FDISK(8) NetBSD System Manager's Manual FDISK(8)
NAME
fdisk - DOS partition maintenance program
SYNOPSIS
fdisk [-aiufBS] [-0 | -1 | -2 | -3] [-b cylinders/heads/sectors] [-s id/start/size] [-c bootcode] [device]
DESCRIPTION
In order for the BIOS to boot the kernel, certain conventions must be ad- hered to. Sector 0 of the disk must contain boot code, a partition table, and a magic number. Newer BIOS programs want the entire first track reserved. BIOS partitions can be used to break the disk up into several pieces. The BIOS brings in sector 0, verifies the magic number, and begins executing the code at the first byte. This code in turn searches the DOS partition table for an `active' partition. If one is found, the boot block from that partition is loaded and replaces the original boot block. 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. The NetBSD 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: NetBSD disklabel disk geometry: cylinders: 769 heads: 15 sectors/track: 33 (495 sectors/cylinder) BIOS geometry: cylinders: 769 heads: 15 sectors/track: 33 (495 sectors/cylinder) Partition table: 0: sysid 169 (NetBSD) start 495, size 380160 (185 MB), flag 0 beg: cylinder 1, head 0, sector 1 end: cylinder 768, head 14, sector 33 1: sysid 164 (reserved) start 378180, size 2475 (1 MB), flag 0 beg: cylinder 764, head 0, sector 1 end: cylinder 768, head 14, sector 33 2: <UNUSED> 3: sysid 99 (GNU HURD or Mach or Sys V/386 (such as ISC UNIX)) start 380656, size 224234 (109 MB), flag 80 beg: cylinder 769, head 0, sector 2 end: cylinder 197, head 14, sector 33 This example 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). The various fields in each partition are: sysid is used to label the partition. NetBSD reserves the magic num- ber 169 decimal (A9 in hex). The number 0 is used to mark a partition as unused. See the -l flag. start, size start address and size of a partition in sectors. flag 80 specifies that this is the active partition. cylinder, head, sector the beginning or ending address of a partition. Note: these numbers are calculated using BIOS's understanding of the disk geometry and saved in the bootblock. At the start the fdisk program will determine whether the disk sector 0 is valid as a boot sector. (This is determined by checking the magic number.) If not, fdisk will initialize the boot code as well as the par- tition table. During this, all four partitions will be marked empty. The flags -a, -i or -u are used to indicate that the partition data is to be updated. The fdisk program will enter an interactive 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 that behavior. Getting the cylinder, head, and sector fields correct is tricky. So by default, they will be calculated for you; you can specify them if you choose. Finally, when all the data for the first sector has been accumulated, fdisk will ask if you really want to rewrite sector 0. Only if you reply affirmatively to this question will fdisk write anything to the disk. Available options: -0 Update partition slot 0. -1 Update partition slot 1. -2 Update partition slot 2. -3 Update partition slot 3. -a Change the active partition. In interactive mode this question will be asked after the partitions have been processed. -b cylinders/heads/sectors Specify the BIOS parameters for cylinders, heads and sectors. It is used only in conjunction with the -u flag. -B Install or update the bootselect code on an i386 machine. -c bootcode Specify the filename that fdisk should read the bootcode from. The default is to read from /usr/mdec/mbr if run on an i386 ma- chine, and leave the bootcode empty for other machines. -f Run fdisk in a non-interactive mode. In this mode, you can only change the disk parameters by using the -b flag. This is provid- ed only so scripts or other programs may use fdisk as part of an automatic installation process. Using the -f flag with -u makes it impossible to specify the starting and ending cylinder, head and sector fields. They will be automatically computed using the BIOS geometry. -i Explicitly request initialization of the master boot code (simi- lar to what fdisk /mbr does under MSDOS), even if the magic num- ber in the first sector is ok. The partition table is left alone by this (but see above). -l Lists known sysid values and exit. -s id/start/size Specify the partition id, start and size. This flag requires the use of a partition selection flag (-0, -1, -2, or -3) -S When used with no other flags print a series of /bin/sh commands for setting variables to the partition information. This is used by the installation scripts. -u Display each partition in turn and interactively ask if you want to edit it. If you reply affirmatively, 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 the information again and ask if it is correct. fdisk will then proceed to the next entry.
NOTES
The automatic calculation of the starting cylinder and other parameters 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 diskla- bel, but fdisk 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. Editing an existing partition is risky, and may 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.
FILES
/usr/mdec/mbr Default location of i386 bootcode /usr/mdec/mbr_bootsel Default location of i386 bootselect code
SEE ALSO
disklabel(8), mbr(8), mbrlabel(8)
BUGS
There are subtleties that the program detects that are not explained in this manual page. NetBSD 1.6.1 December 19, 2000 3
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