RDEV(8)                    Linux Programmer's Manual                   RDEV(8)

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

       rdev [ -rvh ] [ -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 off-
       set 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  ker-
       nel image, might be:


       When  using  the rdev command, the root_device parameter might be some-
       thing 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.

       For the rootflags command, the flags parameter contains extra  informa-
       tion 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.

       -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() system call.

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

Linux 0.99                     20 November 1993                        RDEV(8)