate of the graphics screen. This is usually the last position at which a pixel has been plotted, the second corner given in a `LINE` command, or the centre of the viewport if nothing has been plotted. `fn` is a numeric expression in `[0—3]`.

The coordinate returned depends on the value of `fn`:

`fn` | Return value
--- | ---
0 | viewport x
1 | viewport y
2 | logical x
3 | logical y

##### Parameters

- `fn` is a numeric expression in `[0—3]`.

##### Notes

- In text mode, returns the active coordinate of any previous graphics mode; if no graphics mode has been active, returns 0.

##### Errors

- `fn` has a string value: `Type mismatch`.
- `fn` is not in `[-32768—32767]`: `Overflow`.
- `fn` is not in `[0—3]`: `Illegal function call`.

#### `POINT` (pixel attribute)

`attrib = POINT(x, y)`

Returns the attribute of the pixel at logical coordinate `x`,`y`.

##### Parameters

- `x`, `y` are numeric expressions in `[-32768—32767]`.

##### Notes

- If `x`,`y` is outside the screen, returns -1.

##### Errors

- Function is called in text mode: `Illegal function call`.
- `x` or `y` has a string value: `Type mismatch`.
- `x` or `y` or the physical coordinates they translate into are not in `[-32768—32767]`: `Overflow`.

#### `POS`

`pos = POS(dummy)`

Returns the current cursor column position, in the range `[1—80]`.

##### Parameters

- `dummy` is a valid expression of any type; its value has no effect.

#### `RIGHT$`

`child = RIGHT$(parent, num_chars)`

Returns the rightmost `num_chars` characters of `parent`. If `num_chars` is zero or `parent` is empty, `RIGHT$` returns an empty string. If `num_chars` is greater than the length of `parent`, returns `parent`.

##### Parameters

- `parent` is a string expression.
- `num_chars` is a numeric expression in `[0—255]`.

##### Errors

- `num_chars` has a string value: `Type mismatch`.
- `num_chars` is not in `[-32768—32767]`: `Overflow`.
- `num_chars` is not in `[0—255]`: `Illegal function call`.

#### `RND`

`random = RND[(x)]`

Returns a pseudorandom number in the interval `[0—1)`.

##### Parameters

`x` is a numeric expression.

- If `x` is zero, `RND` repeats the last pseudo-random number.
- If `x` is greater than zero, a new pseudorandom number is returned.
- If `x` is negative, `x` is converted to a single-precision floating-point value and the random number seed is set to the absolute value of its mantissa. The function then generates a new pseudorandom numer with this seed. Since the only the mantissa of `x` is used, any two values whose ratio is a power of 2 will produce the same seed. Note that this procedure for generating a new seed differs from that used by `RANDOMIZE`.

##### Notes

- PC-BASIC's `RND` function generates pseudo-random numbers through a linear congruential generator with modulo 224, multiplier 214013 and increment 2531011. This exactly reproduces the random numbers of GW-BASIC's `RND`.
- It should be noted, however, that this is a very poor random number generator: its parameters imply a recurrence period of 224, meaning that after less than 17 million calls `RND` will wrap around and start running through the exact same series of numbers all over again. `RND` should not be used for cryptography, scientific simulations or anything else remotely serious.

##### Errors

- `x` has a string value: `Type mismatch`.

#### `SCREEN` (function)

`value = SCREEN(row, column [, fn])`

Returns the code point or colour attribute for the character at position `row`, `col`.

##### Parameters

- `row` is a numeric expression in the range `[1—25]`.
- `col` is a numeric expression between 1 and the screen width (40 or 80).
- `fn` is a numeric expression in `[0—255]`. If it is zero or not specified, the code point of the character is returned. If it is non-zero, in text mode the attribute is returned; in other screens, 0 is returned.

##### Errors

- Any parameter has a string value: `Type mismatch`.
- `fn` is not in `[0—255]`: `Illegal function call`.
- `fn` is not in `[-32768—32767]`: `Overflow`.
- `row` is not inside the current `VIEW PRINT` area: `Illegal function call`.
- `KEY ON` and `row=25`: `Illegal function call`.
- `col` is not in `[1, width]`: `Illegal function call`.

#### `SGN`

`sign = SGN(number)`

Returns the sign of `number`: `1` for positive, `0` for zero and `-1` for negative.

##### Parameters

- `number` is a numeric expression.

##### Errors

- `number` has a string value: `Type mismatch`.

#### `SIN`

`sine = SIN(angle)`

Returns the sine of `angle`.

##### Parameters

- `angle` is a numeric expression giving the angle in radians.

##### Notes

- Unless PC-BASIC is run with the `double` option, this function returns a single-precision value.
- The sine returned usually differs from the value returned by GW-BASIC in the least significant figure.

##### Errors

- `angle` has a string value: `Type mismatch`.

#### `SPACE$`

`spaces = SPACE$(number)`

Returns a string of `number` spaces.

##### Parameters

- `number` is a numeric expression in `[0—255]`.

##### Errors

- `number` has a string value: `Type mismatch`.
- `number` is not in `[-32768—32767]`: `Overflow`.
- `number` is not in `[0—255]`: `Illegal function call`.

#### `SQR`

`root = SQR(number)`

Returns the square root of `number`.

##### Parameters

- `number` is a numeric expression.

##### Notes

- Unless PC-BASIC is run with the `double` option, this function returns a single-precision value.

##### Errors

- `number` has a string value: `Type mismatch`

#### `STICK`

`pos = STICK(axis)`

Returns a coordinate of a joystick axis. All coordinates returned are in the range `[1—254]` with `128` indicating the neutral position.

`axis` | Return value
--- | ---
0 | 1st joystick x coordinate
1 | 1st joystick y coordinate
2 | 2nd joystick x coordinate
3 | 2nd joystick y coordinate

##### Parameters

- `axis` is a numeric expression in `[0—3]` and indicates which axis to read.

##### Errors

- `axis` has a string value: `Type mismatch`
- `axis` is not in `[-32768—32767]`: `Overflow`.
- `axis` is not in `[0—3]`: `Illegal function call`.

#### `STR$`

`repr = STR$(number)`

Returns the string representation of `number`.

##### Parameters

- `number` is a numeric expression.

##### Errors

- `number` has a string value: `Type mismatch`.

#### `STRIG` (function)

`result = STRIG(mode)`

Returns the status of the joystick trigger buttons. `STRIG` returns the following results, all Boolean values:

`mode` | Return value
--- | ---
0 | 1st joystick, 1st trigger has been pressed since last poll.
1 | 1st joystick, 1st trigger is currently pressed.
2 | 2nd joystick, 1st trigger has been pressed since last poll.
3 | 2nd joystick, 1st trigger is currently pressed.
4 | 1st joystick, 2nd trigger has been pressed since last poll.
5 | 1st joystick, 2nd trigger is currently pressed.
6 | 2nd joystick, 2nd trigger has been pressed since last poll.
7 | 2nd joystick, 2nd trigger is currently pressed.

##### Parameters

- `mode` is a numeric expression in `[0—7]`.

##### Notes

- The `STRIG` function returns correct results regardless of the `STRIG ON` status or whether `STRIG(0)` has been called first.

##### Errors

- `mode` has a string value: `Type mismatch`.
- `mode` is not in `[-32768—32767]`: `Overflow`.
- `mode` is not in `[0—7]`: `Illegal function call`.

#### `STRING$`

`string = STRING$(length, char)`

Returns a string of `length` times character `char`.

##### Parameters

- If `char` is a numeric expression, it must be in `[0—255]` and is interpreted as the code point of the character.
- If `char` is a string expression, its first character is used.

##### Errors

- `length` has a string value: `Type mismatch`.
- `char` is the empty string: `Illegal function call`.
- `char` or `length` is not in `[-32768—32767]`: `Overflow`.
- `char` or `length` is not in `[0—255]`: `Illegal function call`.

#### `TAN`

`tangent = TAN(angle)`

Returns the tangent of `angle`.

##### Parameters

- `angle` is a numeric expression giving the angle in radians.

##### Notes

- Unless PC-BASIC is run with the `double` option, this function returns a single-precision value.
- The tangent returned usually differs from the value returned by GW-BASIC in the least significant figure.
- For `angle` close to multiples of `π/2`, the tangent is divergent or close to zero. The values returned will have very low precision in these cases.

##### Errors

- `angle` has a string value: `Type mismatch`.

#### `TIME$` (function)

`time = TIME$`

Returns the current BASIC time in the form `"HH:mm:ss"`.

##### Notes

- This function takes no arguments.

#### `TIMER` (function)

`seconds = TIMER`

Returns the number of seconds since midnight on the internal BASIC clock.

##### Notes

- `TIMER` updates in ticks of 1/20 second.
- The least-significant two bytes of `TIMER` are often used as a seed for the pseudorandom number generator through `RANDOMIZE TIMER`. Since these bytes only take values from a limited set, that's not in fact a particularly good random seed. However, the pseudorandom number generator included with GW-BASIC and PC-BASIC is so weak that it should not be used for anything serious anyway.
- This function takes no arguments.

#### `USR`

`value = USR[n](expr)`

Raises `Illegal function call`.

##### Parameters

- `n` is a digit `[0—9]`.
- `expr` is an expression.

##### Notes

- In GW-BASIC, calls a machine-code function and returns its return value.
- This function is not implemented in PC-BASIC.

##### Errors

- `n` is not a digit `[0—9]`: `Syntax error`.

#### `VAL`

`value = VAL(string)`

Returns the numeric value of the string expression `string`. Parsing stops as soon as the first character is encountered that cannot be part of a number. If no characters are parsed, `VAL` returns zero. See the section on numeric literals for the recognised number formats.

##### Notes

- Spaces before or even inside a number are ignored: `VAL(" 1 0")` returns `10`.
- If `string` contains one of the ASCII separator characters `CHR$(28)` (file separator), `CHR$(29)` (group separator) or `CHR$(31)` (unit separator), `VAL` returns zero. This is not the case with `CHR$(30)` (record separator). This behaviour conforms to GW-BASIC.

##### Errors

- `string` has a number value: `Type mismatch`.

#### `VARPTR`

`pointer = VARPTR({name|#file_num})`

Returns the memory address of variable `name` or of the File Control Block of file number `file_num`.

##### Parameters

- `name` is a previously defined variable or fully indexed array element.
- `file_num` is a legal file number.

##### Notes

- `VARPTR` can be used with `PEEK` to read a variable's internal representation.

##### Errors

- `name` has not been previously defined: `Illegal function call`.
- `file_num` has a string value: `Type mismatch`.
- `file_num` is not in `[1, max_files]`, where `max_files` is the maximum number of files as set by the `max-files` option: `Bad file number`.

#### `VARPTR$`

`pointer = VARPTR$(name)`

Returns the memory address of variable `name` in the form of a 3-byte string. The first byte is the length of the record the pointer points to:

2
  for integers
3
  for strings (length + pointer to string space)
4
  for single-precision floats
8
  for double-precision floats

The last two bytes are the pointer address (as returned by `VARPTR`) in little-endian order.

##### Errors

- `name` has not been previously defined: `Illegal function call`.

### Statements

A program line is composed of a line number and one or more statements. If multiple statements are put on one line, they must be separated by colons `:`. Statements may be empty. Each statement has its own idiosyncratic syntax.

Many reference works on GW-BASIC distinguish commands and statements; this distinction stems from the original Dartmouth design of the BASIC language, in which commands were not part of the language and could not be used in programs, but were rather used to control the interpreter itself. However, in GW-BASIC this distinction is less useful and therefore this reference includes what is traditionally thought of as commands in the category of statements.

#### `AUTO`

`AUTO [line_number|.] [, [increment]]`

Start automatic line numbering. Line numbers are automatically generated when `Enter` is pressed. If a program line exists at a generated line number, a `*` is shown after the line number. To avoid overwriting this line, leave it empty and press `Enter`. To stop automatic line numbering, press `Ctrl`+`Break` or `Ctrl`+`C`. The line being edited at that point is not saved. BASIC will return to command mode, even if `AUTO` was run from a program line.

##### Parameters

- Line numbering starts at `line_number`, if specified. If `.` is specified, line numbering starts at the last program line that was stored. Otherwise, line numbering starts at `10`.
- Each next line number is incremented by `increment`, if specified. If a comma is used without specifying an increment, the last increment specified in an `AUTO` command is used. If not, `increment` defaults to `10`.

##### Errors

- `line_number` is not an unsigned-integer value in `[0—65529]`: `Syntax error`.
- When automatic line numbering is enabled and `Enter` is pressed on an empty line with number larger than `65519`: `Undefined line number`.
- `increment` is `0`: `Illegal function call`.

#### `BEEP`

`BEEP`

Beep the speaker at 800Hz for 0.25s.

##### Errors

- If a `Syntax error` is raised, the beep is still produced.

#### `BEEP` (switch)

`BEEP {ON|OFF}`

Switches the internal speaker on or off.

##### Notes

- Only legal with the `syntax={pcjr|tandy}` option.
- On PC-BASIC, both the internal and the external speaker are emulated through the same sound system.

#### `BLOAD`

`BLOAD file_spec [, offset]`

Loads a memory image file into memory.

##### Parameters

- The string expression `file_spec` is a valid file specification indicating the file to read the memory image from.
- `offset` is a numeric expression in the range `[-32768—65535]`. It indicates an offset in the current `DEF SEG` segment where the file is to be stored. If not specified, the offset stored in the `BSAVE` file will be used. If negative, its two's complement will be used.

##### Errors

- The loaded file is not in `BSAVE` format: `Bad file mode`.
- `file_spec` contains disallowed characters: `Bad file number` (on `CAS1:`); `Bad file name` (on disk devices).
- `file_spec` has a numeric value: `Type mismatch`.
- `offset` is not in the range `[-32768—65535]`: `Overflow`.

#### `BSAVE`

`BSAVE file_spec, offset, length`

Saves a region of memory to an image file.

##### Parameters

- The string expression `file_spec` is a valid file specification indicating the file to write to.
- `offset` is a numeric expression in the range `[-32768—65535]` indicating the offset into the current `DEF SEG` segment from where to start reading.
- `length` is a numeric expression in the range `[-32768—65535]` indicating the number of bytes to read.
- If `offset` or `length` are negative, their two's complement will be used.

##### Errors

- `file_spec` has a numeric value: `Type mismatch`.
- `file_spec` contains disallowed characters: `Bad file number` (on `CAS1:`); `Bad file name` (on disk devices).
- `offset` is not in the range `[-32768—65535]`: `Overflow`.
- `length` is not in the range `[-32768—65535]`: `Overflow`.

#### `CALL` and `CALLS`

`{CALL|CALLS} address_var [( p0 [, p1] ... )]`

Does nothing.

##### Notes

- In GW-BASIC, `CALL` or `CALLS` executes a machine language subroutine.
- This statement is not implemented in PC-BASIC.

##### Parameters

- `address_var` is a numeric variable name.
- `p0, p1, ...` are variable names or array elements.

##### Errors

- `address_var` is a string variable: `Type mismatch`.
- `address_var` is a literal or expression: `Syntax error`.

#### `CHAIN`

`CHAIN [MERGE] file_spec [, [line_number_expr] [, ALL] [, DELETE range [, ign]]]`

Loads a program from file into memory and runs it, optionally transferring variables.

- If `ALL` is specified, all variables are transferred. If not, the variables specified in a `COMMON` statement are transferred.
- If `MERGE` is specified, the loaded program is merged into the existing program. To be able to use this, the program file indicated by `file_spec` must be in plain text format.
- If `DELETE` is specified, the `range` of line numbers is deleted from the existing code before the merge. This is pointless without `MERGE`.

##### Parameters

- The string expression `file_spec` is a valid file specification indicating the file to read the program from.
- `line_number_expr` is a numeric expression. It will be interpreted as a line number in the new program and execution will start from this line number. If `line_number_expr` is negative, it will be interpreted as its two's-complement.
- `range` is a line number range of which the closing line number is specified and exists before the merge.
- `ign` is optional and ignored.

##### Notes

- `CHAIN` preserves the `OPTION BASE` setting.
- Only if `ALL` is specified, `DEF FN` definitions are preserved.
- Only if `MERGE` is specified, `DEFINT`, `DEFSTR`, `DEFSNG`, `DEFDBL` definitions are preserved.
- If specified, `ALL` must precede `DELETE`; if unspecified, no comma must be put in its place and only two commas should precede `DELETE`.

##### Errors

- `file_spec` has a numeric value: `Type mismatch`.
- `file_spec` contains disallowed characters: `Bad file number` (on `CAS1:`); `Bad file name` (on disk devices).
- The file specified in `file_spec` cannot be found: `File not found`.
- `MERGE` is specified and the loaded program was not saved in plain-text mode: `Bad file mode`.
- A line number in `range` is greater than 65529: `Syntax error`.
- If a `Syntax error` is raised by `CHAIN`, no lines are deleted and the new program is not loaded.
- The closing line number in `range` does not exist: `Illegal function call`
- If `line_number_expr` does not evaluate to an existing line number in the new program, `Illegal function call` is raised but the load or merge is being performed.
- A loaded text file contains lines without line numbers: `Direct statement in file`.
- A loaded text file contains lines longer than 255 characters: `Line buffer overflow`. Attempting to load a text file that has `LF` rather than `CR LF` line endings may cause this error.

#### `CHDIR`

`CHDIR dir_spec`

Change the current directory on a disk device to `dir_spec`. Each disk device has its own current directory.

##### Parameters

- The string expression `dir_spec` is a valid file specification indicating an existing directory on a disk device.

##### Errors

- No matching path is found: `Path not found`.
- `dir_spec` has a numeric value: `Type mismatch`.
- `dir_spec` is empty: `Bad file name`.

#### `CIRCLE`

`CIRCLE [STEP] (x, y), radius [, [colour] [, [start] [, [end] [, aspect]]]`

Draw an ellipse or ellipse sector.

##### Parameters

- The midpoint of the ellipse is at `(x`,`y)`. If `STEP` is specified, the midpoint is `(x`,`y)` away from the current position.
- `radius` is the radius, in pixels, along the long axis.
- `colour` is the colour attribute.
- If `start` and `end` are specified, a sector of the ellipse is drawn from `start` radians to `end` radians, with zero radians the intersection with the right-hand x axis. If a negative value is specified, the arc sector is connected by a line to the midpoint.
- `aspect` specifies the ratio between the y radius and the x radius. If it is not specified, the standard value for the `SCREEN` mode is used (see there), so as to make the ellipse appear like a circle on the original hardware.

##### Notes

- For `aspect <> 1`, the midpoint algorithm used does not pixel-perfectly reproduce GW-BASIC's ellipses.

##### Errors

- The statement is executed in text mode: `Illegal function call`.
- `start` or `end` is not in `[0—2π]`: `Illegal function call`.
- The statement ends with a comma: `Missing operand`.

#### `CLEAR`

`CLEAR [expr] [, [mem_limit] [, [stack_size] [, video_memory]]]`

Clears all variables, arrays,`DEF FN` user functions and `DEFtype` type definitions. Closes all files. Turns off all sound. Resets `PLAY` state and sets music to foreground. Clears all `ON ERROR` traps. Resets `ERR` and `ERL` to zero. Disables all events. Turns `PEN` and `STRIG` off. Resets the random number generator. Clears the loop stack. Resets the `DRAW` state and the current graphics position.

##### Parameters

- `mem_limit` specifies the upper limit of usable memory. Default is previous memory size. Default memory size is 65534.
- `stack_size` specifies the amount of memory available to the BASIC stack. Default is previous stack size. Default stack size is 512.
- `video_memory` specifies the amount of memory available to the video adapter. This parameter is only legal with one of the options `syntax={pcjr, tandy}`. Instead of using `CLEAR`, the option `video-memory` can also be used to set video memory size.

##### Notes

- The purpose of `expr` is unknown.
- If called inside a `FOR`—`NEXT` or `WHILE`—`WEND` loop, an error will be raised at the `NEXT` or `WEND` statement, since the loop stacks have been cleared.

##### Errors

- Any of the arguments has a string value: `Type mismatch`.
- `mem_limit`, `stack_size` are not in `[-32768—65535]`: `Overflow`.
- `mem_limit` or `stack_size` equal `0`: `Illegal function call`.
- `mem_limit` equals `-1` or `65535`: `Out of memory`.
- `mem_limit` or `expr` are too low: `Out of memory`.
- `expr` is not in `[-32768—32767]`: `Overflow`.
- `expr` is negative: `Illegal function call`.

#### `CLOSE`

`CLOSE [[#] file_0 [, [#] file_1] ...]`

Closes files. If no file numbers are specified, all open files are closed. The hash (`#`) is optional and has no effect.

##### Parameters

- `file_1, file_2, ...` are numeric expressions yielding file numbers.

##### Notes

- No error is raised if the specified file numbers were not open.

##### Errors

- `file_1, file_2, ...` are not in `[-32768—32767]`: `Overflow`.
- `file_1, file_2, ...` are not in `[0—255]`: `Illegal function call`.
- `file_1, file_2, ...` have a string value: `Type mismatch`.
- The statement ends in a comma, `Missing operand`.
- If an error occurs, only the files before the erratic value are closed.

#### `CLS`

`CLS [x][,]`

Clears the screen or part of it. If `x` is not specified, in SCREEN 0 the text view region is cleared; in other screens, the graphics view region is cleared. The comma is optional and has no effect.

##### Parameters

`x` is a numeric valued expression.

- If `x = 0`, the whole screen is cleared.
- If `x = 1`, the graphics view region is cleared.
- If `x = 2`, the text view region is cleared.

The optional argument `x` is not available with `syntax={pcjr|tandy}`.

##### Errors

- `x` is has a string value: `Type mismatch`.
- `x` is not in `[-32768—32767]`: `Overflow` .
- `x` is not in `[0, 1, 2]`: `Illegal function call`.
- No comma is specified but more text follows: `Illegal function call`.
- A comma is specified followed by more: `Syntax error`.
- `syntax=pcjr` is set and an argument is specified: `Syntax error`.
- `syntax=tandy` is set and an argument is specified: `Illegal function call`.
- If an error occurs, the screen is not cleared.

#### `COLOR` (text mode)

`COLOR [foreground] [, [background] [, border]]`

Changes the current foreground and background attributes. All new characters printed will take the newly set attributes. Existing characters on the screen are not affected.

##### Parameters

- `foreground` is a numeric expression in `[0—31]`. This specifies the new foreground attribute. Attributes `16—31` are blinking versions of attributes `0—15`.
- `background` is a numeric expression in `[0—15]`. This specifies the new background attribute. It is taken `MOD 8`: Values `8—15` produce the same colour as `0—7`.
- `border` is a numeric expression in `[0—15]` specifying the border attribute.

##### Textmode attributes (colour)

| Background attribute
--- | ---
0 | 1 | 2 | 3 | 4 | 5 | 6 | 7
FG | 0 | `00` XX | `10` XX | `20` XX | `30` XX | `40` XX | `50` XX | `60` XX | `70` XX
1 | `01` XX | `11` XX | `21` XX | `31` XX | `41` XX | `51` XX | `61` XX | `71` XX
2 | `02` XX | `12` XX | `22` XX | `32` XX | `42` XX | `52` XX | `62` XX | `72` XX
3 | `03` XX | `13` XX | `23` XX | `33` XX | `43` XX | `53` XX | `63` XX | `73` XX
4 | `04` XX | `14` XX | `24` XX | `34` XX | `44` XX | `54` XX | `64` XX | `74` XX
5 | `05` XX | `15` XX | `25` XX | `35` XX | `45` XX | `55` XX | `65` XX | `75` XX
6 | `06` XX | `16` XX | `26` XX | `36` XX | `46` XX | `56` XX | `66` XX | `76` XX
7 | `07` XX | `17` XX | `27` XX | `37` XX | `47` XX | `57` XX | `67` XX | `77` XX
8 | `08` XX | `18` XX | `28` XX | `38` XX | `48` XX | `58` XX | `68` XX | `78` XX
9 | `09` XX | `19` XX | `29` XX | `39` XX | `49` XX | `59` XX | `69` XX | `79` XX
10 | `0a` XX | `1a` XX | `2a` XX | `3a` XX | `4a` XX | `5a` XX | `6a` XX | `7a` XX
11 | `0b` XX | `1b` XX | `2b` XX | `3b` XX | `4b` XX | `5b` XX | `6b` XX | `7b` XX
12 | `0c` XX | `1c` XX | `2c` XX | `3c` XX | `4c` XX | `5c` XX | `6c` XX | `7c` XX
13 | `0d` XX | `1d` XX | `2d` XX | `3d` XX | `4d` XX | `5d` XX | `6d` XX | `7d` XX
14 | `0e` XX | `1e` XX | `2e` XX | `3e` XX | `4e` XX | `5e` XX | `6e` XX | `7e` XX
15 | `0f` XX | `1f` XX | `2f` XX | `3f` XX | `4f` XX | `5f` XX | `6f` XX | `7f` XX
16 | `80` XX | `90` XX | `a0` XX | `b0` XX | `c0` XX | `d0` XX | `e0` XX | `f0` XX
17 | `81` XX | `91` XX | `a1` XX | `b1` XX | `c1` XX | `d1` XX | `e1` XX | `f1` XX
18 | `82` XX | `92` XX | `a2` XX | `b2` XX | `c2` XX | `d2` XX | `e2` XX | `f2` XX
19 | `83` XX | `93` XX | `a3` XX | `b3` XX | `c3` XX | `d3` XX | `e3` XX | `f3` XX
20 | `84` XX | `94` XX | `a4` XX | `b4` XX | `c4` XX | `d4` XX | `e4` XX | `f4` XX
21 | `85` XX | `95` XX | `a5` XX | `b5` XX | `c5` XX | `d5` XX | `e5` XX | `f5` XX
22 | `86` XX | `96` XX | `a6` XX | `b6` XX | `c6` XX | `d6` XX | `e6` XX | `f6` XX
23 | `87` XX | `97` XX | `a7` XX | `b7` XX | `c7` XX | `d7` XX | `e7` XX | `f7` XX
24 | `88` XX | `98` XX | `a8` XX | `b8` XX | `c8` XX | `d8` XX | `e8` XX | `f8` XX
25 | `89` XX | `99` XX | `a9` XX | `b9` XX | `c9` XX | `d9` XX | `e9` XX | `f9` XX
26 | `8a` XX | `9a` XX | `aa` XX | `ba` XX | `ca` XX | `da` XX | `ea` XX | `fa` XX
27 | `8b` XX | `9b` XX | `ab` XX | `bb` XX | `cb` XX | `db` XX | `eb` XX | `fb` XX
28 | `8c` XX | `9c` XX | `ac` XX | `bc` XX | `cc` XX | `dc` XX | `ec` XX | `fc` XX
29 | `8d` XX | `9d` XX | `ad` XX | `bd` XX | `cd` XX | `dd` XX | `ed` XX | `fd` XX
30 | `8e` XX | `9e` XX | `ae` XX | `be` XX | `ce` XX | `de` XX | `ee` XX | `fe` XX
31 | `8f` XX | `9f` XX | `af` XX | `bf` XX | `cf` XX | `df` XX | `ef` XX | `ff` XX

##### Textmode attributes (monochrome)

| Background attribute
--- | ---
0 | 1 | 2 | 3 | 4 | 5 | 6 | 7
FG | 0 | `00` XX | `10` XX | `20` XX | `30` XX | `40` XX | `50` XX | `60` XX | `70` XX
1 | `01` XX | `11` XX | `21` XX | `31` XX | `41` XX | `51` XX | `61` XX | `71` XX
2 | `02` XX | `12` XX | `22` XX | `32` XX | `42` XX | `52` XX | `62` XX | `72` XX
3 | `03` XX | `13` XX | `23` XX | `33` XX | `43` XX | `53` XX | `63` XX | `73` XX
4 | `04` XX | `14` XX | `24` XX | `34` XX | `44` XX | `54` XX | `64` XX | `74` XX
5 | `05` XX | `15` XX | `25` XX | `35` XX | `45` XX | `55` XX | `65` XX | `75` XX
6 | `06` XX | `16` XX | `26` XX | `36` XX | `46` XX | `56` XX | `66` XX | `76` XX
7 | `07` XX | `17` XX | `27` XX | `37` XX | `47` XX | `57` XX | `67` XX | `77` XX
8 | `08` XX | `18` XX | `28` XX | `38` XX | `48` XX | `58` XX | `68` XX | `78` XX
9 | `09` XX | `19` XX | `29` XX | `39` XX | `49` XX | `59` XX | `69` XX | `79` XX
10 | `0a` XX | `1a` XX | `2a` XX | `3a` XX | `4a` XX | `5a` XX | `6a` XX | `7a` XX
11 | `0b` XX | `1b` XX | `2b` XX | `3b` XX | `4b` XX | `5b` XX | `6b` XX | `7b` XX
12 | `0c` XX | `1c` XX | `2c` XX | `3c` XX | `4c` XX | `5c` XX | `6c` XX | `7c` XX
13 | `0d` XX | `1d` XX | `2d` XX | `3d` XX | `4d` XX | `5d` XX | `6d` XX | `7d` XX
14 | `0e` XX | `1e` XX | `2e` XX | `3e` XX | `4e` XX | `5e` XX | `6e` XX | `7e` XX
15 | `0f` XX | `1f` XX | `2f` XX | `3f` XX | `4f` XX | `5f` XX | `6f` XX | `7f` XX
16 | `80` XX | `90` XX | `a0` XX | `b0` XX | `c0` XX | `d0` XX | `e0` XX | `f0` XX
17 | `81` XX | `91` XX | `a1` XX | `b1` XX | `c1` XX | `d1` XX | `e1` XX | `f1` XX
18 | `82` XX | `92` XX | `a2` XX | `b2` XX | `c2` XX | `d2` XX | `e2` XX | `f2` XX
19 | `83` XX | `93` XX | `a3` XX | `b3` XX | `c3` XX | `d3` XX | `e3` XX | `f3` XX
20 | `84` XX | `94` XX | `a4` XX | `b4` XX | `c4` XX | `d4` XX | `e4` XX | `f4` XX
21 | `85` XX | `95` XX | `a5` XX | `b5` XX | `c5` XX | `d5` XX | `e5` XX | `f5` XX
22 | `86` XX | `96` XX | `a6` XX | `b6` XX | `c6` XX | `d6` XX | `e6` XX | `f6` XX
23 | `87` XX | `97` XX | `a7` XX | `b7` XX | `c7` XX | `d7` XX | `e7` XX | `f7` XX
24 | `88` XX | `98` XX | `a8` XX | `b8` XX | `c8` XX | `d8` XX | `e8` XX | `f8` XX
25 | `89` XX | `99` XX | `a9` XX | `b9` XX | `c9` XX | `d9` XX | `e9` XX | `f9` XX
26 | `8a` XX | `9a` XX | `aa` XX | `ba` XX | `ca` XX | `da` XX | `ea` XX | `fa` XX
27 | `8b` XX | `9b` XX | `ab` XX | `bb` XX | `cb` XX | `db` XX | `eb` XX | `fb` XX
28 | `8c` XX | `9c` XX | `ac` XX | `bc` XX | `cc` XX | `dc` XX | `ec` XX | `fc` XX
29 | `8d` XX | `9d` XX | `ad` XX | `bd` XX | `cd` XX | `dd` XX | `ed` XX | `fd` XX
30 | `8e` XX | `9e` XX | `ae` XX | `be` XX | `ce` XX | `de` XX | `ee` XX | `fe` XX
31 | `8f` XX | `9f` XX | `af` XX | `bf` XX | `cf` XX | `df` XX | `ef` XX | `ff` XX

##### Notes

- The syntax and effect of `COLOR` is different in different `SCREEN` modes: `COLOR` (text mode), `COLOR` (`SCREEN 1`), (`SCREEN 3—9`).
- At least one parameter must be provided and the statement must not end in a comma.

##### Errors

- Any of the parameters has a string value: `Type mismatch`.
- Any of the parameters is not in `[-32768—32767]`: `Overflow`.
- `foreground` is not in `[0—31]`, `background` is not in `[0—15]` or `border` is not in `[0—15]`: `Illegal function call`.
- Statement is used in `SCREEN 2`: `Illegal function call`.

#### `COLOR` (`SCREEN 1`)

`COLOR [palette_0] [, palette [, override]]`

Assigns new colours to the palette of attributes.

- `palette_0` is a numeric expression in `[0—255]`. This sets the palette colour associated with attribute 0; by default, the background has this attribute. All pixels with this attribute will change colour. The palette colour value is taken from the 64-colour set. `palette_0` is taken `MOD 64`.
- `palette` is a numeric expression in `[0—255]` that specifies the palette:
  - `palette` odd sets the standard CGA palette (cyan, magenta, grey).
  - `palette` even sets the alternative palette (green, red, brown).
  All pixels with attributes 1,2,3 will change colour to the new palette.
- `override` is a numeric expression in `[0—255]`. If `override` is specified, palette is set as above but using `override` instead of `palette`. `palette` is then ignored.

##### CGA palettes

Attribute | Palette 0 | Palette 1 | Alternate palette
--- | --- | --- | ---
Colour | Lo | Hi | Colour | Lo | Hi | Colour | Lo | Hi
0 | Black |  |  | Black |  |  | Black |  |
1 | Green |  |  | Cyan |  |  | Cyan |  |
2 | Red |  |  | Magenta |  |  | Red |  |
3 | Brown |  |  | White |  |  | White |  |

##### Notes

- The syntax and effect of `COLOR` is different in different `SCREEN` modes: `COLOR` (text mode), `COLOR` (`SCREEN 1`), (`SCREEN 3—9`).
- At least one parameter must be provided and the statement must not end in a comma.

##### Errors

- Any of the parameters has a string value: `Type mismatch`.
- Any of the parameters is not in `[-32768—32767]`: `Overflow`.
- Any of the parameters is not in `[0—255]`: `Illegal function call`.

#### `COLOR` (`SCREEN 3—9`)

`COLOR [foreground] [, palette_0 [, dummy]]`

Changes the current foreground attribute and the colour for attribute 0.

##### Parameters

- `foreground` is a numeric expression in `[1—15]` This sets the new foreground attribute. This applies only to new characters printed or pixels plotted.
- `palette_0` is a numeric expression in `[0—15]` This sets the colour associated with attribute 0; by default, the background has this attribute. All pixels with this attribute will change colour. In `SCREEN 7` and `8`, the `palette_0` colour is taken from the first 8 of the 16-colour EGA set. `palette_0` is taken `MOD 8`. IN `SCREEN 9`, the colour value is taken from the 64-colour set.
- `dummy` is a numeric expression with a value in `[0—255]` The value of `dummy` is ignored.

##### EGA default palette

Attribute | Colour |
--- | --- | ---
0 | Black |
1 | Blue |
2 | Green |
3 | Cyan |
4 | Red |
5 | Magenta |
6 | Brown |
7 | Low-intensity white |
8 | Grey |
9 | Light Blue |
10 | Light Green |
11 | Light Cyan |
12 | Light Red |
13 | Light Magenta |
14 | Light Yellow |
15 | High-intensity white |

##### EGA colour list

0 |  | 8 |  | 16 |  | 24 |  | 32 |  | 40 |  | 48 |  | 56 |
1 |  | 9 |  | 17 |  | 25 |  | 33 |  | 41 |  | 49 |  | 57 |
2 |  | 10 |  | 18 |  | 26 |  | 34 |  | 42 |  | 50 |  | 58 |
3 |  | 11 |  | 19 |  | 27 |  | 35 |  | 43 |  | 51 |  | 59 |
4 |  | 12 |  | 20 |  | 28 |  | 36 |  | 44 |  | 52 |  | 60 |
5 |  | 13 |  | 21 |  | 29 |  | 37 |  | 45 |  | 53 |  | 61 |
6 |  | 14 |  | 22 |  | 30 |  | 38 |  | 46 |  | 54 |  | 62 |
7 |  | 15 |  | 23 |  | 31 |  | 39 |  | 47 |  | 55 |  | 63 |

##### Notes

- The syntax and effect of `COLOR` is different in different `SCREEN` modes: `COLOR` (text mode), `COLOR` (`SCREEN 1`), (`SCREEN 3—9`).
- At least one parameter must be provided and the statement must not end in a comma.

##### Errors

- Any of the parameters has a string value: `Type mismatch`.
- Any of the parameters is not in `[-32768—32767]`: `Overflow`.
- `foreground` is not in `[1—15]`; `background` is not in `[0—15]`; or `dummy` is not in `[0—255]`: `Illegal function call`.

#### `COM`

`COM(port) {ON|OFF|STOP}`

- `ON`: enables `ON COM(port)` event trapping of the emulated serial port.
- `OFF`: disables trapping.
- `STOP`: halts trapping until `COM(port) ON` is used. Events that occur while trapping is halted will trigger immediately when trapping is re-enabled.

##### Parameters

- `port` is a numeric expression with a value of `1` or `2`. This specifies which serial port (`COM1:` or `COM2:`) is trapped. If `port` equals `0`, this statement does nothing.

##### Errors

- `port` a string value: `Type mismatch`.
- `port` is not in `[-32768—32767]`: `Overflow`.
- `port` is not in `[0—3]`: `Illegal function call`.

#### `COMMON`

`COMMON [var_0 [( [index_0] )] [, [var_1 [( [index_1] )] ]] ...]`

Specifies variables to be passed as common variables to the program called with `CHAIN`.

##### Parameters

- `var_0, var_1, ...` are names of scalar or array variables.
- `index_0, index_1, ...` are optional number literals; they are ignored.

##### Notes

- Array elements with square brackets and an index do not cause an error, but are ignored.
- `COMMON` statements are not executed during run time; rather, when a `CHAIN` command is encountered where `ALL` is not specified, all `COMMON` declarations in the program are parsed. As a consequence, the `DEFSTR`, `DEFINT`, `DEFSNG` or `DEFDBL` settings used are those that are active at the time of execution of the `CHAIN` statement.
- `COMMON` declarations need not be reachable in the program flow in order to be used. They may occur anywhere before or after the `CHAIN` statement that uses them.
- Variables may be repeated or occur in multiple `COMMON` declarations.
- If the `COMMON` keyword is not the first element of the statement, the declaration will be ignored. In particular, any `COMMON` declaration that occurs directly after a `THEN` or `ELSE` keyword will not be used. `COMMON` in the second or later statements of a compound statement after `THEN` or `ELSE` will be used regardless of the value of the `IF` condition.

#### `CONT`

`CONT [anything]`

Resumes execution of a program that has been halted by `STOP`, `END`, `Ctrl`+`C`, or `Ctrl`+`Break`.

##### Notes

- Anything after the `CONT` keyword is ignored.
- This statement can only be used in direct mode.
- If a break is encountered in `GOSUB` routine called from a continuing direct line (e.g. `GOSUB 100:PRINT A$`), `CONT` will overwrite the running direct line. As the subroutine `RETURN`s to the position after the `GOSUB` in the old direct line, strange things may happen if commands are given after `CONT`. In GW-BASIC, this can lead to strange errors in non-existing program lines as the parser executes bytes that are not part of a program line. In PC-BASIC, if the new direct line is shorter, execution stops after `RETURN`; but if the direct line is extended beyond the old return position, the parser tries to resume at that return position, with strange effects.

##### Errors

- No program is loaded, a program has not been run, after a program line has been modified or after `CLEAR`: `Can't continue`.
- The break occurred in a direct line: `Can't continue`.
- `CONT` is used in a program: `Can't continue`.

#### `DATA`

`DATA [const_0] [, [const_1]] ...`

Specifies data that can be read by a `READ` statement.

##### Parameters

- `const_0, const_1, ...` are string and number literals or may be empty. String literals can be given with or without quotation marks. If quotation marks are omitted, leading and trailing whitespace is ignored and commas or colons will terminate the data statement.

##### Notes

- `DATA` declarations need not be reachable in the program flow in order to be used. They may occur anywhere before or after the `READ` statement that uses them.
- If the `DATA` keyword is not the first element of the statement, the declaration will be ignored. In particular, any `DATA` declaration that occurs directly after a `THEN` or `ELSE` keyword will not be used. `DATA` in the second or later statements of a compound statement after `THEN` or `ELSE` will be used regardless of the value of the `IF` condition.

##### Errors

- If the type of the literal does not match that of the corresponding `READ` statement, a `Syntax error` occurs on the `DATA` statement.

#### `DATE$` (statement)

`DATE$ = date`

Sets the system date. `date` is a string expression that represents a date in one of the formats: `"mm{-|/}dd{-|/}yy"` or `"mm{-|/}dd{-|/}yyyy"`

Of these,

- `mm` may be one or two characters long and must be in `[1—12]`.
- `dd` may be one or two characters long and must be in `[1—31]`.
- `yyyy` must be in `[1980—2099]`.
- `yy` may be one or two characters long and must be in one of the ranges:
  - `[0—77]`, interpreted as `2000—2077`; or
  - `[80—99]`, interpreted as `1980—1999`.

##### Notes

- The system date is not actually changed; rather, PC-BASIC remembers the offset from the true system date. This avoids requiring user permission to change the system time.
- GW-BASIC appears to accept invalid dates such as `"02-31-2000"`. PC-BASIC raises `Illegal function call` for these.

##### Errors

- `date` has a numeric value: `Type mismatch`.
- `date` is not in the format specified above: `Illegal function call`.

#### `DEF FN`

`DEF FN[ ]name [(arg_0 [, arg_1] ...)] = expression`

Defines a function called `FNname` (or `FN name`: spaces between `FN` and `name` are optional). On calling `FNname( ... )`, `expression` is evaluated with the supplied parameters substituted. Any variable names used in the function that are not in the argument list refer to the corresponding global variables. The result of the evaluation is the return value of `FNname`. The type of the return value must be compatible with the type indicated by `name`.

##### Notes

- This statement may only be used on a program line.
- As the function must be a single expression and PC-BASIC does not have a ternary operator, there is no way to define a recursive function that actually terminates.

##### Parameters

- `name` must be a legal variable name.
- `arg_0, arg_1, ...` must be legal variable names. These are the parameters of the function. Variables of the same name may or may not exist in the program; their value is not affected or used by the defined function.
- `expression` must be a legal PC-BASIC expression.

##### Errors

- The statement is executed directly instead of in a program line: `Illegal direct`.
- If the type of the return value is incompatible with the type of `name`, no error is raised at the `DEF FN` statement; however, a `Type mismatch` will be raised at the first call of `FNname`.

#### `DEFINT`, `DEFDBL`, `DEFSNG`, `DEFSTR`

`{DEFINT|DEFDBL|DEFSNG|DEFSTR} first_0[- last_0] [, first_1[- last_1]] ...`

Sets the type that is assumed if no sigil is specified when a variable name is used. The statement sets the default type for variables starting with a letter from the ranges specified.

The default type is set to:

`DEFINT`
  integer (`%`)
`DEFDBL`
  double (`#`)
`DEFSNG`
  single (`!`)
`DEFSTR`
  string (`$`)

##### Parameters

- `first_0, last_0, ...` are letters of the alphabet. Pairs of letters connected by a dash `-` indicate inclusive ranges.

##### Notes

- `DEFSNG A-Z` is the default setting.

#### `DEF SEG`

`DEF SEG [= address]`

Sets the memory segment to be used by `BLOAD`, `BSAVE`, `CALL`, `PEEK`, `POKE`, and `USR`.

##### Parameters

- `address` is a numeric expression in `[-32768—65535]`.

##### Notes

- If `address` is negative, it is interpreted as its two's complement.
- If `address` is not specified, the segment is set to the GW-BASIC data segment.

##### Errors

- `address` has a string value: `Type mismatch`.
- `address` is not in `[-32768—65535]`: `Overflow`.

#### `DEF USR`

`DEF USR[n] = address`

Does nothing.

##### Parameters

- `n` is a digit between 0 and 9 inclusive.
- `address` is a numeric expression in `[-32768—65535]`.

##### Notes

- In GW-BASIC, this statement sets the starting address of an assembly-language function.
- This statement is not implemented in PC-BASIC.
- If `address` is negative, it is interpreted as its two's complement.

##### Errors

- `n` is not a digit in `[0—9]`: `Syntax error`.
- `address` has a string value: `Type mismatch`.
- `address` is not in `[-32768—65535]`: `Overflow`.

#### `DELETE`

`DELETE [line_number_0|.] [-[line_number_1|.] ]`

Deletes a range of lines from the program. Also stops program execution and returns control to the user.

##### Parameters

- `line_number_0` and `line_number_1` are line numbers in the range `[0—65529]`, specifying the inclusive range of line numbers to delete.
- A `.` indicates the last line edited.
- If the start point is omitted, the range will start at the start of the program.
- If the end point is omitted, the range will end at the end of the program.
- If no range is specified, the whole program will be deleted.

##### Errors

- `line_number_0` or `line_number_1` is greater than `65529`: `Syntax error`.
- The range specified does not include any program lines stored: `Illegal function call`.

#### `DIM`

`DIM name [{(|[} limit_0 [, limit_1] ... {)|]}] [, ... ]`

Allocates memory for one or more arrays. The `DIM` statement also fixes the number of indices of the array. An array can only be allocated once; to re-allocate an array, `ERASE` or `CLEAR` must be executed first. If an array is first used without a `DIM` statement, it is automatically allocated with its maximum indices set at `10` for each index position used. A `DIM` entry with no brackets and no indices performs no operation. Empty brackets are not allowed. The least index allowed is determined by `OPTION BASE`.

##### Parameters

- `name, ...` are legal variable names specifying the arrays to be allocated.
- `limit_0, limit_1, ...` are numeric expressions that specify the greatest index allowed at that position.

##### Notes

- Mixed brackets are allowed.
- The size of arrays is limited by the available BASIC memory.
- The maximum number of indices is, theoretically, `255`. In practice, it is limited by the 255-byte limit on the length of program lines.

##### Errors

- `name` has already been dimensioned: `Duplicate definition`.
- An index is empty: `Syntax error`.
- An index is missing at the end: `Missing operand`.
- `limit_0, limit_1, ...` have a string value: `Type mismatch`.
- `limit_0, limit_1, ...` are not within `[-32768—32767]`: `Overflow`.
- `limit_0, limit_1, ...` are negative: `Illegal function call`.
- The array exceeds the size of available variable space: `Out of memory`.

#### `DRAW`

`DRAW gml_string`

Draws the shape specified by `gml_string`, a string expression in Graphics Macro Language (GML).

##### Graphics Macro Language reference

Movement commands
  `[B][N] movement` where the default is to move and draw; the optional prefixes mean:

  `B` | move but do not plot
  `N` | return to original point after move

  and `movement` is one of:

  `U[n]` | up `n` steps
  `L[n]` | left `n` steps
  `D[n]` | down `n` steps
  `R[n]` | right `n` steps
  `E[n]` | up and right `n` steps
  `F[n]` | down and right `n` steps
  `G[n]` | down and left `n` steps
  `H[n]` | up and left `n` steps
  `M{+|-}x,[+|-]y` | move (`x`,`y`) steps
  `Mx,y` | move to view region coordinate (`x`,`y`)

  where `n` is an integer in `[-32768—32767]` and `x`, `y` are integers in `[0—9999]`. Where optional, `n` defaults to `1`.

Scale commands

  `Sn` | set the step size to `n/4`. The default step size is 1 pixel. `n` is an integer in `[1—255]`
  `TAn` | set the angle to `n` degrees. The default angle is 0 degrees. `n` is an integer in `[-360—360]`
  `An` | set the angle to 0 for `n=0`, 90 for `n=1`, 180 for `n=2`, 270 for `n=3`. `n` is an integer in `[0—3]`

Colour commands

  `Cn` | set the foreground attribute to `n`, where `n` is an integer in `[-32768—32767]` See `COLOR`.
  `Pn,b` | flood fill with attribute `n` and boundary attribute `b`, where `n, b` are integers in `[0—9999]` See `PAINT`.

Subroutine command

  `Xs` | execute a substring

  `s` is one of the following:

  - a string variable name followed by semicolon (`;`)
  - the result of `VARPTR$()` on a string variable

Numeric variables `n`, `x`, `y`, `b` in the commands above can be:

- an integer literal, e.g. `DRAW "U100"`
- a numeric variable name or array element `var` preceded by `=` and followed by `;`. For example, `DRAW "U=VAR;"` or `DRAW "U=A(1);"`
- the result of `VARPTR$(var)` preceded by `=`. For example, `DRAW "U=" + VARPTR$(VAR)`

##### Notes

- The `CLS` statement resets the step size to 1 pixel, angle to 0 degrees and position to the centre of the view region.
- The value `n` in the `TA`, `A` and `C` command can be left out but only if the command is terminated by a semicolon. `n` defaults to `0`.
- In GW-BASIC, the numeric arguments of `U`, `L`, `D`, `R`, `E`, `F`, `G`, `H`, and `C` can be in the range `[-99999—99999]`; however, results for large numbers are unpredictable. This is not implemented in PC-BASIC.

##### Errors

- `gml_string` has a numeric value: `Type mismatch`.
- `gml_string` has errors in the GML: `Illegal function call`.
- A variable referenced in the GML string is of incorrect type: `Type mismatch`.

#### `EDIT`

`EDIT {line_number|.}`

Displays the specified program line with the cursor positioned for editing. `line_number` must be a line that exists in the program, or a period (.) to indicate the last line stored.

##### Errors

- No line_number is specified: `Undefined line number`.
- More characters are written after the line number: `Illegal function call`.
- `line_number` is not in `[0—65529]`: `Illegal function call`.
- The specified line number does not exist: `Undefined line number`.

#### `ELSE`

`ELSE [anything]`

Unless part of an `IF` statement on the same line, anything after `ELSE` is ignored in the same way as after `'` or `:REM`. No colon `:` preceding the `ELSE` statement is necessary. See `IF` for normal usage.

#### `END`

`END`

Closes all files, stops program execution and returns control to the user. No message is printed. It is possible to resume execution at the next statement using `CONT`.

#### `ENVIRON`

`ENVIRON command_string`

Sets a shell environment variable.

##### Parameters

`command_string` is a string expression of one of the following forms:

`"VARIABLE=VALUE"`
  to set `VARIABLE` to `VALUE`;
`"VARIABLE="`
  to unset `VARIABLE`.

##### Errors

- `command_string` has a numeric value: `Type mismatch`.
- `command_string` is not of the required form: `Illegal function call`.

#### `ERASE`

`ERASE array_0 [, array_1] ...`

De-allocates arrays. The data stored in the arrays is lost.

##### Parameters

- `array_0, array_1 ...` are names of existing arrays. The names must be specified without brackets.

##### Errors

- No array names are given: `Syntax error`.
- `array_0, array_1 ...` do not exist: `Illegal function call`.
- If an error occurs, the arrays named before the error occurred are erased.

#### `ERROR`

`ERROR error_number`

Raises the error with number `error_number`.

##### Parameters

- `error_number` is an expression with a numeric value.

##### Errors

- `error_number` has a string value: `Type mismatch`.
- `error_number` is not in `[-32768—32767]`: `Overflow`.
- `error_number` is not in `[1—255]`: `Illegal function call`.

#### `FIELD`

`FIELD [#] file_number [, width_0 AS name_0 [, width_1 AS name_1] ...]`

Assigns variables to the random-access record buffer. The record buffer is a region of memory of length defined by the `OPEN` statement; the default record length is 128 bytes. The `FIELD` statement assigns a portion of this region to one or more fixed-length string variables, so that the value of these strings is whatever happens to be in the record buffer at that location.

##### Notes

- A `FIELD` statement without any variables specified has no effect.
- Another `FIELD` statement on the same file will specify an alternative mapping of the same file buffer; all mappings will be in effect simultaneously.
- A subsequent assignment or `LET` or `MID$` statement on `name_0 , name_1 ...` will dis- associate the string variable from the field buffer.
- Use `LSET`, `RSET` or `MID$` to copy values into a `FIELD` buffer.
- Use `GET` to read values from the file into the field buffer, changing the variables.
- Use `PUT` to write the field buffer to the file.

##### Parameters

- `file_number` is a numeric expression that yields the number of an open random-access file. The `#` is optional and has no effect.
- `width_0, width_1, ...` are numeric expressions giving the length of the string variables
- `name_0 , name_1 ...` are string variables.

##### Errors

- `file_number` is not in `[0—255]`: `Illegal function call`.
- `file_number` is not the number of an open file: `Bad file number`.
- `file_number` is open under a mode other than `RANDOM`: `Bad file mode`.
- The statement ends in a comma: `Missing operand`.
- No file number is specified: `Missing operand`.
- The lengths in a `FIELD` statement add up to a number larger than the record length of the field buffer: `Field overflow`.
- `name_0 , name_1 ...` specify a non-string variable: `Type mismatch`.

#### `FILES`

`FILES [filter_spec]`

Displays the files fitting the specified filter in the specified directory on a disk device. If `filter_spec` is not specified, displays all files in the current working directory.

##### Parameters

- `filter_spec` is a string expression that is much like a file specification, but optionally allows the file name part to contain wildcards.

##### Notes

- The filename filter may contain the following wildcards:

  `?` | Matches any legal file name character.
  `*` | Matches any series of legal file name characters.

- The filter will only match MS-DOS style filenames.
- Matched character series do not stretch across directory separators `\` or extension separators `.`. To match all files with all extensions, use `*.*`.
- Alternatively, if all files in a specified directory are required, end the directory name with a backslash `\`.

##### Errors

- `filter_spec` has a numeric value: `Type mismatch`.
- `filter_spec` is the empty string: `Bad file name`.
- The specified filter does not match any files: `File not found`.

#### `FOR`

`FOR loop_var = start TO stop [STEP step]`

Initiates a `FOR—NEXT` loop.

Initially, `loop_var` is set to `start`. Then, the statements between the `FOR` statement and the `NEXT` statement are executed and `loop_var` is incremented by `step` (if `step` is not specified, by 1). This is repeated until `loop_var` has become greater than `stop`. Execution then continues at the statement following `NEXT`. The value of `loop_var` equals `stop+step` after the loop.

`start`, `stop` and `step` are evaluated only once and the resulting values are used throughout the loop.

##### Parameters

- `loop_var` is an integer or single-precision variable.
- `start`, `stop` and `step` are numeric expressions.

##### Errors

- No `NEXT` statement is found to match the `FOR` statement: `FOR without NEXT` occurs at the `FOR` statement.
- `loop_var` is a string variable or `start`, `stop`, or `end` has a string value: `Type mismatch`.
- `loop_var` is a double-precision variable: `Type mismatch`.
- `loop_var` is an array element: `Syntax error` .
- `loop_var` is an integer variable and a `start`, `stop` or `step` is outside the range `[-32768, 32767]`: `Overflow` .

#### `GET` (files)

`GET [#] file_number [, record_number]`

Read a record from the random-access file `file_number` at position `record_number`. The record can be accessed through the `FIELD` variables or through `INPUT$`, `INPUT` or `LINE INPUT`.

##### Parameters

- `file_number` is a numeric expression that yields the number of an open random-access file. The `#` is optional and has no effect.
- `record_number` is a numeric expression in `[1—33554432]` (`2^25`), and is interpreted as the record number.

##### Notes

- If the record number is beyond the end of the file, the file buffer is filled with null bytes.
- The record number is stored as single-precision; this precision is not high enough to distinguish single records near the maximum value of `2^25`.

##### Errors

- `record_number` is not in `[1—33554432]`: `Bad record number`.
- `file_number` is not in `[0—255]`: `Illegal function call`.
- `file_number` is not the number of an open file: `Bad file mode`.
- `file_number` is open under a mode other than `RANDOM`: `Bad file mode`.
- `file_number` is not specified: `Missing operand`.

#### `GET` (communications)

`GET [#] com_file_number [, number_bytes]`

Read `number_bytes` bytes from the communications buffer opened under file number `com_file_number`. The record can be accessed through the `FIELD` variables or through `INPUT$`, `INPUT` or `LINE INPUT`.

##### Parameters

- `file_number` is a numeric expression that yields the number of a file open to a `COM` device. The `#` is optional and has no effect.
- `number_bytes` is a numeric expression between `1` and the `COM` buffer length, inclusive.

##### Notes

- If `bytes` is `32768` or greater, GW-BASIC hangs. This functionality is not implemented in PC-BASIC.
- In GW-BASIC, `Device I/O error` is raised for overrun error, framing error, and break interrupt. `Device fault` is raised if DSR is lost during I/O. `Parity error` is raised if parity is enabled and incorrect parity is encountered. This is according to the manual; it is untested.

##### Errors

- `bytes` is less than 1: `Bad record number`
- `bytes` is less than `32768` and greater than the `COM` buffer length: `Illegal function call`.
- `com_file_number` is not specified: `Missing operand`.
- `com_file_number` is not in `[0—255]`: `Illegal function call`.
- `com_file_number` is not the number of an open file: `Bad file number`.
- If the serial input buffer is full, i.e. `LOF(com_file_number) = 0`, and `LOC(com_file_number) = 255`: `Communication buffer overflow`
- If the carrier drops during `GET`, hangs until the `Ctrl`+`Break` key is pressed.

#### `GET` (graphics)

`GET (x0, y0) - [STEP] (x1, y1), array_name`

Stores a rectangular area of the graphics screen in an array. The area stored is a rectangle parallel to the screen edges, bounded by the top-left and bottom-right coordinates `x0`,`y0` and `x1`,`y1`. If `STEP` is specified, `x1`,`y1` is an offset from `x0`,`y0`. The area is such that these corner points are inside it.

The image stored in the array can then be put on the screen using PUT. For the purposes of `GET`, any array is considered a string of bytes. The byte size of an array can be calculated as `number_elements * byte_size` with `byte_size` equal to `2` for integers (`%`), 4 for single (`!`) and 8 for double (`#`). Array byte size for string is 3, but string arrays are not allowed in `GET`. For calculating the number of elements, keep in mind that `OPTION BASE 0` is the default; in which case an array with maximum index 10 has 11 elements. This works through in multidimensional arrays.

The array format is as follows:

Byte | Contains
--- | ---
0, 1 | Number of x pixels, unsigned int. In `SCREEN 1`, this value is doubled.
2, 3 | Number of y pixels, unsigned int.
4— | Pixel data. Data is arranged in 2-byte words. The first 16-bit word holds the bit 0 of the first 16 pixels on the top row. The second word holds the second bit, etc. Data is word-aligned at the end of each row. Thus, in a screen mode with 4 bits per pixel, the first row takes at least 8 bytes (4 words), even if it consists of only one pixel. The number of bits per pixel depends on the `SCREEN` mode.

##### Parameters

- `array_name` is the name of a numeric array dimensioned with enough space to store the area.
- `x0`, `y0`, `x1`, `y1` are numeric expressions.

##### Notes

- In PCjr/Tandy mode, in `SCREEN 6`, `GET` stores an area of twice the width of the specified rectangle.

##### Errors

- The array does not exist: `Illegal function call`.
- `array_name` refers to a string array: `Type mismatch`.
- The area is too large for the array: `Illegal function call`.
- `x0`, ... `y1` are string expressions: `Type mismatch`.
- `x0`, ... `y1` are not in `[-32768—32767]`: `Overflow`.
- `x0`, ... `y1` are outside the current `VIEW` or `WINDOW`: `Illegal function call`

#### `GOSUB`

`GO[ ]SUB line_number [anything]`

Jumps to a subroutine at `line_number`. The next `RETURN` statement jumps back to the statement after `GOSUB`. Anything after `line_number` until the end of the statement is ignored. If executed from a direct line, `GOSUB` runs the subroutine and the following `RETURN` returns execution to the direct line.

##### Parameters

- `line_number` is an existing line number literal.
- Further characters on the line are ignored until end of statement.

##### Notes

- If no `RETURN` is encountered, no problem.
- One optional space is allowed between `GO` and `SUB`; it will not be retained in the program.

##### Errors

- If `line_number` does not exist: `Undefined line number`.
- If `line_number` is greater than `65529`, only the first 4 characters are read (e.g. `6553`)

#### `GOTO`

`GO[ ]TO line_number [anything]`

Jumps to `line_number`. Anything after `line_number` until the end of the statement is ignored. If executed from a direct line, `GOTO` starts execution of the program at the specified line.

##### Parameters

- `line_number` is an existing line number literal.
- Further characters on the line are ignored until end of statement.

##### Notes

- Any number of optional spaces is allowed between `GO` and `TO`, but they will not be retained in the program.
- If `line_number` is greater than `65529`, only the first 4 characters are read (e.g. `GOTO 65530` is executed as `GOTO 6553`)

##### Errors

- `line_number` does not exist: `Undefined line number`.

#### `IF`

`IF truth_value [,] {THEN|GOTO} [compound_statement_true|line_number_true [anything]] [ELSE [compound_statement_false|line_number_false [anything]]]`

If `truth_value` is non-zero, executes `compound_statement_true` or jumps to `line_number_true` . If it is zero, executes `compound_statement_false` or jumps to `line_number_false` .

##### Parameters

- `truth_value` is a numeric expression.
- `line_number_false` and `line_number_true` are existing line numbers.
- `compound_statement_false` and `compound_statement_true` are compound statements, consisting of at least one statement, optionally followed by further statements separated by colons `:`. The compound statements may contain nested `IF—THEN—ELSE` statements.

##### Notes

- The comma is optional and ignored.
- `ELSE` clauses are optional; they are bound to the innermost free `IF` statement if nested. Additional `ELSE` clauses that have no matching `IF` are ignored.
- All clauses must be on the same program line.
- `THEN` and `GOTO` are interchangeable; which one is chosen is independent of whether a statement or a line number is given. `GOTO PRINT 1` is fine.
- As in `GOTO`, anything after the line number is ignored.

##### Errors

- If `truth_value` has a string value: `Type mismatch`.
- `truth_value` equals `0` and `line_number_false` is a non-existing line number, or `truth_value` is nonzero and `line_number_true` is a non-existing line number: `Undefined line number`.

#### `INPUT` (console)

`INPUT [;] [prompt {;|,}] var_0 [, var_1] ...`

Prints `prompt` to the screen and waits for the user to input values for the specified variables. The semicolon before the prompt, if present, stops a newline from being printed after the values have been entered. If the prompt is followed by a semicolon, it is printed with a trailing `?`. If the prompt is followed by a comma, no question mark is added.

##### Parameters

- `prompt` is a string literal.
- `var_0, var_1, ...` are variable names or fully indexed array elements.

##### Notes

- Values entered must be separated by commas. Leading and trailing whitespace is discarded.
- String values can be entered with or without double quotes (`"`).
- If a string with a comma, leading or trailing whitespace is needed, quotes are the only way to enter it.
- Between a closing quote and the comma at the end of the entry, only white- space is allowed.
- If quotes are needed in the string itself, the first character must be neither a quote nor whitespace. It is not possible to enter a string that starts with a quote through `INPUT`.
- If a given `var_n` is a numeric variable, the value entered must be number literal.
- Characters beyond the 255th character of the screen line are discarded.
- If user input is interrupted by `Ctrl`+`Break`, `CONT` will re-execute the `INPUT` statement.

##### Errors

- If the value entered for a numeric variable is not a valid numeric literal, or the number of values entered does not match the number of variables in the statement, `?Redo from start` is printed and all values must be entered again.
- A `Syntax error` that is caused after the prompt is printed is only raised after the value shave been entered. No values are stored.

#### `INPUT` (files)

`INPUT # file_num, var_0 [, var_1] ...`

Reads string or numeric variables from a text file or the `FIELD` buffer of a random access file.

##### Parameters

- `file_num` is the number of a file open in `INPUT` mode or a random-access file open in `RANDOM` mode.
- `var_0, var_1, ...` are variable names or fully indexed array elements.

##### Notes

- The `#` is mandatory. There may or may not be whitespace between `INPUT` and `#`.
- String values can be entered with or without double quotes (`"`).
- Numeric values are terminated by `` , `LF`, `CR`, `,`.
- Unquoted strings are terminated by `LF`, `CR`, `,`.
- Quoted strings are terminated by the closing quote.
- Any entry is terminated by `EOF` character or its 255th character.
- Leading and trailing whitespace is discarded.
- If the entry cannot be converted to the requested type, a zero value is returned.
- If `file_num` is open to `KYBD:`, `INPUT#` reads from the keyboard until a return or comma is encountered (as in a file). Arrow keys and delete are passed as their control characters (not scancodes!) preceded by `CHR$(&hFF)`.

##### Errors

- Input is requested after the end of a text file has been reached or an `EOF` character has been encountered: `Input past end`.
- The last character of the field buffer is read: `Field overflow`.
- `file_num` has a string value: `Type mismatch`.
- `file_num` is greater than `32767`: `Overflow`.
- `file_num` is less than zero: `Illegal function call`.
- `file_num` is not an open file: `Bad file number`.
- `file_num` is not open for `INPUT` or `RANDOM`: `Bad file mode`.
- `file_num` is open to a `COM` port and this is the first `INPUT`, `LINE INPUT` or `INPUT$` call on that port since the buffer has filled up completely (i.e. `LOF(file_num)` has become zero): `Communication buffer overflow`.

#### `IOCTL`

`IOCTL [#] file_num, control_string`

Raises `Illegal function call`.

##### Notes

- In GW-BASIC, `IOCTL` sends a control string to a device.
- This statement is not implemented in PC-BASIC.

##### Errors

- `file_num` has a string value: `Type mismatch`.
- `file_num` is not in `[-32768—32767]`: `Overflow`.
- `file_num` is not an open file: `Bad file number`.
- Otherwise: `Illegal function call`

#### `KEY` (macro list)

`KEY {ON|OFF|LIST}`

Turns the list of function-key macros on the bottom of the screen `ON` or `OFF`. If `LIST` is specified, prints a list of the 10 (or 12 with `syntax=tandy`) function keys with the function-key macros defined for those keys to the console.

Most characters are represented by their symbol equivalent in the current codepage. However, some characters get a different representation, which is a symbolic representation of their effect as control characters on the screen.

Code point | Replacement | Usual glyph
--- | --- | ---
`&h07` | `&h0E` | ♫
`&h08` | `&hFE` | ■
`&h09` | `&h1A` | →
`&h0A` | `&h1B` | ←
`&h0B` | `&h7F` | ⌂
`&h0C` | `&h16` | ▬
`&h0D` | `&h1B` | ←
`&h1C` | `&h10` | ►
`&h1D` | `&h11` | ◄
`&h1E` | `&h18` | ↑
`&h1F` | `&h19` | ↓

#### `KEY` (macro definition)

`KEY key_id, string_value`

Defines the string macro for function key `key_id`. Only the first 15 characters of `string_value` are stored.

##### Parameters

- `key_id` is a numeric expression in the range `[1—10]` (or `[1—12]` when `syntax=tandy`).
- `string_value` is a string expression.

##### Notes

- If `key_id` is not in the prescribed range, the statement is interpreted as an event-trapping `KEY` statement.
- If `string_value` is the empty string or the first character of `string_value` is `CHR$(0)`, the function key macro is switched off and subsequent catching of the associated function key with `INKEY$` is enabled.

##### Errors

- `key_id` is not in `[-32768—32767]`: `Overflow`.
- `key_id` is not in `[1—255]`: `Illegal function call`.
- `key_id` has a string value: `Type mismatch`.

#### `KEY` (event switch)

`KEY (key_id) {ON|OFF|STOP}`

Controls event trapping of the key with identifier `key_id`. Event trapping is switched `ON` or `OFF`. `STOP` suspends event trapping until a `KEY() ON` is executed. Up to one event can be triggered during suspension, provided that event handling was switched on prior to suspension. The event triggered during suspension is handled immediately after the next `KEY() ON` statement.

##### Parameters

`key_id` is a numeric expression in `[1—20]`. Keys are:

1 | `F1`
2 | `F2`
3 | `F3`
4 | `F4`
5 | `F5`
6 | `F6`
7 | `F7`
8 | `F8`
9 | `F9`
10 | `F10`
11 | `↑`
12 | `←`
13 | `→`
14 | `↓`

Keys 15 to 20 are defined using the event trapping `KEY` definition statement.

##### Notes

- With `syntax=tandy`, key 11 is `F11` and key 12 is `F12`. Pre-defined keys 11—14 shift to 13—16.

##### Errors

- `key_id` is not in `[-32768—32767]`: `Overflow`.
- `key_id` is not in `[0—20]`: `Illegal function call`.
- `key_id` has a string value: `Type mismatch`.

#### `KEY` (event definition)

`KEY key_id, two_char_string`

Defines the key to trap for `key_id`.

##### Parameters

- `key_id` is a numeric expression in `[15—20]` (or `[17—20]` when `syntax=tandy`).
- `two_char_string` is a string expression of length 2. The first character is interpreted as a modifier while the second character is interpreted as a scancode. The modifier character is a bitwise `OR` combination of the following flags:

  `CHR$(&h80)` | Extended
  `CHR$(&h40)` | `Caps Lock`
  `CHR$(&h20)` | `Num Lock`
  `CHR$(&h08)` | `Alt`
  `CHR$(&h04)` | `Ctrl`
  `CHR$(&h02)` | `Shift` (either side)
  `CHR$(&h01)` | `Shift` (either side)

  For the unmodified key, the modifier character is `CHR$(0)`.

##### Notes

- If `key_id` is not in the prescribed range, no error is raised; such values are ignored. In GW-BASIC strange things can happen in this case: screen anomalies and crashes suggestive of unintended memory access.
- If `key_id` is in `[1—10]` (or `[1—12]` when `syntax=tandy`), the statement is interpreted as a function-key macro definition.

##### Errors

- `key_id` is not in `[-32768—32767]`: `Overflow`.
- `key_id` is not in `[1—255]`: `Illegal function call`.
- `key_id` has a string value: `Type mismatch`.
- `two_char_string` is longer than two: `Illegal function call`.
- `two_char_string` has a numeric value: `Type mismatch`.

#### `KILL`

`KILL filter_spec`

Deletes one or more files on a disk device.

##### Parameters

- The string expression `filter_spec` is a valid file specification indicating the files to delete. Wildcards are allowed. See `FILES` for a description of wildcards.

##### Notes

- Be very careful with the use of wildcards in this statement: the DOS matching rules may not be the same as what is usual on your operating system, which could result in unexpected files being deleted.
- This statement may not delete hidden file and files that do not have short names which are legal DOS names. However, this behaviour is not guaranteed so you must not depend on it.

##### Errors

- `filter_spec` is a numeric value: `Type mismatch`.
- A file with a base name equal to that of a file matching `filter_spec` is open: `File already open`
- No file matches `filter_spec` : `File not found`
- The user has no write permission: `Permission denied`
- If a syntax error occurs after the closing quote, the file is removed anyway.

#### `LCOPY`

`LCOPY [num]`

Does nothing.

##### Parameters

- `num` is a numeric expression in `[0—255]`.

##### Notes

- This statement does nothing in GW-BASIC. Presumably, it is left over from a statement in older versions of MS Basic that would copy the screen to the printer.

##### Errors

- `num` is not in `[-32768—32767]`: `Overflow`.
- `num` is not in `[0—255]`: `Illegal function call`.
- `num` has a string value: `Type mismatch`.

#### `LET`

`[LET] name = expression`

Assigns the value of `expression` to the variable or array element `name`.

##### Parameters

- `name` is a variable that may or may not already exist.
- The type of `expression` matches that of `name`: that is, all numeric types can be assigned to each other but strings can only be assigned to strings.

##### Errors

- `name` and `expression` are not of matching types: `Type mismatch`.

#### `LINE`

`LINE [[STEP] (x0, y0)] - [STEP] (x1, y1) [, [attr] [, [B [F]] [, pattern]]]`

Draws a line or a box in graphics mode. If `B` is not specified, a line is drawn from `(x0, y0)` to `(x1, y1)`, endpoints inclusive. If `B` is specified, a rectangle is drawn with sides parallel to the screen and two opposing corners specified by `(x0, y0)` and `(x1, y1)`. If the starting point is not given, the current graphics position is used as a staring point. If `STEP` is specified, `(x0, y0)` is an offset from the current position and `(x1, y1)` is an offset from `(x0, y0)`. `LINE` moves the current graphics position to the last given endpoint. If `F` is specified with `B`, the rectangle is filled with the specified attribute. `F` and `B` may be separated by zero or more spaces.

##### Parameters

- `attr` is a numeric expression in `[0—255]`, which specifies the colour attribute of the line. If it is not given, the current attribute is used.
- `pattern` is a numeric expression in `[-32768—32767]`. This is interpreted as a 16-bit binary pattern mask applied to consecutive pixels in the line: a 1 bit indicates a pixel plotted; a 0 bit indicates a pixel left untouched. The pattern starts at the most significant bit, which is applied to the topmost endpoint. If a box is drawn, the pattern is applied in the following counter-intuitive sequence: `(x1, y1)—(x0, y1)`, `(x1, y0)—(x0, y0)`, then `(x1, y0)—(x1, y1)`, `(x0, y0)—(x0, y1)` if `y0<y1` and `y0`, `y1` reversed if `y1<y0`. When drawing a filled box, `LINE` ignores the pattern.

##### Notes

- If a coordinate is outside the screen boundary, it is replaced with -1 (if less than 0) or the screen dimension (if larger than the screen dimension).

##### Errors

- The statement ends in a comma and it is the first or third: `Missing operand`. If it is the second: `Syntax error`.
- Any of the coordinates is not in `[-32768—32767]`: `Overflow`.
- Any of the parameters has a string value: `Type mismatch`.

#### `LINE INPUT` (console)

`LINE INPUT [;] [prompt_literal {;|,}] string_name`

Displays the prompt given in `prompt_literal` and reads user input from the keyboard, storing it into the variable `string_name`. All input is read until `Enter` is pressed; the first 255 characters are stored. If the `;` is given right after `LINE INPUT`, the `Enter` ending user input is not echoed to the screen.

##### Parameters

- `prompt_literal` is a string literal. It makes no difference whether it is followed by a comma or a semicolon.
- `string_name` is a string variable or array element.

##### Notes

- If user input is interrupted by `Ctrl`+`Break`, `CONT` will re-execute the `LINE INPUT` statement.
- Unlike `INPUT`, `LINE INPUT` does not end the prompt with `?`.

#### `LINE INPUT` (files)

`LINE INPUT # file_num, string_name`

Reads string or numeric variables from a text file or the `FIELD` buffer of a random access file. All input is read until `Enter` is pressed; the first 255 characters are stored. `file_num` must be the number of a file open in `INPUT` mode or a random-access file open in `RANDOM` mode.

##### Parameters

- `string_name` is a string variable or array element.

##### Notes

- The `#` is mandatory. There may or may not be whitespace between `INPUT` and `#`.
- Input is only terminated by a `CR`.
- If `file_num` is open to `KYBD:`, `LINE INPUT#` reads from the keyboard until a return or comma is encountered (as in a file). Arrow keys and delete are passed as their control characters (not scancodes!) preceded by `CHR$(&hFF)`.

##### Errors

- Input is requested after the end of a text file has been reached or an `EOF` char has been encountered: `Input past end`.
- The last character of the field buffer is read: `Field overflow`.
- `file_num` is not an open file: `Bad file number`.
- `file_num` is less than zero: `Illegal function call`.
- `file_num` is not in `[-32768—32767]`: `Overflow`.
- `file_num` is not open for `INPUT` or `RANDOM`: `Bad file mode`.
- `file_num` has a string value: `Type mismatch`.
- `file_num` is open to a `COM` port and this is the first `INPUT`, `LINE INPUT` or `INPUT$` call on that port since the buffer has filled up completely (i.e. `LOF(file_num)` has become zero): `Communication buffer overflow`.

#### `LIST`

`LIST [line_number_0|.] [-[line_number_1|.]] [, file_spec [anything]]`

Prints the program to the screen or a file, starting with `line_number_0` up to and including `line_number_1`. Also stops program execution and returns control to the user. If the `LIST` statement ends with a file specification, anything further is ignored. In all cases, any further statements in a compound after `LIST` will be ignored, both in a program and in direct mode.

When listing to the screen, the same control characters are recognised as in the `PRINT` statement.

##### Notes

- In GW-BASIC 3.23, `LIST` will not show line numbers `65531`—`65535` inclusive. By default, PC-BASIC's `LIST` does show these lines. However, showing them can be disabled with the option `hide-listing=65530`.

##### Parameters

- `line_number_0` and `line_number_1` are line numbers in the range `[0—65529]` or a `.` to indicate the last line edited. The line numbers do not need to exist; they specify a range. If the range is empty, nothing is printed.
- The string expression `file_spec` is a valid file specification indicating the file to list to. If this file already exists, it will be overwritten.

##### Errors

- A line number is greater than `65529`: `Syntax error`.
- `file_spec` has a numeric value: `Type mismatch`.
- `file_spec` ends in a colon but is not a device name or drive letter: `Bad file number`.
- `file_spec` contains disallowed characters: `Bad file number` (on `CAS1:`); `Bad file name` (on disk devices).

#### `LLIST`

`LLIST [line_number_0|.] [-[line_number_1|.]]`

Prints the program to the screen, starting with `line_number_0` up to and including `line_number_1`. Also stops program execution and returns control to the user. Any further statements on a line after `LLIST` will be ignored, both in a program and in direct mode.

##### Notes

- In GW-BASIC 3.23, `LLIST` will not show line numbers `65531`—`65535` inclusive. By default, PC-BASIC's `LLIST` does show these lines. However, showing them can be disabled with the option `hide-listing=65530`.

##### Parameters

- `line_number_0` and `line_number_1` are line numbers in the range `[0—65529]`. or a `.` to indicate the last line edited. The line numbers do not need to exist; they specify a range. If the range is empty, nothing is printed.

##### Errors

- A line number is greater than `65529`: `Syntax error`.

#### `LOAD`

`LOAD file_spec [, R]`

Loads the program stored in a file into memory. Existing variables will be cleared and any program in memory will be erased. `LOAD` implies a `CLEAR`.

If `,R` is specified, keeps all data files open and runs the specified file.

##### Parameters

- The string expression `file_spec` is a valid file specification indicating the file to read the program from.

##### Errors

- `file_spec` has a numeric value: `Type mismatch`.
- `file_spec` contains disallowed characters: `Bad file number` (on `CAS1:`); `Bad file name` (on disk devices).
- The file specified in `file_spec` cannot be found: `File not found`.
- A loaded text file contains lines without line numbers: `Direct statement in file`.
- A loaded text file contains lines longer than 255 characters: `Line buffer overflow`. Attempting to load a text file that has `LF` rather than `CR LF` line endings may cause this error.

#### `LOCATE`

`LOCATE [row] [, [col] [, [cursor_visible] [, [start_line] [, [stop_line] [,]]]]]`

Positions the cursor at `row`, `col` on the screen and changes the cursor shape and visibility. `cursor_visible` may be 0 or 1. If `cursor_visible` is 0, it makes the cursor invisible; if it is 1, makes the cursor visible. This works only while a program is running. The cursor shape is adjusted within a character cell to start from `start_line` and end on `end_line` where `start_line` and `end_line` are in `[0—31]`. If `start_line` or `end_line` is greater than the character cell height (15), substitute 15.

##### Notes

- On emulated VGA cards, the cursor shape parameters are interpreted in a complicated way that is intended to maintain functional compatibility with CGA.
- In GW-BASIC, cursor shape is preserved after pressing `Ins` twice. The insert-mode cursor is different from the usual half-block. In PC-BASIC, insert mode resets the cursor shape to default.
- Cursor shape and visibility options have no effect in graphics mode.
- Locate accepts a 5th comma at the end, which is ignored.

##### Errors

- Any parameter has a string value: `Type mismatch`.
- Any parameter is not in `[-32768—32767]`: `Overflow`.
- `row` is outside the current view area: `Illegal function call`.
- `col` is greater than the current width: `Illegal function call`.
- `cursor_visible` is not in `[0, 1]` (`[0—255]` on Tandy/PCjr): `Illegal function call`.

#### `LOCK`

`LOCK [#] file_number [, record_0]` `LOCK [#] file_number, [record_0] TO record_1`

Locks a file or part of a file against access by other users. On a `RANDOM` file, `record_0` is the first record locked and `record_1` is the last record locked. On any other kind of file `record_0` and `record_1` have no effect. If `record_0` is not specified, it is assumed to be 1. If no records are specified, the whole file is locked.

##### Parameters

- `file_number` is a numeric expression in `[0—255]`.
- `record_0` and `record_1` are numeric expressions in `[1—2^25-2]`.

##### Notes

- In GW-BASIC under MS-DOS, the `LOCK` command requires `SHARE.EXE` to be loaded. The maximum number of locks is specified in the MS-DOS `SHARE` command. If `SHARE` has not been activated or all locks are used, `LOCK` raises `Permission denied`. PC-BASIC behaves as if `SHARE` has been activated with unlimited locks.
- If `file_number` is open for `RANDOM`, `LOCK` and `UNLOCK` statements must match in terms of `record_0` and `record_1`. An non-matching `UNLOCK` will raise `Permission denied`.
- To check if another open file is the same file, PC-BASIC only looks at the base name of the file, i.e. its DOS name without directories. As a consequence, if a file `"test.txt"` is open and locked, an attempt to lock a file `"dir\test.txt"` will fail, even if these are different files. Conversely, if two file names are different but point to the same file in the file system (for example due to file system links), then these will be considered as different files by BASIC.

##### Errors

- Any parameter has a string value: `Type mismatch`.
- `file_num` is not in `[-32768—32767]`: `Overflow`.
- `file_num` is not in `[0—255]`: `Illegal function call`.
- `file_num` is not an open file: `Bad file number`.
- `LOCK` (part of) a file with the same name as a file already locked: `Permission denied`.
- `record_0` or `record_1` is not in `[1—2^25-2]`: `Bad record number`.

#### `LPRINT`

See `PRINT`.

#### `LSET`

`LSET string_name = expression`

Copies a string value into an existing string variable or array element. The value will be left-justified and any remaining characters are replaced by spaces.

##### Parameters

- `string_name` is a string variable or array element.
- `expression` is a string expression.

##### Notes

- If `expression` has a value that is longer than the length of the target variable, it is truncated at the tail to the length of the target variable.
- If `string_name` has not been allocated before, this statement has no effect.
- Use `LSET`, `RSET` or `MID$` to copy values into a `FIELD` buffer.
- If `LET` is used on a `FIELD` variable instead of `L|RSET`, the variable is detached from the field and a new, normal string variable is allocated.

##### Errors

- `string_name` is not a string variable: `Type mismatch`.
- `expression` does not have a string value: `Type mismatch`.

#### `MERGE`

`MERGE file_spec`

Overlays the lines of a program from a plain-text program file into the existing program. The loaded lines overwrite existing lines if they have the same line number.

##### Parameters

- The string expression `file_spec` is a valid file specification indicating the file to read the program from.

##### Errors

- `file_spec` cannot be found: `File not found`.
- `file_spec` contains disallowed characters: `Bad file number` (on `CAS1:`); `Bad file name` (on disk devices).
- `file_spec` was not saved as plain text: `Bad file mode`.
- A loaded text file contains lines without line numbers: `Direct statement in file`.
- A loaded text file contains lines longer than 255 characters: `Line buffer overflow`. Attempting to load a text file that has `LF` rather than `CR LF` line endings may cause this error.

#### `MID$` (statement)

`MID$(string_name, position [, length]) = substring`

Replaces part of `string_name` with `substring`.

##### Parameters

- `string_name` is a valid string variable name.
- `position` is a numeric expression between 1 and the string length, inclusive.
- `length` is a numeric expression in `[0—255]`.

##### Notes

- No whitespace is allowed between `MID$` and `(`.
- If `substring` is longer than `length`, only the first `length` characters are used.
- If `substring` is shorter than `length`, only `LEN(substring)` characters are replaced.

##### Errors

- `position` is greater than the length of `string_name`: `Illegal function call`, except if `length` is specified as 0.
- `position` is not in `[1—255]`: `Illegal function call`.
- `length` is not in `[0—255]`: `Illegal function call`.
- `position` or `length` are not in `[-32768—32767]`: `Overflow`.

#### `MKDIR`

`MKDIR dir_spec`

Creates a new directory on a disk device.

##### Parameters

- The string expression `dir_spec` is a valid file specification that specifies the path of the new directory on a disk device.

##### Errors

- `dir_spec` is not a string: `Type mismatch`.
- The parent directory does not exist: `Path not found`.
- The directory name already exists on that path: `Path/File access error`.
- The user has no write permission: `Permission denied`.

#### `MOTOR`

`MOTOR [num]`

Does nothing.

##### Parameters

- `num` is a numeric expression in `[0—255]`.

##### Notes

- In GW-BASIC, this statement turns on the cassette motor if `num` is nonzero or omitted, and turns it off if `num` is zero. This is not implemented in PC-BASIC.

##### Errors

- `num` has a string value: `Type mismatch`.
- `num` is not in `[-32768—32767]`: `Overflow`.
- `num` is not in `[0—255]`: `Illegal function call`.

#### `NAME`

`NAME old_name AS new_name`

Renames the disk file `old_name` into `new_name`.

##### Parameters

- The string expressions `old_name` and `new_name` are valid file specifications giving the path on a disk device to the old and new filenames, respectively.

##### Notes

- `new_name` will be modified into all-uppercase 8.3 format.

##### Errors

- `old_name` or `new_name` have number values: `Type mismatch`.
- `old_name` does not exist: `File not found`.
- A file with a base name equal to that of `old_name` or `new_name` is open: `File already open`.
- `new_name` exists: `File already exists`.

#### `NEW`

`NEW`

Stops execution of a program, deletes the program in memory, executes `CLEAR` and `RESTORE` and returns control to the user.

#### `NEXT`

`NEXT [var_0 [, var_1] ...]`

Iterates a `FOR—NEXT` loop: increments the loop variable and jumps to the `FOR` statement. If no variables are specified, next matches the most recent `FOR` statement. Several nested `NEXT` statements can be consolidated into one by using the variable list. If one or more variables are specified, their order must match the order of earlier `FOR` statements.

##### Parameters

- `var_0, var_1, ...` are numeric variables which are loop counters in a `FOR` statement.

##### Errors

- No `FOR` statement is found to match the `NEXT` statement and variables: `NEXT without FOR`.
- `var_0, var_1, ...` are string variables: `NEXT without FOR`.
- The (implicit or explicit) loop variable is an integer variable and is taken outside the range `[-32768, 32767]` when incremented after the final iteration: `Overflow` .

#### `NOISE`

`NOISE source, volume, duration`

Generates various kinds of noise.

##### Parameters

- `source` is a numeric expression in `[0—7]`. It indicates the type of noise:

  source | type | top of frequency band (Hz)
  --- | --- | ---
  0 | periodic | 6991
  1 | periodic | 3495
  2 | periodic | 1747
  3 | periodic | last tone played on voice 2
  0 | white noise | 6991
  1 | white noise | 3495
  2 | white noise | 1747
  3 | white noise | last tone played on voice 2

- `volume` is a numeric expression in `[0—15]`.
- `duration` is a numeric expression.

Volume and duration are determined in the same way as for the `SOUND` statement; see there.

##### Notes

- This statement is only available if `syntax={pcjr|tandy}` is set.

##### Errors

- `SOUND ON` has not been executed: `Illegal function call`.
- `duration` is not in `[-65535—65535]`: `Illegal function call`.
- `volume` is not in `[0—15]`: `Illegal function call`.
- `source` is not in `[0—7]`: `Illegal function call`.

#### `ON` (calculated jump)

`ON n {GOTO|GOSUB} line_number_0 [, line_number_1] ...`

Jumps to the `n`th line number specified in the list. If `n` is 0 or greater than the number of line numbers in the list, no jump is performed. If `GOTO` is specified, the jump is unconditional; if `GOSUB` is specified, jumps to a subroutine.

##### Parameters

- `n` is a numeric expression in `[0—255]`. The expression must not start with the `STRIG`, `PEN`, `PLAY` or `TIMER` function keywords; if you need these functions, the expression must be bracketed.
- `line_number_0, line_number_1, ...` are existing line numbers in the program.

##### Errors

- `n` has a string value: `Type mismatch`.
- `n` is not in `[-32768—32767]`, `Overflow` .
- `n` is not in `[0—255]`: `Illegal function call`.
- The line number jumped to does not exist: `Undefined line number`.

#### `ON` (event trapping)

`ON {COM(n)|KEY(n)|STRIG(n)|PEN|PLAY(n)|TIMER(x)} GOSUB line_number`

Defines an event trapping subroutine. The type of event is given by one of the following keywords:

`COM(n)` | The event is triggered if data is present in the input buffer of the `COMn:`. `n` is the port number in `[1,2]`.
`KEY(n)` | The event is triggered if key `n` is pressed. `n` is the key number `[1—20]` defined in the `KEY` statement.
`STRIG(n)` | They event is triggered if fire button `n` is pressed. `n` in `[0,2,4,6]` refer to the two fire triggers on two joysticks.
`PEN` | The event is triggered if the light pen is on the screen. (In PC-BASIC, the light pen is emulated by default by the right mouse button).
`PLAY(n)` | The event is triggered if there are exactly `n` notes left on the music background queue. `n` is a numeric expression in `[1—32]`.
`TIMER(x)` | The event is triggered every `x` seconds after the `TIMER ON` statement. `x` is a numeric expression in `[1—86400]`.

##### Notes

- Event trapping for your chosen event first needs to be enabled using one of the statements: `COM(n) ON`, `KEY(n) ON`, `STRIG(n) ON`, `PEN ON`, `PLAY ON`, `TIMER ON`
- Events are only trapped when a program is running.

##### Errors

- `n` or `x` has a string value: `Type mismatch`.
- `n` is not in `[-32768—32767]`: `Overflow`.
- `n` or `x` is outside the specified range: `Illegal function call`.

#### `ON ERROR`

`ON ERROR GOTO {line_number|0}`

Turns error trapping on or off. When `line_number` is set, any error causes the error handling routine starting at that line number to be called; no message is printed and program execution is not stopped. The error handling routine is ended by a `RESUME` statement. While in an error handling routine, events are paused and error trapping is disabled. After the `RESUME` statement, any triggered events are picked up in the following order: `KEY`, `TIMER`, `PLAY` - the order of the others is unknown. Unlike event trapping, error trapping remains active when no program is running. `ON ERROR GOTO 0` turns off error trapping.

##### Parameters

- `line_number` is an existing line number in the program.

##### Notes

- It is not possible to start the error handler at line number `0`.

##### Errors

- `line_number` does not exist: `Undefined line number`.

#### `OPEN`

`OPEN mode_char, [#] file_num, file_spec [, rec_len]` `OPEN file_spec [FOR {INPUT|OUTPUT|APPEND|RANDOM}] [ACCESS {READ|WRITE|READ WRITE}] [SHARED|LOCK {READ|WRITE|READ WRITE}] AS [#] file_num [LEN = rec_len]`

Opens a data file on a device.

##### Parameters

- The string expression `file_spec` is a valid file specification.
- `file_num` is a numeric expression in `[1—max_files]`, where `max_files` is the maximum file number (default 3).
- `rec_len` is a numeric expression in `[1—128]`: the record length.
- `mode_char` is a string expression of which the first character is one of `["I", "O", "A", "R"]`.

##### Access modes

The `FOR` modes or `mode_char` are as follows:

`mode_char` | `FOR` | Effect
--- | --- | ---
`"I"` | `INPUT` | Opens a text file for reading and positions the file pointer at the start.
`"O"` | `OUTPUT` | Truncates a text file at the start and opens it for writing. Any data previously present in the file will be deleted.
`"A"` | `APPEND` | Opens a text file for writing at the end of any existing data.
`"R"` | `RANDOM` | Opens a file for random access; the file is divided in records of length `rec_len`. If `LEN` is not specified, the record length defaults to 128. The file contents can be accessed using `GET` and `PUT` of the `FIELD` buffer; the `FIELD` buffer can be accessed through `FIELD` variables or through `PRINT#` and `INPUT#` statements.

If no `FOR` mode or `mode_char` is specified, the file is opened for `RANDOM`.

If both `FOR` and `ACCESS` are specified, any `ACCESS` mode is allowed for `RANDOM` but for the other modes the access must match as follows:

`FOR` | default `ACCESS` | allowed `ACCESS`
--- | --- | ---
`INPUT` | `READ` | `READ`
`OUTPUT` | `WRITE` | `WRITE`
`APPEND` | `READ WRITE` | `READ WRITE`
`RANDOM` | `READ WRITE` | all

##### Sharing and locks

If neither `SHARED` nor `LOCK` are specified. Inside this process, a file may be opened multiple times for `INPUT` or `RANDOM` but only once for `OUTPUT` or `APPEND`, as long as it is again opened in default mode. It may not be opened in `SHARED` or any `LOCK` modes.

If `SHARED`, `LOCK READ`, `LOCK WRITE` or `LOCK READ WRITE` is specified, whether two `OPEN` statements may access the same file depends on one's `LOCK` status and the other's `ACCESS` status and vice versa. For two `OPEN` statements as follows: `OPEN "file" lock_1 AS 1
OPEN "file" ACCESS acc_2 SHARED AS 2` the following combinations are allowed:

Access allowed | `acc_2`
--- | ---
`READ` | `WRITE` | `READ WRITE`
`lock_1` | `SHARED` | yes | yes | yes
`LOCK READ` | no | yes | no
`LOCK WRITE` | yes | no | no
`LOCK READ WRITE` | no | no | no

In GW-BASIC under MS-DOS with `SHARE.EXE` active, these locks should be enforced across a network as well as inside a single BASIC process. Without `SHARED` and `LOCK`, the file is locked exclusively for use by the GW-BASIC process. By contrast, in PC-BASIC, the locks are only implemented internally. Whether other processes may access the file will depend on the host OS.

To check if another open file is the same file, PC-BASIC only looks at the base name of the file, i.e. its DOS name without directories. As a consequence, if a file `"test.txt"` is open and locked, an attempt to lock a file `"dir\test.txt"` will fail, even if these are different files. Conversely, if two file names are different but point to the same file in the file system (for example due to file system links), then these will be considered as different files by BASIC.

##### File specifications

A file specification `file_spec` is a non-empty string expression of the form `"[device:]parameters"`, where `device` is a PC-BASIC device and the form of the `parameters` is specific to the type of device. If `device` is omitted, the current device (one of the disk devices or `CAS1:`) is used.

Disk devices `A:`—`Z:` and `@:`

  `parameters` must specify a valid file path of the form `[\][dirname\] ... filename`.

  PC-BASIC follows DOS file system conventions. Directory names are separated with backslashes `\` (even if the host OS separates paths with forward slashes). File and directory names consist of a 8-character name and 3-character extension. Names are case insensitive. Permissible characters for both filename and extension are the printable ASCII characters in the range `&h20`–`&h7E` excluding the characters `" * + . , / : ; < = > ? \ [ ] |`. Spaces are allowed but leading and trailing spaces are ignored. The names `AUX`, `CON`, `PRN` and `NUL` are reserved as device aliases and are not legal names for files or directories on a disk device.

  A path starting with a backslash is interpreted as an absolute path, starting at the root of the specified disk device. Otherwise, the path is interpreted as relative to the current directory on the specified device. The special directory name `..` refers to the parent directory of a preceding path, or the parent directory of the current directory if no path is given. The special directory name `.` refers to the same directory as given by the preceding path, or the current directory if no preceding path is given.

  If the file name provided does not contain any dots, the `LOAD`, `SAVE`, `BLOAD`, `BSAVE`, `CHAIN`, `MERGE`, `RUN`, and `LIST` statements append the default extension `.BAS`. To refer to a file name without an extension, the file specification should end in a dot `.`. For other statements, appending a dot is allowed but not required.

  ##### Compatibility notes

  Unlike PC-BASIC, some versions of MS-DOS allow certain characters in the range `&h7F`–`&hFF`. However, their permissibility and interpretation depends on the console code page, which may be different from the display code page that affects GW-BASIC. Depending on its console code page, MS-DOS will replace accented letters by their unaccented uppercase variant. Some DOS implementations will remove spaces from filenames; notably, this is the case on DOSBox.

  In order to allow access to files whose name on the host system does not conform to DOS standards while maintaining compatibility with GW-BASIC, PC-BASIC will follow these steps to match DOS-style file names to host file names:

  1. Look for a file with the name as provided. This can be a long file name which may contain non-permissible characters and which will be case sensitive if your file system is.
  2. If such a file is not found, it will truncate the name provided to all-uppercase 8.3 format and look for an exact match. The truncated name consists of the first 8 characters before the first dot, followed by the first three characters after the first dot. If the resulting file name contains non-permissible characters, an error will be raised.
  3. Look for 8.3 names in mixed case which match the name provided in a case-insensitive way. Such files are searched in lexicographic order. File names longer than 8.3 will not be matched, unless their name is entered exactly. On Windows, the name matched can be a short filename as well as a long filename provided it is of 8.3 length — it may, for example, contain spaces and thus not be a valid Windows short file name.

  If the file name provided ends in a single dot and contains no other dots, PC-BASIC will first match the name as provided; if this is not found, it will match the name as provided but without the single dot. The 8.3 format of such a file name will match file names with and without the dot, in lexicographic order.

  If no matching file is found for an output file name, a new file will be created with an all-uppercase 8.3 file name.

Cassette device `CAS1:`
  `parameters` can be a file name of up to eight characters. Cassette file names are case sensitive, have no path or extension, may be empty and do not need to be unique. They may contain any character in the range `&h20`–`&hFF`. On the cassette device, when called in direct mode, `OPEN`, `CHAIN`, `MERGE`, `LOAD` and `BLOAD` will print a message to the console for each file found while winding the tape. The message consists of the filename followed by a dot and the file type and concluded with a status message. The file type is one of the following:
  `A`
    Program file in text format
  `B`
    Program file in tokenised format
  `D`
    Data file
  `M`
    `BSAVE` memory image
  `P`
    Program file in protected format
  If the file does not match the file specification and required file type, the status is `Skipped`; if the file matches, the status is `Found`. When called from a program, these statements do not print messages to the console. If the `device` was specified explicitly, `parameters` may also be empty. In this case the first file of the appropriate type is opened.
Console and parallel devices `SCRN:`, `KYBD:`, and `LPTn:`
  These devices do not allow further device parameters.
Serial devices `COMn:`

  When opening a `COM` port, the `file_spec` has the form `"COMn:[speed[,parity[,data[,stop[,RS][,CS[n]][,DS[n]][,CD[n]][,LF][,PE]]]]]"` The first four parameters after the device colon must be given in the order specified but the named parameters can be given in any order. The meaning of the parameters is:

  Parameter | Default | Meaning
  --- | --- | ---
  `speed` | `300` | Baud (bps) rate for the connection. `speed` is one of `[75, 110, 150, 300, 600, 1200, 1800, 2400, 4800, 9600]`.
  `parity` | `E` | Parity bit convention. `parity` is one of `[S, M, O, E, N]`.
  `parity` | Meaning | Effect
  --- | --- | ---
  `S` | SPACE | Parity bit always set to 0.
  `M` | MARK | Parity bit always set to 1.
  `O` | ODD | Parity bit set so that character parity is odd.
  `E` | EVEN | Parity bit set so that character parity is even.
  `N` | NONE | No parity bit transmitted or received.
  `data` | `7` | Data bits per byte. `data` must be one of `[4, 5, 6, 7, 8]`. A byte consists of the data bits plus parity bit, if any. Byte size must be in the range `[5—8]`: if `data` is `4`, `parity` must not be `N`; if `data` is `8`, `parity` must be `N`.
  `stop` | `1` | The number of stop bits. `stop` must be `1` or `2`. Default is `2` if `speed` is `75` or `110`; `1` otherwise.
  `RS` | no | Suppress Request To Send.
  `CS[n]` | `CS1000` | Set Clear To Send timeout to `n` milliseconds. If `n` is `0` or not given, disable CTS check. Default is `CS0` if `RS` is set; `CS1000` otherwise.
  `DS[n]` | `DS1000` | Set Data Set Ready timeout to `n` milliseconds. If `n` is `0` or not given, disable DSR check.
  `CD[n]` | `CD0` | Set Carrier Detect timeout to `n` milliseconds. If `n` is `0` or not given, disable CD check.
  `LF` | no | Send a line feed after each carriage return.
  `PE` | no | Enable parity checking (This setting is ignored by PC-BASIC).

##### Notes

- If a `COM` port is opened for `RANDOM`, access is byte-for-byte rather than through `FIELD` records; `PRINT#` and `INPUT#` access the port directly. `rec_len` sets the number of bytes read by the `GET` and `PUT` statements.
- For `INPUT`, `OUTPUT` and `APPEND` modes, `LEN` may be specified but is ignored.
- If I/O is attempted contravening the `FOR` mode specified, the `PRINT` or `INPUT` statement will raise `Bad file mode`.
- If `RANDOM` I/O is attempted contravening the `ACCESS` mode specified, the `PUT` or `GET` statement will raise `Path/File access error`.
- The `#` is optional and has no effect.

##### Errors

- `file_spec` is empty or a non-existent device: `Bad file number`.
- `FOR APPEND ACCESS WRITE` is specified: `Path/File access error`.
- `FOR` and `ACCESS` mismatch in other ways: `Syntax error`.
- The `COM:` `file_spec` parameters do not follow the specification: `Bad file name`.
- The `CAS1:` `file_spec` contains disallowed characters: `Bad file number`.
- A file with the same name is already open for `OUTPUT` or `APPEND`: `File already open`. This is only raised for `COMn:`, `CASn:` and disk devices.
- `rec_len` or `file_num` have string values: `Type mismatch`.
- `file_spec` or `mode_char` have number values: `Type mismatch`.
- `file_num` is not in `[-32768—32767]`: `Overflow`.
- `file_num` is not in `[0—255]`: `Illegal function call`.
- `file_num` is not in `[1—max_files]`: `Bad file number`.
- `rec_len` is not in `[-32768—32767]`: `Overflow`.
- `rec_len` is not in `[1—128]`: `Illegal function call`.
- `mode_char` is empty or the first character is not in `["I", "O", "A", "R"]`: `Bad file mode`.

#### `OPTION BASE`

`OPTION BASE n`

Sets the starting index of all arrays to `n`.

##### Parameters

- `n` is a literal digit `0` or `1`. Expressions are not allowed.

##### Notes

- If `OPTION BASE` has not been called, the first array allocation defaults to starting index 0.

##### Errors

- `n` is not a digit `0` or `1`: `Syntax error`.
- `OPTION BASE 1` is called but an array has already been allocated before: `Duplicate definition`.
- `OPTION BASE` is called more than once with different starting index: `Duplicate definition`.

#### `OUT`

`OUT port, value`

Sends a byte to an emulated machine port.

The following machine ports are emulated in PC-BASIC:

`port` | Effect
--- | ---
`&h201` | resets the game port (joystick port)
`&h3C5` | sets the write bitmask for `SCREEN 7`, `8`, `9` colour planes. `bitmask = 2 ^ value`.
`&h3CF` | sets the read colour plane to `value`.
`&h3D8` | if `value = &h1A`, enable composite colorburst. if `value = &h1E`, disable composite colorburst. Requires `video={cga, tandy, pcjr}`.

##### Notes

- Only a limited number of machine ports are emulated.
- In GW-BASIC under MS-DOS, the sequence needed to set the colour plane mask is: `OUT &h3C4, 2
  OUT &h3C5, 2 ^ plane` The sequence needed to set the colour plane is: `OUT &h3CE, 4
  OUT &h3CF, plane` The initial `OUT` statements currently have no effect in PC-BASIC.

##### Parameters

- `port` is a numeric expression in `[-32768—65535]`.
- `value` is a numeric expression in `[0—255]`.

##### Errors

- `port` or `value` has a string value: `Type mismatch`.
- `port` is not in `[-32768—65535]`: `Overflow`.
- `value` is not in `[-32768—32767]`: `Overflow`.
- `value` is not in `[0—255]`: `Illegal function call`.

#### `PAINT`

`PAINT [STEP] (x, y) [, attrib [, border [, background]]]`

Flood-fills the screen with a colour or pattern, starting from the given seed point.

##### Parameters

- `x`, `y` are numeric expressions in the range `[-32768—32767]` If `STEP` is specified, `x` `y` are offsets from the current position. If the seed point is outside the visible screen area, no flood fill is performed.
- `attrib` is an expression that specifies the fill attribute or pattern. If not specified, the current foreground attribute is used.
- If `attrib` has a number value, it must be in `[0—255]`; it specifies the colour attribute used to fill.
- If `attrib` has a string value, it specifies a tile pattern (see below).
- `border` is a numeric expression in `[0—255]`. It specifies the attribute of the fill boundary (see below).
- `background` is a string expression that represents a background tile pattern to ignore when determining boundaries (see below).

##### Tile patterns

A tile pattern can be specified by a string of up to 255 characters. The interpretation of the string depends on the number of bits per pixel and on the current screen mode.

1 bit per pixel (e.g. `SCREEN 2`)
  Here is an example:

  `76543210` | Byte value
  --- | ---
  `*.......` | `&h80`
  `.*......` | `&h40`
  `..*.....` | `&h20`
  `...*....` | `&h10`
  `....*...` | `&h08`
  `.....*..` | `&h04`
  `......*.` | `&h02`

  This diagonal stripe pattern can thus be produced with `PAINT (0, 0), CHR$(128)+CHR$(64)+CHR$(32)+CHR$(16)+CHR$(8)+CHR$(4)+CHR$(2)`
`SCREEN 7`, `8`, `9`
  The tile pattern is always 8 pixels wide. The first character in the pattern string contains the first bit of each of these 8 pixels, the second character contains the second bits, etc. For example, in a 2-bits-per-pixel mode, four colour attributes can be used in the pattern. To create a diagonal stripe pattern of the same shape, in attribute `&h03`, we now need a tile string that is twice as long:

  Attribute bit | `76543210` | Byte value
  --- | --- | ---
  0 | `*.......` | `&h80`
  1 | `*.......` | `&h80`
  0 | `.*......` | `&h40`
  1 | `.*......` | `&h40`
  0 | `..*.....` | `&h20`
  1 | `..*.....` | `&h20`
  0 | `...*....` | `&h10`
  1 | `...*....` | `&h10`
  0 | `....*...` | `&h08`
  1 | `....*...` | `&h08`
  0 | `.....*..` | `&h04`
  1 | `.....*..` | `&h04`
  0 | `......*.` | `&h02`
  1 | `......*.` | `&h02`

  If the pattern string is truncated before all bits of the last line have been defined, the remaining bits will be zero.
`SCREEN 1`, `3`, `4`, `5` , `6`
  Each row of the tile pattern represents a screen row. Colours are encoded in consecutive bits; the more bits per pixel, the narrower the pattern is. For 2 bits per pixel, the pattern is 4 pixels wide; for 4 bits per pixel it is 2 pixels wide. The following pattern string encodes a diagonal dotted stripe in two colours:

  `3210` | `76543210` | Byte value
  --- | --- | ---
  `2000` | `*.......` | `&h80`
  `1000` | `.*......` | `&h40`
  `0200` | `..*.....` | `&h20`
  `0100` | `...*....` | `&h10`
  `0020` | `....*...` | `&h08`
  `0010` | `.....*..` | `&h04`
  `0002` | `......*.` | `&h02`

The tile pattern is anchored to the screen; imagine a grid starting at `(0,0)` and covering the screen. Whenever an area is tile-filled, the tiles are put into this grid. In this way, adjacent areas will have continuous tiling even if they were filled from different seed points.

##### Boundaries

A solid flood fill stops at pixels that have the same attribute as the fill or that have the specified border attribute, if specified. A tiling flood fill stops at the specified border attribute; if no border attribute is specified, it stops at the current foreground attribute. A tiling flood fill also stops at scan line intervals that are the same as the tiling pattern for that line, unless a background pattern is specified and the interval also equals the background pattern for that line.

##### Errors

- In `SCREEN 7`, `8`, `9`, if `background` equals `attrib` up to the length of `attrib`: `Illegal function call`.
- `background` has a number value: `Illegal function call`.
- `border`,`x`, or `y` have a string value: `Type mismatch`.
- `border`,`x`, or `y` are not in `[-32768—32767]`: `Overflow`.
- `border` is not in `[0—255]`: `Illegal function call`.
- `attrib` is numeric and not in `[-32768—32767]`: `Overflow`.
- `attrib` is numeric and not in `[0—255]`: `Illegal function call`.

#### `PALETTE`

`PALETTE [attrib, colour]`

Assigns a colour to an attribute. All pixels with that attribute will change colour immediately. If no parameters are specified, `PALETTE` resets to the initial setting.

##### Parameters

- `attrib` is a numeric expression between 0 and the current palette size, less one.
- `colour` is a numeric expression between -1 and the maximum number of colours for the current screen mode, less one. If `colour` equals -1, the palette remains unchanged.

##### Errors

- `attrib` or `colour` has a string value: `Type mismatch`.
- `attrib` or `colour` is not in `[-32768—32767]`: `Overflow`
- `attrib` or `colour` is not in range: `Illegal function call`

#### `PALETTE USING`

`PALETTE USING int_array_name {(|[} start_index {)|]}`

Assigns new colours to all attributes.

##### Parameters

- `int_array_name` is a single- or multidimensional array of integers (`%`) that will supply the new values for the palette.
- `start_index` is a numeric expression that indicates at which index in the array to start mapping to the palette.

##### Notes

- Array values are assigned to palette entries in the order in which they are stored in memory. See Arrays for details about the layout of arrays in memory.
- If an array entry has value `-1`, the matching attribute is left unchanged.

##### Errors

- `int_array_name` has not been allocated: `Illegal function call`. The array will not be automatically allocated.
- `int_array_name` is not an integer array: `Type mismatch`.
- `int_array_name` is too short: `Illegal function call`.
- `start_index` has a string value: `Type mismatch`.
- `start_index` is not in `[-32768—32767]`: `Overflow`
- `start_index` is outside array dimensions: `Subscript out of range`

#### `PCOPY`

`PCOPY src, dst`

Copies the screen page `src` to `dst`. All text and graphics on `dst` is replaced by those of `src`.

##### Parameters

- `src` and `dst` are numeric expressions between 0 and the current video mode's number of pages, less one.

##### Errors

- `src` or `dst` has a string value: `Type mismatch`.
- `src` or `dst` is not in `[-32768—32767]`: `Overflow`.
- `src` or `dst` is out of range: `Illegal function call`.

#### `PEN` (statement)

`PEN {ON|OFF|STOP}`

Controls event trapping and read access of the light pen (emulated through the mouse in PC-BASIC). `PEN ON` switches pen reading and trapping on. `PEN OFF` switches it off. `PEN STOP` suspends `PEN` event trapping until `PEN ON` is executed. Up to one event can be triggered during suspension, provided that event handling was switched on prior to suspension. The event triggered during suspension is handled immediately after the next `PEN ON` statement.

#### `PLAY` (event switch)

`PLAY {ON|OFF|STOP}`

- `ON`: enables `ON PLAY` event trapping of the music queue.
- `OFF`: disables trapping.
- `STOP`: halts trapping until `PLAY ON` is used. Events that occur while trapping is halted will trigger immediately when trapping is re-enabled.

#### `PLAY` (music statement)

`PLAY [mml_string_0] [, [mml_string_1] [, mml_string_2]]`

Plays the tune defined by the Music Macro Language strings `mml_string_0, ...`.

Unless `syntax={tandy | pcjr}` is set, only the single-voice syntax is available. The three separate MML strings correspond to the three voices of the PCjr/Tandy sound adapter. The notes in these strings are played synchronously.

##### Parameters

- `mml_string_0`, `mml_string_1`, `mml_string_2` are string expressions in MML.
- At least one parameter must be provided and the statement must not end in a comma.

##### Music Macro Language reference

###### Notes and Pauses

Command | Effect
--- | ---
`{A|B|C|D|E|F|G}[#|+|-][m]` | Play a note. `+` or `#` indicates sharp. `-` indicates flat. `m` is a numeric literal and indicates duration of an `m`th note. `m` is in the range `[0—64]`. If `m=0` or omitted, use the default length.
`Nn` | Play note `n`, in the range `[0—84]` (7 octaves). `n = 0` means rest.
`On` | Set the current octave to `n`, in the range `[0—6]`. Default is 4.
`>` | Increase the current octave by 1, with a maximum of 6.
`<` | Decrease the current octave by 1, with a minimum of 0.
`Pn` | Pause for the duration of an `n`th note. `n` is in the range `[0—64]`. If `n=0`, this has no effect.

###### Timing commands

Command | Effect
--- | ---
`.` | Increase the duration of the preceding note by 1/2 times its normal duration. Periods can be repeated to increase duration further.
`Ln` | Set the duration of following note to an `n`th note. (`n=4` is a quarter note, etc.) `n` is in the range `[1—64]`.
`MN` | Normal: 7/8 of the duration is sound, with 1/8 silence. Default mode.
`ML` | Legato: full duration is sound.
`MS` | Staccato: 3/4 of the duration is sound, with 1/4 silence.
`Tn` | Sets the tempo to `n` `L4`s per minute. `n` is in the range `[32—255]`. Default is 120.

###### Background-mode commands

These commands affect `SOUND`, `PLAY` and `BEEP`

Command | Effect
--- | ---
`MB` | Turns on background mode; sound commands exit without waiting for the music to finish. The music keeps playing while other commands are executed. There can be up to 32 notes in the background music queue; if more notes are played, `PLAY` will block until there are only 32 left. Note that the gaps between notes in the default articulation and in staccato are counted as separate notes on the queue.
`MF` | Turns off background mode; sound commands block. Default mode.

###### Subroutine command

Command | Effect
--- | ---
`Xs` | Execute substring. `s` is one of the following: a string variable name followed by a`;` the result of `VARPTR$()` on a string variable

###### Volume control

Volume control is available on `syntax={tandy | pcjr}` only:

Command | Effect
--- | ---
`Vn` | Set the volume to `n`, in the range `[-1—15]`. `-1` means full volume. If `SOUND ON` has not been executed, this has no effect.

###### MML Parameters

Numeric variables `n` in the commands above can be:

- an integer literal, e.g. `PLAY "L4G"`
- a numeric variable name or array element `var` preceded by `=` and followed by `;`. For example, `PLAY "L=VAR;G"` or `PLAY "L=A(1);G"`
- the result of `VARPTR$(var)` preceded by `=`. For example, `PLAY "L=" + VARPTR$(VAR) + "G"`

Note that only number literals may follow named notes and dereferencing variables or arrays is not allowed there. It is an error to write `PLAY "G=VAR;"` or `PLAY "G=" + VARPTR$(VAR)`. Use `PLAY "G4"` or `PLAY "L=VAR;G"` or `PLAY "L=" + VARPTR$(VAR) + "G"` instead.

##### Errors

- `mml_string` has a numeric value: `Type mismatch`.
- `mml_string` has errors in the MML: `Illegal function call`.
- A variable in an MML string is of incorrect type: `Type mismatch`.
- No MML string is specified: `Missing operand`.
- If `SOUND ON` has not been executed, using the three-voice syntax will raise `Syntax error`.

#### `POKE`

`POKE address, value`

Sets the value of the memory byte at `segment * 16 + address` to `value`, where `segment` is the current segment set with `DEF SEG`.

##### Parameters

- `address` is a numeric expression in `[-32768—65535]`. Negative values are interpreted as their two's complement.
- `value` is a numeric expression in `[0—255]`.

##### Notes

- The memory is only partly emulated in PC-BASIC. See Memory model for supported addresses. Outside emulated areas of memory, this statement has no effect.

##### Errors

- `address` or `value` has a string value: `Type mismatch`.
- `address` is not in `[-32768—65535]`: `Overflow`.
- `value` is not in `[-32768—32767]`: `Overflow`.
- `value` is not in `[0—255]`: `Illegal function call`.

#### `PSET` and `PRESET`

`{ PSET | PRESET } [STEP] (x, y) [, attrib]`

Change the attribute of a pixel on the screen at position `(x, y)`. If `STEP` is specified, `(x, y)` is an offset from the current position.

If `attrib` is between 0 and the screen mode's palette size, the pixel is changed to attribute `attrib`. If `attrib` is larger than the palette size, the pixel's attribute is changed to the highest legal attribute value. If `attrib` is not specified, `PSET` changes the attribute to the current foreground attribute while `PRESET` changes it to zero.

##### Parameters

- `x`, `y` are numeric expressions in `[-32768—32767]`.
- `attrib` is a numeric expression in `[0—255]`.

##### Errors

- `x` or `y` has a string value: `Type mismatch`.
- `attrib`, `x` or `y` or the physical coordinates they translate into are not in `[-32768—32767]`: `Overflow`.
- `attrib` is not in `[0—255]`: `Illegal function call`.

#### `PRINT` and `LPRINT`

`{LPRINT|{PRINT|?} [# file_num,]} [expr_0|;|,|SPC(n)|TAB(n)] ... [USING format; uexpr_0 [{;|,} uexpr_1] ... [;|,]]`

Writes expressions to the screen, printer, or file. If `LPRINT` is used, output goes to `LPT1:`. If `file_num` is specified, output goes to the file open under that number. `?` is a shorthand for `PRINT`.

When writing a string expression to the screen, the following control characters have special meaning. Other characters are shown as their corresponding glyph in the current codepage.

Code point | Control character | Effect
--- | --- | ---
`&h07` | `BEL` | Beep the speaker.
`&h08` | `BS` | Erase the character in the previous column and move the cursor back.
`&h09` | `HT` | Jump to the next 8-cell tab stop.
`&h0A` | `LF` | Go to the leftmost column in the next row; connect the rows to one logical line.
`&h0B` | `VT` | Move the cursor to the top left of the screen.
`&h0C` | `FF` | Clear the screen.
`&h0D` | `CR` | Go to the leftmost column in the next row.
`&h1C` | `FS` | Move the cursor one column to the right.
`&h1D` | `GS` | Move the cursor one column to the left.
`&h1E` | `RS` | Move the cursor one row up.
`&h1F` | `US` | Move the cursor one row down.

Expressions can optionally be separated by one or more of the following keywords:

Keyword | Effect
--- | ---
`;` | Attaches two expressions tight together; strings will be printed without any space in between, numbers will have one space separating them, in addition to the space or minus sign that indicate the sign of the number.
`,` | The expression after will be positioned at the next available zone. The output file is divided in 14-character zones; if the width of the file is not a multiple of 14, the remaining spaces are unused and the first zone of the next line is used instead. If the file has a width of less than 14 characters, the zones are determined as if the file were wrapping continuously.
`SPC(n)` | Produces `n` spaces, where `n` is a numeric expression. if `n` is less than zero, it defaults to zero. If `n` is greater than the file width, it is taken modulo the file width.
`TAB(n)` | Moves to column `n`, where `n` is a numeric expression. if `n` is less than zero, it defaults to zero. If `n` is greater than the file width, it is taken modulo the file width. If the current column is greater than `n`, TAB moves to column `n` on the next line.

If the print statement does not end in one of these four separation tokens, a newline is printed after the last expression. String expressions can be separated by one or more spaces, which has the same effect as separating by semicolons.

##### Format string syntax

A `USING` declaration occurs at the end of an `[L]PRINT[#]` statement and writes a formatted string to the screen, printer or file. The following tables list the format tokens that can be used inside the `format` string.

`_` | Escape character; causes the next character in the format string to be printed as is rather than interpreted as a format token.

For string expressions:

`!` | Prints the first character of a string.
`\\` | Prints 2 or more characters of a string. A greater number of characters is selected by separating the `\`s by spaces.
`&` | Prints the whole string.

For numeric expressions, the format string specifies a width and alignment.

`#` | Indicate a position for a digit.
`.` | Indicate the decimal point.
`,` | Before the decimal point: cause digits to be grouped in threes separated by commas. After the decimal point it is not a token. Provides one digit position.

The number of characters in the field must not exceed 24.

Tokens preceding the number field:

`+` | Cause the sign to be printed for positive as well as negative numbers. The sign is to be printed to the left of the number.
`**` | Cause any leading spaces to be replaced with `*`s. Provides two digit positions.
`$$` | Cause a `$` to be printed to the left of the number. Provides one digit position.

Tokens trailing the number field:

`+` | Cause the sign to be printed for positive as well as negative numbers. The sign will be printed to the right of the number.
`-` | Cause the sign for negative numbers to be printed to the right of the number. Note that `-` preceding the field is not a token but printed literally.
`^^^^` | Specify that scientific notation `E+00` is to be used.

Numeric expressions are always fully printed, even if they do not fit in the positions specified. If the number does not fit in the allowed space, a `%` is printed preceding it.

- If there are more expressions than format fields, the format string is wrapped around.
- Expressions may be separated with semicolons or commas; the effect is the same.
- If the `USING` declaration ends in a comma or semicolon, no newline is printed at the end.
- After a `USING` declaration, other elements of the `PRINT` syntax such as `SPC(` and `TAB(` can not be used.

##### Parameters

- `expr_0, expr_1, ...` are expressions of any type.
- `format` is a string expression that specifies the output format.
- `uexpr_0, uexpr_1, ...` are expressions matching a token in the format string.

##### Notes

- If an error is raised, the output before the error was encountered is printed as normal.
- In GW-BASIC, when formatting a number with a dollar sign, if the number is in the range `[-10000—-32767]` and does not fit in the width of the number field, the minus sign is omitted. This is not implemented in PC-BASIC.

##### Errors

- `n` has a string value: `Type mismatch`.
- `n` is not in `[-32768—65535]`: `Overflow`.
- The format string contains no tokens: `Illegal function call`.
- An expression doesn't match the corresponding format token type: `Type mismatch`.
- A number field in the format string exceeds 24 characters: `Illegal function call`.
- A number field in the format string contains no `#` characters: `Illegal function call`.

#### `PUT` (files)

`PUT [#] file_number [, record_number]`

Writes a record to the random-access file `file_number` at position `record_number`.

##### Parameters

- `file_number` is a numeric expression that yields the number of an open random-access file. The `#` is optional and has no effect.
- `record_number` is a numeric expression in `[1—33554432]` (`2^25`) and is interpreted as the record number.

##### Notes

- The record number is stored as single-precision; this precision is not high enough to distinguish single records near the maximum value of `2^25`.

##### Errors

- `record_number` is not in `[1—33554432]`: `Bad record number`.
- `file_number` is not in `[0—255]`: `Illegal function call`.
- `file_number` is not the number of an open file: `Bad file mode`.
- `file_number` is open under a mode other than `RANDOM`: `Bad file mode`.
- `file_number` is not specified: `Missing operand`.

#### `PUT` (communications)

`PUT [#] com_file_number [, number_bytes]`

Writes `number_bytes` bytes to the communications buffer opened under file number `com_file_number`. `number_bytes` is a numeric expression between 1 and the `COM` buffer length, inclusive.

##### Notes

- In GW-BASIC, `Device I/O error` is raised for overrun error, framing error, and break interrupt. `Device fault` is raised if DSR is lost during I/O. A `Parity error` is raised if parity is enabled and incorrect parity is encountered. This is according to the manual; it is untested.

##### Errors

- `bytes` is less than 1: `Bad record number`.
- `bytes` is less than `32768` and greater than the `COM` buffer length: `Illegal function call`.
- `com_file_number` is not specified: `Missing operand`.
- `com_file_number` is not in `[0—255]`: `Illegal function call`.
- `com_file_number` is not the number of an open file: `Bad file number`.
- The serial input buffer is full, i.e. `LOF(com_file_number) = 0` and `LOC(com_file_number)=255`: `Communication buffer overflow`.

#### `PUT` (graphics)

`PUT (x0, y0), array_name [, {PSET|PRESET|AND|OR|XOR}]`

Displays an array to a rectangular area of the graphics screen. Usually, `PUT` is used with arrays that have been stored using `GET`. See `GET` for the format of the array.

The keywords have the following effect:

`PSET` | Overwrite the screen location with the new image
`PRESET` | Overwrite the screen location with the inverse image
`AND` | Combines the old and new attributes with bitwise AND
`OR` | Combines the old and new attributes with bitwise OR
`XOR` | Combines the old and new attributes with bitwise XOR

##### Parameters

- `array_name` is a numeric array.
- `x0`, `y0` are numeric expressions.

##### Errors

- The array does not exist: `Illegal function call`.
- `array_name` refers to a string array: `Type mismatch`.
- `x0`, `y0` are string expressions: `Type mismatch`.
- `x0`, `y0` are not in `[-32768—32767]`: `Overflow`.
- `x0`, `y0` is outside the current `VIEW` or `WINDOW`: `Illegal function call`

#### `RANDOMIZE`

`RANDOMIZE [expr]`

Seeds the random number generator with `expr`. If no seed is specified, `RANDOMIZE` will prompt the user to enter a random seed. The user-provided value is rounded to an integer. The random seed is formed of the last two bytes of that integer or `expr`. If `expr` is a float (4 or 8 bytes), these are `XOR`ed with the preceding 2. The first 4 bytes of a double are ignored. The same random seed will lead to the same sequence of pseudorandom numbers being generated by the `RND` function.

##### Parameters

- `expr` is a numeric expression.

##### Notes

- For the same seed, PC-BASIC produces the same pseudorandom numbers as GW-BASIC 3.23.
- The random number generator is very poor and should not be used for serious purposes. See `RND` for details.

##### Errors

- `expr` has a string value: `Illegal function call`.
- The user provides a seed outside `[-32768—32767]` at the prompt: `Overflow`.

#### `READ`

`READ var_0 [, var_1] ...`

Assigns data from a `DATA` statement to variables. Reading starts at the current `DATA` position, which is the `DATA` entry immediately after the last one read by previous `READ` statements. The `DATA` position is reset to the start by the `RUN` and `RESTORE` statements.

##### Parameters

- `var_0`, `var_1` are variables or array elements.

##### Errors

- Not enough data is present in `DATA` statements: `Out of DATA`.
- The type of the variable is not compatible with that of the data entry being read: a `Syntax error` occurs on the `DATA` line.

#### `REM`

`{REM|'} [anything]`

Ignores everything until the end of the line. The `REM` statement is intended for comments. Everything after `REM` will be stored in the program unaltered and uninterpreted. `'` (apostrophe) is an alias for `:REM'`; it can be placed at any point in the program line and will ensure that the rest of the line is ignored.

Note that a colon `:` does not terminate the `REM` statement; the colon and everything after it will be treated as part of the comment.

#### `RENUM`

`RENUM [new|.] [, [old|.] [, increment]]`

Replaces the line numbers in the program by a systematic enumeration starting from `new` and increasing by `increment`. If `old` is specified, line numbers less than `old` remain unchanged. `new`, `old` are line numbers; the dot `.` signifies the last line edited. `increment` is a line number but must not be a dot or zero.

##### Notes

- Line numbers afer the following keywords will be renumbered: `AUTO`, `EDIT`, `ELSE`, `ERL`, `DELETE`, `GOSUB`, `GOTO`, `LIST`, `LLIST`, `RENUM`, `RESTORE`, `RESUME`, `RETURN`, `RUN`, `THEN`.
- Any line numbers in `CHAIN` statements will not be renumbered; note that these line numbers refer to another program.
- All arguments of `RENUM` or `AUTO` statements in a program will be renumbered, including any line number offsets or increments, even though that does not make much sense.
- A zero line number following the keywords `ERROR GOTO` will not be renumbered.
- If a referenced line number does not exist in the program, a message `Undefined line ref in old_line` is printed. Here, `old_line` is the line number prior to renumbering. The referenced line number will be left unchanged, but the line's old line number will be renumbered.

##### Errors

- Any of the parameters is not in `[0—65529]`: `Syntax error`.
- Any of the newly generated line numbers is greater than `65529`: `Illegal function call`. The line numbers up to the error have not been changed.
- `increment` is empty or zero: `Illegal function call`.
- `old` is specified and `new` is less than or equal to an existing line number less than old: `Illegal function call`.

#### `RESET`

`RESET`

Closes all open files.

##### Notes

- Official GW-BASIC documentation and many other sources state that `RESET` closes all files on disk devices. However, in reality GW-BASIC 3.23 also closes files on tape and any other device, making this statement identical to `CLOSE` with no arguments. PC-BASIC follows this behaviour.

#### `RESTORE`

`RESTORE [line]`

Resets the `DATA` pointer. `line` is a line number. If `line` is not specified, the `DATA` pointer is reset to the first `DATA` entry in the program. If it is specified, the `DATA` pointer is reset to the first `DATA` entry in or after `line`.

##### Errors

- `line` is not an existing line number: `Undefined line number`.

#### `RESUME`

`RESUME [0|NEXT|line]`

Continues normal execution after an error handling routine. If `0` or no option is specified, re-executes the statement that caused the error. If `NEXT` is specified, executes the statement following the one that caused the error. If `line` is specified, it must be a valid line number.

##### Errors

- `RESUME` is encountered outside of an error trapping routine: `RESUME without error`.
- The program ends inside an error trapping routine without a `RESUME` or `END` statement: `No RESUME`.
- `line` is not an existing line number: `Undefined line number`.

#### `RETURN`

`RETURN [line]`

Returns from a `GOSUB` subroutine. If `line` is not specified, `RETURN` jumps back to the statement after the `GOSUB` that jumped into the subroutine. If `line` is specified, it must be a valid line number. `RETURN` jumps to that line (and pops the `GOSUB` stack). When returning from an error trapping routine, `RETURN` re-enables the event trapping which was stopped on entering the trap routine.

##### Errors

- `line` is not an existing line number: `Undefined line number`.

#### `RMDIR`

`RMDIR dir_spec`

Removes an empty directory on a disk device.

##### Parameters

- The string expression `dir_spec` is a valid file specification that specifies the path and name of the directory.

##### Errors

- `dir_spec` has a numeric value: `Type mismatch`.
- `dir_spec` is an empty string: `Bad file name`.
- No matching path is found: `Path not found`.
- Directory to remove is not empty: `Path/File access error`.

#### `RSET`

`RSET string_name = expression`

Copies a string value into an existing string variable or array element. The value will be right-justified and any remaining characters are replaced by spaces.

##### Parameters

- `string_name` is a string variable or array element.
- `expression` is a string expression.

##### Notes

- If `expression` has a value that is longer than the length of the target variable, it is truncated at the tail to the length of the target variable.
- If `string_name` has not been allocated before, this statement has no effect.
- Use `LSET`, `RSET` or `MID$` to copy values into a `FIELD` buffer.
- If `LET` is used on a `FIELD` variable instead of `L|RSET`, the variable is detached from the field and a new, normal string variable is allocated.

##### Errors

- `string_name` is not a string variable: `Type mismatch`.
- `expression` does not have a string value: `Type mismatch`.

#### `RUN`

`RUN [line_number [anything]|file_spec [, R]]`

Executes a program. Existing variables will be cleared and any program in memory will be erased. `RUN` implies a `CLEAR` If `,R` is specified after `file_spec`, files are kept open; if not, all files are closed.

##### Parameters

- `line_number` is a valid line number in the current program. If specified, execution starts from this line number. The rest of the `RUN` statement is ignored in this case.
- The string expression `file_spec`, if specified, is a valid file specification indicating the file to read the program from.

##### Errors

- `line_number` is not a line number in the current program: `Undefined line number`.
- `file_spec` cannot be found: `File not found`.
- `file_spec` is an empty string: `Bad file number`.
- A loaded text file contains lines without line numbers: `Direct statement in file`.

#### `SAVE`

`SAVE file_spec [, {A|P}]`

Stores the current program in a file.

- If `,A` is specified, the program will be saved in plain text format. In this case, program execution will stop and control will be returned to the user.
- If `,P` is specified, the program will be saved in protected format. When a protected program is loaded in GW-BASIC, it cannot be `LIST`ed or `SAVE`d in non-protected format.
- If neither is specified, the program will be saved in tokenised format.

##### Parameters

- The string expression `file_spec` is a valid file specification indicating the file to store to.

##### Errors

- `file_spec` has a number value: `Type mismatch`.
- `file_spec` is an empty string: `Bad file number`.
- `file_spec` contains disallowed characters: `Bad file number` (on `CAS1:`); `Bad file name` (on disk devices).
- `hide-protected` is enabled, the current program is protected and `,P` is not specified: `Illegal function call`.

#### `SCREEN` (statement)

`SCREEN [mode] [, [colorburst] [, [apage] [, [vpage] [, erase]]]]`

Change the video mode, composite colorburst, active page and visible page. Video modes are described in the Video Modes section.

##### Parameters

- `mode` is a numeric expression that sets the screen mode.
- `colorburst` is a numeric expression. See notes below.
- `apage` is a numeric expression that sets the active page.
- `vpage` is a numeric expression that sets the visible page.
- `erase` is a numeric expression in the range `[0, 1, 2]`. It is only legal with `syntax={pcjr, tandy}`. See notes below.

##### Video modes

The video modes are as follows:

`SCREEN 0` Text mode
  80x25 or 40x25 characters of 8x16 pixels
  16 attributes picked from 64 colours
  Attributes 16-31 are blinking versions of 0-15
  4 pages `ega`
`SCREEN 1` CGA colour
  320x200 pixels
  40x25 characters of 8x8 pixels
  4 attributes picked from 16 colours; 2 bits per pixel
  1 page `ega` 2 pages `pcjr` `tandy`
`SCREEN 2` CGA monochrome
  640x200 pixels
  80x25 characters of 8x8 pixels
  2 attributes picked from 16 colours; 1 bit per pixel
  1 page `ega` 2 pages `pcjr` `tandy`
`SCREEN 3` Low-res 16-colour `pcjr` `tandy`
  160x200 pixels
  20x25 characters of 8x8 pixels
  16 attributes picked from 16 colours; 4 bits per pixel
  2 pages
`SCREEN 3` Hercules monochrome `hercules`
  720x348 pixels
  80x25 characters of 9x14 pixels (with bottom line truncated by 2 px)
  2 attributes; 1 bit per pixel
  2 pages
`SCREEN 3—255` Altissima risoluzione `olivetti`
  640x400 pixels
  80x25 characters of 8x16 pixels
  2 attributes of which one picked from 16 colours; 2 bits per pixel
  1 page
`SCREEN 4` Med-res 4-colour `pcjr` `tandy`
  320x200 pixels
  40x25 characters of 8x8 pixels
  4 attributes picked from 16 colours; 2 bits per pixel
  2 pages
`SCREEN 5` Med-res 16-colour `pcjr` `tandy`
  320x200 pixels
  40x25 characters of 8x8 pixels
  16 attributes picked from 16 colours; 4 bits per pixel
  1 page

  Note: a minimum of 32768 bytes of video memory must be reserved to use this video mode. Use the statement `CLEAR ,,,32768!` or the option `video-memory=32768`.

`SCREEN 6` High-res 4-colour `pcjr` `tandy`
  640x200 pixels
  80x25 characters of 8x8 pixels
  4 attributes picked from 16 colours; 2 bits per pixel
  1 page

  Note: a minimum of 32768 bytes of video memory must be reserved to use this video mode. Use the statement `CLEAR ,,,32768!` or the option `video-memory=32768`.

`SCREEN 7` EGA colour `ega`
  320x200 pixels
  40x25 characters of 8x8 pixels
  16 attributes picked from 16 colours; 4 bits per pixel
  8 pages
`SCREEN 8` EGA colour `ega`
  640x200 pixels
  80x25 characters of 8x8 pixels
  16 attributes picked from 16 colours; 4 bits per pixel
  4 pages
`SCREEN 9` EGA colour `ega`
  640x350 pixels
  80x25 characters of 8x14 pixels
  16 attributes picked from 64 colours; 4 bits per pixel
  2 pages
`SCREEN 10` EGA monochrome `ega` `monitor=mono`
  640x350 pixels
  80x25 characters of 8x14 pixels
  4 attributes picked from 9 pseudocolours; 2 bits per pixel
  2 pages

##### NTSC Composite Colorburst

On CGA, Tandy and PCjr, `colorburst` has the following effects, depending on the type of monitor - RGB (default) or composite:

`mode` | `colorburst` | CGA mode | Effect (composite) | Effect (RGB)
--- | --- | --- | --- | ---
0 | 0 | 0, 2 | greyscale | default palette
0 | 1 | 1, 3 | colour | default palette
1 | 0 | 4 | colour | default palette
1 | 1 | 5 | greyscale | alternate palette

On `SCREEN 2`, `colorburst` has no effect; on a composite monitor, colour artifacts can be enabled on this screen through `OUT` (see there). On `SCREEN 3` and up, `colorburst` has no effect.

##### Erase

By default, if the `mode` changes or the `colorburst` changes between zero and non-zero, the old page and the new page of the screen are cleared. On `syntax={pcjr, tandy}`, the `erase` parameter can be used to change this behaviour. Its values are as follows:

`erase` | Effect
--- | ---
`0` | Do not erase any screen page
`1` (default) | If the `mode` changes or the `colorburst` changes between zero and non-zero, the old page and the new page of the screen are cleared.
`2` | If the `mode` changes or the `colorburst` changes between zero and non-zero, all pages of the screen are cleared.

##### Notes

- At least one parameter must be specified.
- Composite colour artifacts are emulated only crudely in PC-BASIC, and not at all in `SCREEN 1`.

##### Errors

- No parameters are specified: `Missing operand`.
- Any parameter has a string value: `Type mismatch`.
- Any parameter is not in `[-32768—32767]`: `Overflow`.
- `mode` is not an available video mode number for your video card setting: `Illegal function call`.
- `vpage`, `apage` are not between 0 and the number of pages for the chosen video mode, less one: `Illegal function call`.
- `colorburst` is not in `[0—255]`: `Illegal function call`.
- `erase` is not in `[0, 1, 2]`: `Illegal function call`.

#### `SHELL`

`SHELL [command]`

Starts an operating system subshell on the console. If `command` is specified, the command is executed on the shell and execution returns to the program.

To enable this statement, the `shell` option must be set to a valid command interpreter.

##### Parameters

- `command` is a string expression.

##### Notes

- Be careful when enabling this command, as it allows the running BASIC program full access to your files and operating system.

##### Errors

- `shell` option is not specified: `Illegal function call`.
- `command` has a number value: `Type mismatch`.
- All output from the operating system subshell, including error messages, is displayed on the PC-BASIC screen.

#### `SOUND` (tone)

`SOUND frequency, duration [, volume [, voice]]`

Produces a sound at `frequency` Hz for `duration/18.2` seconds. On PCjr and Tandy, the volume and voice channel can additionally be specified.

If `PLAY "MB"` has been executed, `SOUND` plays in the background. If `PLAY "MF"` has been executed, sound plays in the foreground and the interpreter blocks until the sound is finished. Foreground mode is default. Unlike `PLAY`, the sound played by the most recent `SOUND` statement always plays in the background, even if `PLAY "MF"` has been entered. In background mode, each `SOUND` statement counts as 1 toward the length of the queue reported by the `PLAY` function.

##### Parameters

- `frequency` is a numeric expression in `[37—32767]` or `0` (for `syntax={advanced | pcjr}`) or in `[-32768—32767]` (for `syntax=tandy`).
- `duration` is a numeric expression in `[0—65535]`.
- `volume` is a numeric expression in `[-1—15]`. 0 is silent, 15 is full volume; every step less reduces the volume by 2 dB. -1 is also full volume. (For `syntax={pcjr | tandy}`).
- `voice` is a numeric expression in `[0—2]`, indicating which of the three tone voice channels is used for this sound. (For `syntax={pcjr | tandy}`)

##### Notes

- On PCjr and Tandy, Frequencies below 110 Hz are played as 110 Hz.
- If `duration` is zero, any active background sound is stopped and the sound queue is emptied.
- If `duration` is zero, `volume` and `voice` must not be specified.
- If `duration` is less than `.022` but nonzero, the sound will be played in background and continue indefinitely until another sound statement is executed. This is also the behaviour for negative `duration`.
- If `frequency` equals `32767` or `0`, a silence of length `duration` is queued.

##### Errors

- Any argument has a string value: `Type mismatch`.
- `frequency` is not in its allowed range, and `duration` is not zero: `Illegal function call`.
- `duration` is zero and more than two arguments are specified: `Syntax error`.
- `syntax={ pcjr | tandy }` is not set and more than two arguments are specified: `Syntax error`.
- `frequency` is not in `[-32768—32767]`: `Overflow`.
- `duration` is not in `[-65535—65535]`: `Illegal function call`.
- `volume` is not in `[0—15]`: `Illegal function call`.
- `voice` is not in `[0—2]`: `Illegal function call`.

#### `SOUND` (switch)

`SOUND {ON|OFF}`

Switches the external speaker on or off and toggles the availability of advanced sound capabilities on PCjr and Tandy. This includes 3-voice sound, noise generation and volume control. Clears the background music queue.

##### Notes

- Only available with `syntax={pcjr | tandy}`.
- On PC-BASIC, both the internal and the external speaker are emulated through the same sound system.

##### Errors

- This statement is used and `syntax={ pcjr | tandy }` is not set: `Syntax error`.

#### `STOP`

`STOP`

Breaks program execution, prints a `Break` message on the console and returns control to the user. Files are not closed. It is possible to resume program execution at the next statement using `CONT`.

#### `STRIG` (switch)

`STRIG {ON|OFF}`

Has no effect.

#### `STRIG` (event switch)

`STRIG[ ](button) {ON|OFF|STOP}`

Switches event trapping of the joystick trigger `button` `ON` or `OFF`. `STRIG (button) STOP` suspends event trapping until `STRIG (button) ON` is executed. Up to one event can be triggered during suspension, provided that event handling was switched on prior to suspension. The event triggered during suspension is handled immediately after the next `STRIG (button) ON` statement.

`button` | return value
--- | ---
0 | 1st joystick 1st trigger
2 | 2nd joystick 1st trigger
4 | 1st joystick 2nd trigger
6 | 2nd joystick 2nd trigger

##### Parameters

- `button` is a numeric expression in `[0, 2, 4, 6]`.

##### Errors

- `button` has a string value: `Type mismatch`.
- `button` is not in `[-32768—32767]`: `Overflow`.
- `button` is not in `[0, 2, 4, 6]`: `Illegal function call`.

#### `SWAP`

`SWAP var_0, var_1`

Exchanges variables `var_0` and `var_1`.

##### Notes

- The variables are exchanged by reference. If, for example, `var_0` is a `FIELD` variable and `var_1` is not, then `SWAP` will reverse those roles.

##### Parameters

- `var_0` and `var_1` are variables or array elements of the same type. `var_1` must have been previously defined.

##### Errors

- `var_1`is undefined: `Illegal function call`. Note that no error is raised if `var_0` is undefined, and that after this error both variables will be defined.
- The types of `var_0` and `var_1` are not the same: `Type mismatch`.

#### `SYSTEM`

`SYSTEM`

Exits the interpreter.

##### Notes

- `SYSTEM` quits the PC-BASIC interpreter immediately without further interaction. Any unsaved program or data will be lost.

#### `TERM`

`TERM`

Load and run the program defined by the `term` option. By default, as on the IBM PCjr, this is a built-in serial terminal emulator application. This statement is only available with `syntax={pcjr|tandy}`.

##### Errors

- If `term` is not set, this statement raises `Internal error`.
- If `syntax` is not set to `pcjr` or `tandy`, this keyword is not present. Calling `TERM` will raise `Syntax error`.

#### `TIME$` (statement)

`TIME$ = time`

Sets the current BASIC time to `time`.

##### Parameters

- Time is a string expression of the form `"HH{:|.}mm{:|.}ss"` where `0 <= HH < 24`, `0 <= mm < 60` and `0 <= ss < 60`. Each position may have one or two characters.

##### Notes

- PC-BASIC stores an offset to the system time and uses this for future calls to `TIME$` and `DATE$` functions in the same interpreter session. The system time is not changed, unlike GW-BASIC under MS-DOS.

##### Errors

- `time` has a numeric value: `Type mismatch`.
- `time` is not of the correct form: `Illegal function call`.

#### `TIMER` (statement)

`TIMER {ON|OFF|STOP}`

- `ON`: enables `ON TIMER` event trapping of the timer clock.
- `OFF`: disables trapping.
- `STOP`: halts trapping until `TIMER ON` is used. Events that occur while trapping is halted will trigger immediately when trapping is re-enabled.

#### `TRON` and `TROFF`

`{TRON|TROFF}`

Turns line number tracing on or off. If line number tracing is on, BASIC prints a tag `[100]` to the console when program line `100` is executed, and so forth.

##### Notes

- Tracing is turned off by the `NEW` and `LOAD` statements.

#### `UNLOCK`

`UNLOCK [#] file_number [, record_0]` `UNLOCK [#] file_number, [record_0] TO record_1`

Unlocks a file or part of it that has previously been locked with `LOCK`.

##### Parameters

- `file_number` is a numeric expression in `[0—255]`.
- `record_0` and `record_1` are numeric expressions in `[1—2^25-2]`.

##### Errors

- Any parameter has a string value: `Type mismatch`.
- `file_number` is not in `[-32768—32767]`: `Overflow`.
- `file_number` is not in `[0—255]`: `Illegal function call`.
- `file_number` is not an open file: `Bad file number`.
- If `file_number` is open for `RANDOM`, `LOCK` and `UNLOCK` statements must match in terms of `record_0` and `record_1`. An non-matching `UNLOCK` will raise `Permission denied`.
- `record_0` or `record_1` is not in `[1—2^25-2]`: `Bad record number`.

#### `VIEW`

`VIEW [[SCREEN] (x0, y0)-(x1, y1) [, [fill] [, border]]]`

Defines a graphics viewport. Graphics drawn outside the viewport will not be shown. `(x0, y0)`, `(x1, y1)` are absolute screen coordinates of two opposing corners of the area.

Unless `SCREEN` is specified, after a `VIEW` statement the coordinate system is shifted such that `(0, 0)` becomes the top left coordinate of the viewport. If `VIEW` is called without arguments, the viewport is reset to the whole screen.

##### Parameters

- `fill` is an attribute. The viewport will be filled with this attribute.
- `border` is an attribute. A border will be drawn just outside the viewport with this attribute.

##### Errors

- Any of the parameters has a string value: `Type mismatch`.
- Any of the coordinates is not in `[-32768—32767]`: `Overflow`.
- Any of the coordinate pairs is outside the physical screen: `Illegal function call`.

#### `VIEW PRINT`

`VIEW PRINT top_row TO bottom_row`

Defines the text scrolling area of the screen. `LOCATE` statements, cursor movement and scrolling will be limited to the scrolling area.

##### Parameters

- `top_row` and `bottom_row` are numeric expressions in `[1—24]`.

##### Notes

- If `syntax={pcjr | tandy}` and `KEY OFF` is set, `bottom_row` may be 25. Otherwise, screen row 25 cannot be part of the scrolling area.

##### Errors

- `top_row` or `bottom_row` is not in `[1—24]`: `Illegal function call`.

#### `WAIT`

`WAIT port, and_mask [, xor_mask]`

Waits for the value of `(INP(port) XOR xor_mask) AND and_mask` to become nonzero. Event handling is suspended until `WAIT` returns. If `xor_mask` is not specified, it defaults to `0`.

##### Notes

- A limited number of machine ports are emulated in PC-BASIC. See `INP`.

##### Errors

- Any parameter has a string value: `Type mismatch`.
- `port` is not in `[-32768—65535]`: `Overflow`.
- `and_mask` or `xor_mask` are not in `[0—255]`: `Type mismatch`.

#### `WEND`

`WEND`

Iterates a `WHILE—WEND` loop: jumps to the matching `WHILE` statement, where its condition can be checked.

##### Notes

- `WHILE—WEND` loops can be nested. `WEND` jumps to the most recent `WHILE` statement that has not been closed by another `WEND`.

##### Errors

- All previous `WHILE` statements have been closed by another `WEND` or no `WHILE` statement has been executed before: `WEND without WHILE`.

#### `WHILE`

`WHILE expr`

Initiates a `WHILE—WEND` loop. If `expr` evaluates to zero, `WHILE` jumps to the statement immediately after the matching `WEND`. If not, execution continues.

##### Parameters

- `expr` is a numeric expression.

##### Errors

- No matching `WEND` is found: `WHILE without WEND`.
- `expr` has a string value: `Type mismatch`.

#### `WIDTH` (console)

`WIDTH num_columns [, [num_rows] [,]]`

Sets the screen width to 20, 40 or 80 columns.

##### Notes

- When changing screen width in graphics mode, the video mode is changed. The following changes occur:
  `SCREEN 1` (40) ↔ `SCREEN 2` (80)
  `SCREEN 7` (40) ↔ `SCREEN 8` (80)
  `SCREEN 7` (40) ← `SCREEN 9` (80)
- Screen width value 20 is only allowed on Tandy and PCjr. Changing to this width changes to `SCREEN 3`. Additionally, the following changes occur:
  `SCREEN 3` (20) → `SCREEN 1` (40)
  `SCREEN 3` (20) → `SCREEN 2` (80)
  `SCREEN 4` (40) → `SCREEN 2` (80)
  `SCREEN 5` (40) ↔ `SCREEN 6` (80)

##### Parameters

- `num_columns` is either a literal `20`, `40` or `80` or a numeric expression in parentheses. The trailing comma is optional and has no effect.
- `num_rows` is optional and must equal 25. If `syntax={pcjr | tandy}` is set, `num_rows` may be in `[0—25]` but its value is ignored.

##### Errors

- `num_columns` is a string expression: `Type mismatch`.
- `num_columns` is not in `[-32768—32767]`: `Overflow`.
- `num_columns` is not in `[0—255]`: `Illegal function call`.
- `num_columns` is not a literal and not bracketed: `Illegal function call`.
- `num_rows` is not in its accepted range: `Illegal function call`.

#### `WIDTH` (devices and files)

`WIDTH {#file_num,|device_name,|LPRINT} num_columns`

Sets the line width for a file or a device. When a write operation passes beyond the column width, a `CR LF` sequence is inserted.

If a device is specified, it does not need to have a file open to it; the width setting will be the default width next time a file is opened to that device.

If `device_name` is `"LPT1:"` or `LPRINT` is specified, the device width setting affects `LPRINT` and `LLIST`.

If `device_name` is `"SCRN:"`, `"KYBD:"`, or omitted, the screen width is changed. In this case, `num_columns` must be one of 20, 40 or 80. See the notes at `WIDTH` (console) for side effects.

##### Parameters

- `file_num` is a numeric expression which is the number of an open file.
- `device_name` is a string expression that is one of `"KYBD:"`, `"SCRN:"`, `"LPT1:"`, `"LPT2:"`, `"LPT3:"`, `"COM1:"`, `"COM2:"`, `"CAS1:"`
- `num_columns` is a numeric expression.

##### Errors

- `device_name` is not one of the allowed devices: `Bad file name`.
- `device_name` is `"SCRN:"`, `"KYBD:"` and `num_columns` is not 20, 40 or 80: `Illegal function call`.
- `file_num` or `num_columns` are strings: `Type mismatch`.
- `file_num` or `num_columns` are not in `[-32768—32767]`: `Overflow`.
- `file_num` or `num_columns` are not in `[0—255]`: `Illegal function call`.
- `file_num` is not an open file: `Bad file mode`.

#### `WINDOW`

`WINDOW [[SCREEN] (x0, y0)-(x1, y1)]`

Define logical coordinates for the viewport. If `SCREEN` is not specified, the bottom left of the screen is mapped to the lower coordinates; the top right of the screen is mapped to the higher coordinates. If `SCREEN` is specified, the top left of the screen is mapped to the lower coordinates; the bottom right of the screen is mapped to the higher coordinates.

If `WINDOW` is called without arguments, the logical coordinates are reset to the viewport coordinates.

##### Parameters

- `x0`, `y0`, `x1`, `y1` are numeric expressions.

##### Errors

- Any of the coordinates have a string value: `Type mismatch`.
- `x0 = x1` or `y0 = y1`: `Illegal function call`.

#### `WRITE`

`WRITE [# file_num,] [expr_0 [{,|;} expr_1] ... ]`

Writes values to a file or the screen in machine-readable form. Values are separated by commas and the line is ended with a `CR LF` sequence. Strings are delimited by double quotes `"`. No padding spaces are inserted.

When writing to the screen, the same control characters are recognised as for the `PRINT` statement.

##### Parameters

- `expr_0, expr_1, ...` are expressions whose value is to be printed.

##### Errors

- `file_num` has a string value: `Type mismatch`.
- `file_num` is open for `INPUT`: `Bad file mode`.

### Errors and Messages

#### Errors

`1` | `NEXT without FOR` A `NEXT` statement has been encountered for which no matching `FOR` can be found.
`2` | `Syntax error` The BASIC syntax is incorrect. A statement or expression has been mistyped or called in one of many incorrect ways. This error is also raised on a `DATA` line if a `READ` statement encounters a data entry of an incorrect format.
`3` | `RETURN without GOSUB` A `RETURN` statement has been encountered for which no `GOSUB` call has been made.
`4` | `Out of DATA` A `READ` statement is attempting to read more data entries than can be found from the current `DATA` location onward.
`5` | `Illegal function call` A statement, function or operator has been called with parameters outside the accepted range. This error is also raised for a large variety of other conditions – check the reference for the statement or function called.
`6` | `Overflow` A numeric expression result or intermediate value is too large for the required number format.
`7` | `Out of memory` There is not enough free BASIC memory to complete the operation. Too much memory is consumed by the program; variables, arrays and strings, or execution stacks for loops, subroutines or user-defined functions.
`8` | `Undefined line number` A reference is made to a line number that does not exist in the program.
`9` | `Subscript out of range` An array index (subscript) is used that is outside the range reserved for that array by the `DIM` statement.
`10` | `Duplicate Definition` A `DIM` statement is used on an array that has been dimensioned before (either implicitly or explicitly) or `OPTION BASE` is called in a way that conflicts with an earlier implicit or explicit definition of the starting index.
`11` | `Division by zero` An attempt is made to divide a number by zero or by a number that is too small to distinguish from zero within the number format's precision.
`12` | `Illegal direct` A `DEF FN` statement is being used in direct mode.
`13` | `Type mismatch` The expression used is of a type that cannot be converted to the required type for the function or statement. Most commonly, this is raised if a string argument is supplied to a statement or function that expects a number, or vice versa.
`14` | `Out of string space` There is not enough free BASIC memory to store the string variable.
`15` | `String too long` A string expression result or intermediate value is longer than 255 characters.
`16` | `String formula too complex`
`17` | `Can't continue` The `CONT` statement is used in circumstances where continuing program execution is not possible.
`18` | `Undefined user function` The `FN` function is called with a function name for which no definition was made by a `DEF FN` statement.
`19` | `No RESUME` The program terminates inside an error trapping routine that has not been closed with `RESUME` or `END`.
`20` | `RESUME without error` A `RESUME` statement is encountered while the program is not executing an error trapping routine.
`21` | unused
`22` | `Missing operand` An operator expression misses an operand or a function or statement is not supplied with sufficient parameters.
`23` | `Line buffer overflow` An `INPUT` or `LINE INPUT` statement encountered an input string longer than 255 characters or the plain-text program file being loaded by `LOAD`, `CHAIN` or `MERGE` contains a line with more than 255 characters. Attempting to load a text file that has `LF` rather than `CR LF` line endings may cause this error.
`24` | `Device Timeout` The handshake has failed on a serial device or a tape device has reached the end of tape.
`25` | `Device Fault`
`26` | `FOR without NEXT` A `FOR` statement has been encountered for which no matching `NEXT` statement can be found.
`27` | `Out of paper` An attempt is made to write to a printer which is out of paper or to another parallel device which has raised an out-of-paper condition.
`28` | unused
`29` | `WHILE without WEND` A `WHILE` statement has been encountered for which no matching `WEND` statement can be found.
`30` | `WEND without WHILE` A `WEND` statement has been encountered for which no matching `WHILE` statement can be found.
`31—49` | unused
`50` | `FIELD overflow` An attempt is made to read, write, or define a `FIELD` variable beyond the length of the random-access file buffer.
`51` | `Internal error` The `TERM` statement is executed but no terminal manager program has been defined.
`52` | `Bad file number` A file number is accessed to which no file is open, or the file number used in an `OPEN` statement is outside the range of allowable file numbers, or (confusingly) the file specification is empty, malformed or contains illegal characters.
`53` | `File not found` A named file on a disk device cannot be found.
`54` | `Bad file mode` The requested file mode in an `OPEN` statement does not exist or is unsupported for the given device, or the file function called is not supported for this device, or the function or statement called requires a file opened for `RANDOM` and the file is not.
`55` | `File already open` An attempt is made to open a file to a file number that is already in use; or an attempt is made to open a file for `OUTPUT` or `APPEND` on a serial, disk or cassette device when a file (or, on a disk device, a file with the same name) is already open for `OUTPUT` or `APPEND` on that device; or a `KILL` or `NAME` statement is executed on a disk file when a file with the same name is open on the same device.
`56` | unused
`57` | `Device I/O error` An I/O error has occured during input/output to a device. This includes faming errors, CRC check failures and unexpected end-of-tape on cassette devices.
`58` | `File already exists` The proposed new name of a disk file in a `NAME` statement is already in use.
`59—60` | unused
`61` | `Disk full` There is insufficient free space on the disk device to complete the operation.
`62` | `Input past end` An attempt is made to retrieve input from a file that has passed its end of file.
`63` | `Bad record number` A random-access file record number is referenced that is outside the permitted range.
`64` | `Bad file name` The file name or other device parameter string in a file specification is malformed or contains illegal characters.
`65` | unused
`66` | `Direct statement in file` A line with no line number is encountered in a plain-text program file.
`67` | `Too many files`
`68` | `Device Unavailable` An attempt is made to access a device that does not exist or is not enabled.
`69` | `Communication buffer overflow` A serial device is receiving more data than fits in its buffer.
`70` | `Permission Denied` The requested access to a file is not granted due to `LOCK` restrictions, operating system locking, or insufficient operating system file permissions.
`71` | `Disk not Ready` The disk device is not ready for access. For example, there is no diskette in a floppy drive or the drive lock is open.
`72` | `Disk media error`
`73` | `Advanced Feature`
`74` | `Rename across disks` An attempt is made to use the `NAME` statement to move a file from one disk device to another.
`75` | `Path/File access error` An attempt is made to create a directory that already exists or to remove a directory that is not empty.
`76` | `Path not found` An `OPEN`, `MKDIR`, `RMDIR`, or `CHDIR` statement is executed referring to a (parent) path that does not exist on the disk device.
`77` | `Deadlock`

Any error code that does not have a message associated to it will generate the message `Unprintable error`.

If an error occurs in direct mode, the error message is printed as above. If the error occurs in a program, the message is supplemented with the line number in which the error occurred. For example, `Illegal function call in 100` indicates that the illegal function call took place in line number `100`.

If a `Syntax error` occurs during program execution, the error message is followed by a listing of the program line in which the error occurred, wth the cursor positioned at the location where the error was raised.

A `Division by zero` error or, in a floating point calculation, an `Overflow`, will not interrupt execution unless it occurs within an error handling routine. The error message will be printed on the console and the result of the offending calculation will be taken to be the maximum value that fits in the appropriate floating-point variable. `Overflow` in an integer calculation will always interrupt execution like other errors.

#### Other messages

`Break`
  Execution of a compound statement or program has been interrupted by a `CONT` statement or by a user keyboard interrupt (such as `Ctrl`+`Break`). If the interrupt happens in a program, the `Break` message will be supplemented with the line number in which the interrupt occurred.
`?Redo from start`
  The input provided on the console for an `INPUT` statement does not match the expected format. The number or type of inputs is not correct. Re-enter all inputs.
`Undefined line ref_num in line_num`
  The `RENUM` statement encountered a reference to the line number `ref_num` which is not defined in the program. The reference occurs on line number `line_num`. The undefined line number reference will not be renumbered.
`filename Found.`
  A file matching the requested specification has been found on the cassette device. This message only occurs in direct mode.
`filename Skipped.`
  A file not matching the requested specification has been encountered on the cassette device. This message only occurs in direct mode.