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



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

SYNOPSIS
       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 ] ] ]

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 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:

	      /vmlinux
	      /vmunix
	      /boot/bzImage-2.4.0
	      /dev/fd0
	      /dev/fd1

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

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  architec-
       tures  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() system call.

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)



Linux 0.99		       20 November 1993			      RDEV(8)

UNIX/Linux commands referenced on this page:

1. rdev
2. size
3. vidmode
4. rootflags
5. file
6. i386
7. which
8. at
9. mount
10. as
11. find