Man page for rdev

June 14, 2007 – 6:18 pm

RDEV


Section: Linux Programmer’s Manual (8)
Updated: 20 November 1993
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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 kernels.)

–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@nessie.cs.id.ethz.ch)
Modified by Peter MacDonald (pmacdona@sanjuan.UVic.CA)
rootflags support added by Stephen Tweedie (sct@dcs.ed.ac.uk)



 

Index



NAME

SYNOPSIS

DESCRIPTION

OPTIONS

BUGS

HISTORY

AUTHORS



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  2. man mkinitrd – Man page for mkinitrd
  3. man initrd – Man page for initrd
  4. man xmdomain – Man page for xmdomain
  5. man kexec_load – Man page for kexec_load
  6. man mkdumprd – Man page for mkdumprd
  7. man sulogin – Man page for sulogin
  8. man grubby – Man page for grubby
  9. man mkswap – Man page for mkswap
  10. man grub-install – Man page for grub-install

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