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RDEV(8) Linux Programmer’s Manual RDEV(8)
NAME
rdev - query/set image root device, RAM disk size, or video mode
SYNOPSIS
rdev [ -Rrvh ] [ -o offset ] [ image [ value [ offset ] ] ]
rdev [ -o offset ] [ image [ root_device [ offset ] ] ]
ramsize [ -o offset ] [ image [ size [ offset ] ] ]
vidmode [ -o offset ] [ image [ mode [ offset ] ] ]
rootflags [ -o offset ] [ image [ flags [ offset ] ] ]
DESCRIPTION
With no arguments, rdev outputs an /etc/mtab line for the current root file system.
With no arguments, ramsize, vidmode, and rootflags print usage information.
In a bootable image for the Linux kernel on i386, there are several pairs of bytes
which specify the root device, the video mode, and the size of the RAM disk. These
pairs of bytes, by default, begin at offset 504 (decimal) in the kernel image:
498 Root flags
(500 and 502 Reserved)
504 RAM Disk Size
506 VGA Mode
508 Root Device
(510 Boot Signature)
rdev will change these values.
Typical values for the image parameter, which is a bootable Linux kernel image,
might be:
/vmlinux
/vmunix
/boot/bzImage-2.4.0
/dev/fd0
/dev/fd1
When using the rdev command, the root_device parameter might be something like:
/dev/hda1
/dev/hdf13
/dev/sda2
/dev/sdc4
/dev/ida/c0d0p1
One may also specify the device by a comma-separated pair of decimal integers
major,minor.
For the ramsize command, the size parameter specifies the size of the RAM disk in
kilobytes. 2.0.x kernels and newer dynamically allocate the ramdisk and do not need
this setting.
For the rootflags command, the flags parameter contains extra information used when
mounting root. Currently the only effect of these flags is to force the kernel to
mount the root filesystem in readonly mode if flags is non-zero.
For the vidmode command, the mode parameter specifies the video mode:
-3 = Prompt
-2 = Extended VGA
-1 = Normal VGA
0 = as if "0" was pressed at the prompt
1 = as if "1" was pressed at the prompt
2 = as if "2" was pressed at the prompt
n = as if "n" was pressed at the prompt
If the value is not specified, the image will be examined to determine the current
settings.
OPTIONS
-r Causes rdev to act like ramsize. (Not relevant for 2.0.x and newer ker-
nels.)
-R Causes rdev to act like rootflags.
-v Causes rdev to act like vidmode.
-h Provides help.
BUGS
The rdev utility, when used other than to find a name for the current root device,
is an ancient hack that works by patching a kernel image at a magic offset with
magic numbers. It does not work on architectures other than i386. Its use is
strongly discouraged. Use a boot loader like SysLinux or LILO instead.
HISTORY
At offset 502 there used to be the device number of the swap device (in Linux
0.12), and "rdev -s" or "swapdev" would set this. However, since Linux 0.95 this
constant is not used any longer, and the swap device is specified using the
swapon(2) system call.
At offset 504 there used to be the size of the ramdisk in kilobytes. One would
specify a size, and this much was grabbed off the top of memory. In Linux 1.1.39
it became also possible to set this value on the kernel command line. In Linux
1.3.48 the ramdisk setup was changed. Ramdisk memory is now taken from the buffer
cache, so that the ramdisk can grow dynamically. The interpretation of the ramdisk
word was changed to a word of which the high order bit is a prompt flag (1: prompt
for ramdisk: "VFS: Insert ramdisk floppy and press ENTER" - this is needed with a
two-floppy boot), the next bit a load flag (1: load ramdisk), and the low order 11
bits give the starting block number of the root filesystem image (so that one can
have a single floppy boot). See also linux/Documentation/ramdisk.txt.
AUTHORS
Originally by Werner Almesberger (almesber AT nessie.ch)
Modified by Peter MacDonald (pmacdona AT sanjuan.CA)
rootflags support added by Stephen Tweedie (sct AT dcs.uk)
Linux 0.99 20 November 1993 RDEV(8)
Generated by $Id: phpMan.php,v 4.55 2007/09/05 04:42:51 chedong Exp $ Author: Che Dong
On Apache/1.3.41 (Unix) PHP/5.2.5 mod_perl/1.30 mod_gzip/1.3.26.1a
Under GNU General Public License
2009-01-10 09:03 @38.103.63.58 CrawledBy CCBot/1.0 (+http://www.commoncrawl.org/bot.html)