PREP(8) PREP(8)
NAME
prep, fdisk, format, mbr - prepare disks, floppies and
flashes
SYNOPSIS
disk/prep [ -bcfnprw ] [ -a name ]... [ -s sectorsize ]
plan9partition
disk/fdisk [ -abfprw ] [ -s sectorsize ] disk
disk/format [ -dfvx ] [ -b bootblock ] [ -c csize ] [ -l
label ] [ -r nresrv ] [ -t type ] disk [ file... ]
disk/mbr [ -9 ] [ -m mbrfile ] disk
DESCRIPTION
A partition table is stored on a non-floppy disk to specify
the division of the physical disk into a set of logical
units. On PCs, the partition table is stored at the end of
the master boot record of the disk. Partitions of type 0x39
are Plan 9 partitions. The names of PC partitions are cho-
sen by convention from the type: dos, plan9, etc. Second
and subsequent partitions of the same type on a given disk
are given unique names by appending a number (or a period
and a number if the name already ends in a number).
Plan 9 partitions (and Plan 9 disks on non-PCs) are them-
selves divided, using a textual partition table, called the
Plan 9 partition table, in the second sector of the parti-
tion (the first is left for architecture-specific boot data,
such as PC boot blocks). The table is a sequence of lines
of the format part name start end, where start and end name
the starting and ending sector. Sector 0 is the first sec-
tor of the Plan 9 partition or disk, regardless of its posi-
tion in a larger disk. Partition extents do not contain the
ending sector, so a partition from 0 to 5 and a partition
from 5 to 10 do not overlap.
The Plan 9 partition often contains a number of convention-
ally named subpartitions. They include:
9fat A small FAT file system used to hold configuration
information (such as plan9.ini and plan9.nvr) and
kernels. This typically begins in the first sector
of the partition, and contains the partition table
as a ``reserved'' sector. See the discussion of the
-r option to format.
arenas A venti(8) arenas partition.
bloom A venti(8) bloom-filter partition.
cache A cfs(4) file system cache.
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fossil A fossil(4) file system.
fs A kfs(4) file system.
fscfg A few-sector partition used to store an fs(3) con-
figuration.
isect A venti(8) index section.
nvram A one-sector partition used to simulate non-volatile
RAM on PCs.
other A non-archived fossil(4) file system.
swap A swap(8) swap partition.
fdisk and prep
Fdisk edits the PC partition table and is usually invoked
with a disk like /dev/sdC0/data as its argument, while prep
edits the Plan 9 partition table and is usually invoked with
a disk partition like /dev/sdC0/plan9 as its argument.
Fdisk works in units of disk ``cylinders'': the cylinder
size in bytes is printed when fdisk starts. Prep works in
units of disk sectors, which are almost always 512 bytes.
Fdisk and prep share most of their options:
-a Automatically partition the disk. Fdisk will create a
Plan 9 partition in the largest unused area on the disk,
doing nothing if a Plan 9 partition already exists. If
no other partition on the disk is marked active (i.e.
marked as the boot partition), fdisk will mark the new
partition active. Prep's -a flag takes the name of a
partition to create. (See the list above for partition
names.) It can be repeated to specify a list of parti-
tions to create. If the disk is currently unparti-
tioned, prep will create the named partitions on the
disk, attempting to use the entire disk in a sensible
manner. The partition names must be from the list given
above.
-b Start with a blank disk, ignoring any extant partition
table.
-p Print a sequence of commands that when sent to the disk
device's ctl file will bring the partition table infor-
mation kept by the sd(3) driver up to date. Then exit.
Prep will check to see if it is being called with a disk
partition (rather than an entire disk) as its argument;
if so, it will translate the printed sectors by the
partition's offset within the disk. Since fdisk oper-
ates on a table of unnamed partitions, it assigns names
based on the partition type (e.g., plan9, dos, ntfs,
linux, linuxswap) and resolves collisions by appending a
numbered suffix. (e.g., dos, dos.1, dos.2).
-r In the absence of the -p and -w flags, prep and fdisk
enter an interactive partition editor; the -r flag runs
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the editor in read-only mode.
-s sectorsize
Specify the disk's sector size. In the absence of this
flag, prep and fdisk look for a disk ctl file and read
it to find the disk's sector size. If the ctl file can-
not be found, a message is printed and a sector size of
512 bytes is assumed.
-w Write the partition table to the disk and exit. This is
useful when used in conjunction with -a or -b.
If neither the -p flag nor the -w flag is given, prep and
fdisk enter an interactive partition editor that operates on
named partitions. The PC partition table distinguishes
between primary partitions, which can be listed in the boot
sector at the beginning of the disk, and secondary (or
extended) partitions, arbitrarily many of which may be
chained together in place of a primary partition. Primary
partitions are named pn, secondary partitions sn. The num-
ber of primary partitions plus number of contiguous chains
of secondary partitions cannot exceed four.
The commands are as follows. In the descriptions, read
``sector'' as ``cylinder'' when using fdisk.
a name [ start [ end ] ]
Create a partition named name starting at sector
offset start and ending at offset end. The new
partition will not be created if it overlaps an
extant partition. If start or end are omitted,
prep and fdisk will prompt for them. In fdisk,
the newly created partition has type ``PLAN9;'' to
set a different type, use the t command (q.v.).
Start and end may be expressions using the opera-
tors +, -, *, and /, numeric constants, and the
pseudovariables . and $. At the start of the pro-
gram, . is set to zero; each time a partition is
created, it is set to the end sector of the new
partition. It can also be explicitly set using
the . command. When evaluating start, $ is set to
one past the last disk sector. When evaluating
end, $ is set to the maximum value that end can
take on without running off the disk or into
another partition. Numeric constants followed by
`k', `m', `g', or `t' (or upper-case equivalents)
are scaled to the respective size in kilo-, mega-,
giga-, or tera-bytes. Finally, the expression n%
evaluates to (n×disksize)/100. As examples, `a .
.+20%' creates a new partition starting at . that
takes up a fifth of the disk, `a . .+21G' creates
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a new partition starting at . that takes up 21
gigabytes (21×230 bytes), and `a 1000 $' creates a
new partition starting at sector 1000 and extend-
ing as far as possible.
. newdot Set the value of the variable . to newdot, which
is an arithmetic expression as described in the
discussion of the a command.
d name Delete the named partition.
h Print a help message listing command synopses.
p Print the disk partition table. Unpartitioned
regions are also listed. The table consists of a
number of lines containing partition name, begin-
ning and ending sectors, and total size. A ' is
prefixed to the names of partitions whose entries
have been modified but not written to disk. Fdisk
adds to the end of each line a textual partition
type, and places a * next to the name of the
active partition (see the A command below).
P Print the partition table in the format accepted
by the disk's ctl file, which is also the format
of the output of the -p option.
w Write the partition table to disk. Prep will also
inform the kernel of the changed partition table.
The write will fail if any programs have any of
the disk's partitions open. If the write fails
(for this or any other reason), prep and fdisk
will attempt to restore the partition table to its
former state.
q Quit the program. If the partition table has been
modified but not written, a warning is printed.
Typing q again will quit the program.
Fdisk also has the following commands.
A name Set the named partition active. The active par-
tition is the one whose boot block is used when
booting a PC from disk.
e Print the names of empty slots in the partition
table, i.e., the valid names to use when creat-
ing a new partition.
t [ type ] Set the partition type. If it is not given,
fdisk will display a list of choices and then
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prompt for it.
format and pbs
Format prepares for use the disk partition or the floppy
diskette in the file named disk, for example /dev/sdC0/9fat
or /dev/fd0disk. The options are:
-f Do not physically format the disc. Used to install a
FAT file system on a previously formatted disc. If disk
is not a floppy device, this flag is a no-op.
-t specify a density and type of disk to be prepared. The
possible types are:
3½DD 3½" double density, 737280 bytes
3½HD 3½" high density, 1474560 bytes
5¼DD 5¼" double density, 368640 bytes
5¼HD 5¼" high density, 1146880 bytes
hard fixed disk
The default when disk is a floppy drive is the highest
possible on the device. When disk is a regular file,
the default is 3½HD. When disk is an sd(3) device, the
default is hard.
-d initialize a FAT file system on the disk.
-b use the contents of bootblock as a bootstrap block to
be installed in sector 0.
The remaining options have effect only when -d is specified:
-c use a FAT cluster size of csize sectors when creating
the FAT.
-l add a label when creating the FAT file system.
-r mark the first nresrv sectors of the partition as
``reserved''. Since the first sector always contains
the FAT parameter block, this really marks the nresrv-1
sectors starting at sector 1 as ``reserved''. When
formatting the 9fat partition, -r 2 should be used to
jump over the partition table sector.
Again under -d, any files listed are added, in order, to the
root directory of the FAT file system. The files are con-
tiguously allocated. If a file is named 9load, it will be
created with the SYSTEM attribute set so that dossrv(4)
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keeps it contiguous when modifying it.
Format checks for a number of common mistakes; in particu-
lar, it will refuse to format a 9fat partition unless -r is
specified with nresrv larger than two. It also refuses to
format a raw sd(3) partition that begins at offset zero in
the disk. (The beginning of the disk should contain an
fdisk partition table with master boot record, not a FAT
file system or boot block.) Both checks are disabled by the
-x option. The -v option prints debugging information.
The file /386/pbs is an example of a suitable bootblock to
make the disk a boot disk. It gets loaded by the BIOS at
0x7C00, reads the first sector of the root directory into
address 0x7E00, and looks for a directory entry named 9LOAD.
If it finds such an entry, it uses single sector reads to
load the file into address 0x10000 and then jumps to the
loaded file image. The file /386/pbslba is similar, but
because it uses LBA addressing (not supported by older
BIOSes), it can access more than the first 8.5GB of the
disk. /386/pbsraw is suitable for CDs.
mbr
Mbr installs a new boot block in sector 0 (the master boot
record) of a disk such as /dev/sdC0/data. If mbrfile con-
tains more than one sector of `boot block', the rest will be
copied into the first track of the disk, if it fits. This
boot block should not be confused with the boot block used
by format, which goes in sector 0 of a partition. Typi-
cally, the boot block in the master boot record scans the PC
partition table to find an active partition and then exe-
cutes the boot block for that partition. The partition boot
block then loads a bootstrap program such as 9load(8), which
then loads the operating system. If MS-DOS or Windows is
already installed on your disk, the master boot record
already has a suitable boot block. Otherwise, /386/mbr is
an appropriate mbrfile. It detects and uses LBA addressing
when available from the BIOS (the same could not be done in
the case of pbs due to space considerations). If the
mbrfile is not specified, a boot block is installed that
prints a message explaining that the disk is not bootable.
The -9 option initialises the partition table to consist of
one plan9 partition which spans the entire disc starting at
the end of the first track.
EXAMPLES
Initialize the kernel disk driver with the partition infor-
mation from the FAT boot sectors. If Plan 9 partitions
exist, pass that partition information as well.
for(disk in /dev/sd??) {
if(test -f $disk/data && test -f $disk/ctl)
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disk/fdisk -p $disk/data >$disk/ctl
for(part in $disk/plan9*)
if(test -f $part)
disk/prep -p $part >$disk/ctl
}
Create a Plan 9 boot floppy on a previously formatted
diskette.
disk/format -b /386/pbs -df /dev/fd0disk \
/386/9load /tmp/plan9.ini /386/9pcf.gz
Initialize the blank disk /dev/sdC0/data.
disk/mbr -m /386/mbr /dev/sdC0/data
disk/fdisk -baw /dev/sdC0/data
disk/prep -bw -a^(9fat nvram fossil cache swap) /dev/sdC0/plan9
disk/format -b /386/pbslba -d -r 2 /dev/sdC0/9fat \
/386/9load /386/9pcf /tmp/plan9.ini
Create a bootable USB disk or flash-memory device to be
booted via the BIOS and containing only a FAT Plan 9 parti-
tion.
mount /srv/usb /n/usb
ls -d /n/usb/sdU* # note the name, normally sdU0.0
disk/partfs /n/usb/sdU0.0/data
cd /dev/sdXX
disk/mbr -m /386/mbr data
disk/fdisk -baw data
disk/prep -bw -a^(9fat nvram fossil) plan9
cp /386/9loadusb /tmp/9load # force format to use the name `9load'
disk/format -b /386/pbslba -d -r 2 9fat /tmp/9load \
/386/9pccpuf /tmp/plan9.ini
FILES
/386/mbr
/386/mbr.bootmgr self-configuring `smart boot manager'
SOURCE
/sys/src/cmd/disk/prep
/sys/src/boot/pc
/n/sources/extra/bootmgr.tgz nasm assembler source for
/386/mbr.bootmgr
SEE ALSO
floppy(3), sd(3), usb(4), 9load(8), mk9660(8), mkusbboot(8),
partfs(8)
BUGS
Format can create FAT12 and FAT16 file systems, but not
FAT32 file systems. The boot block can only read from FAT12
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and FAT16 file systems.
If `prep -p' doesn't find a Plan 9 partition table, it will
emit commands to delete all extant partitions. Similarly,
`fdisk -p' will delete all partitions, including `data', if
there are no partitions defined in the MBR.
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