FDISK(8)

NAME
fdisk - DOS partition maintenance program

SYNOPSIS
fdisk [-i] [-u] [-a] [-0123] [disk]

-u  Is used for updating (editing) sector 0 of the disk.

-i  Initializes sector 0 of the disk. This implies -u.

-a  Change the active partition only.

-0123
    Operate on a single fdisk entry only.

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.

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. 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 0 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 1 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 2 is:
    	<UNUSED>
    	The data for partition 3 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. 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.

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.