FDISK(8)

NAME
fdisk - DOS partition maintenance program

SYNOPSIS
fdisk [-i] [-u] [-a] [-1234] [disk]
fdisk [-f configfile] [-i] [-v] [-t] [disk]

PROLOGUE
In order for the BIOS to boot the kernel, certain conventions must be adhered to. Sector 0 of the disk must contain boot code, a partition table, and a magic number. BIOS partitions can be used to break the disk up into several pieces. The BIOS brings in sector 0 (does it really use the code?) and verifies the magic number. It then searches the 4 BIOS partitions described by sector 0 to determine which of them is active. This boot then brings in the secondary boot block from the active partition and runs it. Under DOS, you could have one or more partitions with one active. The DOS fdisk program can be used to divide space on the disk into partitions and set one active.

DESCRIPTION
The FreeBSD program fdisk serves a similar purpose to the DOS program. The first form is used to display partition information or to interactively edit the partition table. The second is used to write a partition table using a configfile and is designed to be used by other scripts/programs.

Options are: Is used for updating (editing) sector 0 of the disk. Ignored if -f is given. Initialize sector 0 of the disk. This implies -u, unless -f is given. Change the active partition only. Ignored if -f is given. Operate on a single fdisk entry only. Ignored if -f is given. Set partition values using the file configfile. The configfile always modifies existing partitions, unless -i is also given, in which case all existing partitions are deleted (marked as "unused") before the configfile is read. The configfile can be "-", in which case stdin is read. See CONFIGURATION FILE, below, for file syntax.

WARNING: when -f is used, you are not asked if you really want to write the partition table (as you are in the interactive mode). Use with caution! Test mode; do not write partition values. Generally used with the -f option to see what would be written to the partition table. Implies -v. Be verbose. When -f is used, fdisk prints out the partition table that is written to the disk.

The final disk name can be provided as a ‘bare’ disk name only, e.g. ‘sd0’, or as a fully qualified device node under /dev. If omitted, the disks ‘wd0’, ‘sd0’, and ‘od0’ are being searched in that order, until one is being found responding.

When called with no arguments, it prints the sector 0 partition table. An example follows:

    	******* Working on device /dev/rwd0 *******
    	parameters extracted from in-core disklabel are:
    	cylinders=769 heads=15 sectors/track=33 (495 blks/cyl)
    	parameters to be used for BIOS calculations are:
    	cylinders=769 heads=15 sectors/track=33 (495 blks/cyl)

    	Warning: BIOS sector numbering starts with sector 1
    	Information from DOS bootblock is:
    	The data for partition 1 is:
    	sysid 165,(FreeBSD/NetBSD/386BSD)
        	    start 495, size 380160 (185 Meg), flag 0
    		beg: cyl 1/ sector 1/ head 0;
    		end: cyl 768/ sector 33/ head 14
    	The data for partition 2 is:
    	sysid 164,(unknown)
        	    start 378180, size 2475 (1 Meg), flag 0
    		beg: cyl 764/ sector 1/ head 0;
    		end: cyl 768/ sector 33/ head 14
    	The data for partition 3 is:
    	<UNUSED>
    	The data for partition 4 is:
    	sysid 99,(ISC UNIX, other System V/386, GNU HURD or Mach)
        	    start 380656, size 224234 (109 Meg), flag 80
    		beg: cyl 769/ sector 2/ head 0;
    		end: cyl 197/ sector 33/ head 14

The disk is divided into three partitions that happen to fill the disk. The second partition overlaps the end of the first. (Used for debugging purposes)

sysid
    is used to label the partition. FreeBSD reserves the magic number 165 decimal (A5 in hex).

start and size
    fields provide the start address and size of a partition in sectors.

flag 80
    specifies that this is the active partition.

cyl, sector and head
    fields are used to specify the beginning address and end address for the partition.

Note:
    these numbers are calculated using BIOS's understanding of the disk geometry and saved in the bootblock.

The flags -i or -u are used to indicate that the partition data is to be updated, unless the -f option is used. If the -f option is not used, the fdisk program will enter a conversational mode. This mode is designed not to change any data unless you explicitly tell it to. Fdisk selects defaults for its questions to guarantee the above behavior.

It displays each partition and ask if you want to edit it. If you say yes, it will step through each field showing the old value and asking for a new one. When you are done with a partition, fdisk will display it and ask if it is correct. Fdisk will then proceed to the next entry.

Getting the cyl, sector, and head fields correct is tricky. So by default, they will be calculated for you; you can specify them if you choose.

After all the partitions are processed, you are given the option to change the active partition. Finally, when the all the data for the first sector has been accumulated, you are asked if you really want to rewrite sector 0. Only if you answer yes, will the data be written to disk.

The difference between the -u flag and -i flag is that the -u flag just edits the fields as they appear on the disk. While the -i flag is used to "initialize" sector 0; it will setup the last BIOS partition to use the whole disk for FreeBSD; and make it active.

NOTES
The automatic calculation of starting cylinder etc. uses a set of figures that represent what the BIOS thinks is the geometry of the drive. These figures are by default taken from the incore disklabel, but the program initially gives you an opportunity to change them. This allows the user to create a bootblock that can work with drives that use geometry translation under the BIOS.

If you hand craft your disk layout, please make sure that the FreeBSD partition starts on a cylinder boundary. A number of decisions made later may assume this. (This might not be necessary later.)

Editing an existing partition will most likely cause you to lose all the data in that partition.

You should run this program interactively once or twice to see how it works. This is completely safe as long as you answer the last question in the negative. There are subtleties that the program detects that are not fully explained in this manual page.

CONFIGURATION FILE
When the -f option is given, a disk's partition table can be written using values from a configfile. The syntax of this file is very simple. Each line is either a comment or a specification, and whitespace (except for newlines) are ignored:

    # comment ... Lines beginning with a "#" are comments and are ignored.

    g spec1 spec2 spec3 Set the BIOS geometry used in partition calculations. There must be three values specified, with a letter preceding each number:

    cnum
        Set the number of cylinders to num.

    hnum
        Set the number of heads to num.

    snum
        Set the number of sectors/track to num.

    These specs can occur in any order, as the leading letter determines which value is which; however, all three must be specified.

    This line must occur before any lines that specify partition information.

    It is an error if the following is not true:

            1 <= number of cylinders
            1 <= number of heads <= 256
            1 <= number of sectors/track < 64

    The number of cylinders should be less than or equal to 1024, but this is not enforced, although a warning will be output. Note that bootable FreeBSD partitions (the "/" filesystem) must lie completely within the first 1024 cylinders; if this is not true, booting may fail. Non-bootable partitions do not have this restriction.

    Example (all of these are equivalent), for a disk with 1019 cylinders, 39 heads, and 63 sectors:

            g       c1019   h39     s63
            g       h39     c1019   s63
            g       s63     h39     c1019

    p partition type start length Set the partition given by partition (1-4) to type type, starting at sector start for length sectors.

    Only those partitions explicitly mentioned by these lines are modified; any partition not referenced by a "p" line will not be modified. However, if an invalid partition table is present, or the -i option is specified, all existing partition entries will be cleared (marked as unused), and these "p" lines will have to be used to explicitly set partition information. If multiple partitions need to be set, multiple "p" lines must be specified; one for each partition.

    These partition lines must occur after any geometry specification lines, if one is present.

    The type is 165 for FreeBSD partitions. Specifying a partition type of zero is the same as clearing the partition and marking it as unused; however, dummy values (such as "0") must still be specified for start and length.

    Note: the start offset will be rounded upwards to a head boundary if necessary, and the end offset will be rounded downwards to a cylinder boundary if necessary.

    Example: to clear partition 4 and mark it as unused:

            p       4       0       0       0

    Example: to set partition 1 to a FreeBSD partition, starting at sector 1 for 2503871 sectors (note: these numbers will be rounded upwards and downwards to correspond to head and cylinder boundaries):

            p       1       165     1       2503871

    a partition Make partition the active partition. Can occur anywhere in the config file, but only one must be present.

    Example: to make partition 1 the active partition:

            a       1

SEE ALSO
disklabel(8)

BUGS
The entire program should be made more user-friendly.

Throughout this man page, the term ‘partition’ is used where it should actually be ‘slice’, in order to conform with the terms used elsewhere.

You cannot use this command to completely dedicate a disk to FreeBSD. The disklabel(8) command must be used for this.