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 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)

AVAILABILITY

       The rdev command is part of the util-linux-ng package and is available from ftp://ftp.kernel.org/pub/linux/utils/util-linux-ng/.

Linux 0.99                                                       20 November 1993                                                          RDEV(8)