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

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

       rdev [ -rsvh ] [	-o offset ] [ image [ value [ offset ] ] ]
       rdev [ -o offset	] [ image [ root_device	[ offset ] ] ]
       swapdev [ -o offset ] [ image [ swap_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, swapdev, ramsize, vidmode, and root-
       flags print usage information.

       In a bootable image for the Linux kernel, there are  several  pairs  of
       bytes  which  specify  the root device, the video mode, the size	of the
       RAM disk, and the swap device.  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 ker-
       nel image, are as follows:


       When using the rdev, or swapdev commands, the root_device  or  swap_de-
       vice parameter are as follows:


       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.

       -s     Causes rdev to act like swapdev.

       -r     Causes rdev to act like ramsize.

       -R     Causes rdev to act like rootflags.

       -v     Causes rdev to act like vidmode.

       -h     Provides help.

       For historical reasons, there are two methods for  specifying  alterna-
       tive values for the offset.

       The user	interface is cumbersome, non-intuitive,	and should probably be
       re-written from scratch,	allowing multiple kernel image	parameters  to
       be changed or examined with a single command.

       If  LILO	 is used, rdev is no longer needed for setting the root	device
       and the VGA mode, since these parameters	that rdev modifies can be  set
       from  the  LILO prompt during a boot.  However, rdev is still needed at
       this time for setting the RAM disk size.	 Users are encouraged to  find
       the LILO	documentation for more information, and	to use LILO when boot-
       ing their systems.

       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)


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