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       rdev - query/set image root device, RAM disk size, or video mode


       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]]]


       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:


       When  using  the  rdev  command,  the  root_device  parameter  might be
       something like:


       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.


       -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.


       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.


       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.


       Originally by Werner Almesberger (
       Modified by Peter MacDonald (pmacdona@sanjuan.UVic.CA)
       rootflags support added by Stephen Tweedie (


       The rdev command is part of the util-linux-ng package and is  available