---
name: "swapdev"
section: "8"
platform: "linux"
package: "util-linux"
version: "2.10f-5.1"
release: "Debian 2.2 (Potato)"
platforms: ["linux"]
url: "https://new.carta.tech/rdev"
---

# swapdev(8)

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

Part of [rdev(8)](/rdev/8.md).

## NAME

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

## SYNOPSIS

```
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 ] ] ]
```

## DESCRIPTION

With no arguments, **rdev** outputs an */etc/mtab* line for the current root file system. With no arguments, **swapdev**, **ramsize**, **vidmode**, and **rootflags** 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 kernel image, are as follows:

```
/vmlinux
/vmlinux.test
/vmunix
/vmunix.test
/dev/fd0
/dev/fd1
```

When using the **rdev**, or **swapdev** commands, the *root\_device* or *swap\_device* parameter are as follows:

```
/dev/hda[1-8]
/dev/hdb[1-8]
/dev/sda[1-8]
/dev/sdb[1-8]
```

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 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

- **-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.

## BUGS

For historical reasons, there are two methods for specifying alternative 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 booting their systems.

## 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)
```