Bootable DOS USB stick

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Many firmware loading programs on PCs require a DOS environment to function. Moreover, most computers no longer have floppy drives, and many do not even have CD drives. Therefore, it can be useful to have a bootable USB stick which loads DOS. This page describes how to prepare such a stick using tools available in Portage.

Preparing the disk


It is advisable to wipe the USB stick before loading it with a new environment. If the USB stick is at /dev/sdb, this can be done with

root #dd if=/dev/zero of=/dev/sdb bs=1M count=COUNT

where COUNT is replaced with the size of the USB stick in MiB (can be found in dmesg when you plug in the stick).

Double-check the device filename for your USB stick, and that your USB stick has no vital data! This operation has no safeguards to prevent you from destroying your data -- on any block device.


The stick must now be partitioned. Most any partitioning tool can be used, such as cfdisk. In this article we assume that the first partition is to be used for this application. This partition should be marked bootable and its type set to FAT16.

Boot sector

Using ms-sys from the package sys-block/ms-sys, we now prepare the boot sector on the USB stick with

root #ms-sys -s /dev/sdb

This writes a public domain boot sector compatible with DOSEMU which we will use below.


Finally, using mkfs.fat from the package sys-fs/dosfstools, format this partition as FAT16 with

root #mkfs.fat -F16 /dev/sdb1
The maximum file system size under FAT16 is 4GiB. It should also possible to load a DOSEMU environment from FAT32, though this author has not tried it.

Loading a DOS environment

We will now use DOSEMU to create a DOS environment in the USB stick. If DOSEMU is not already on your system, install app-emulation/dosemu.

Configuring DOSEMU

  • Add the USB partition you prepared to the list of "hdimages" in the DOSEMU configuration file ~/.dosemurc:
FILE ~/.dosemurc
$_hdimage = "drives/* /tmp /dev/sdb1"
  • Make sure the USB stick is not already mounted, e.g through your desktop environment file manager, as DOSEMU requires block-level access. This means that DOSEMU must be invoked as root, unless the permissions of the device file have been modified.

Setting up DOS with DOSEMU

  • After starting DOSEMU, the disk should appear as F:. Make the USB partition a DOS boot disk with
C:\>sys F:

This should provide a working commandline environment for running your firmware software.

  • To have a more user-friendly and powerful DOS environment, it is helpful to copy over tools from DOSEMU into the disk:
C:\>xcopy /S /N Z: F:

In order to prevent problems with booting and conflicts between drivers and firmware software, this author recommends editing config.sys and autoexec.bat to be minimal. For example:

FILE config.sys
FILE autoexec.bat
@echo off
path c:\bin;c:\gnu;c:\dosemu
set HELPPATH=c:\help
set TEMP=c:\tmp 
prompt $P$G

To edit these files within DOSEMU, use the tool edit. If editing from Linux, take care to make sure that the files retain their DOS-style CR+LF newlines.

The path statement in the autoexec.bat example above assumes that your BIOS sees your USB stick as a hard disk, such that your stick will be C: under DOS. If it is seen as a floppy, it will become A:.

Finishing up

Checking with QEMU

You can check that the USB stick is bootable without actually rebooting your computer by using QEMU:

root #qemu -hda /dev/sdb1

Keep in mind that this is no guarantee that your BIOS will boot the computer off the USB stick successfully, only a check that you've done things correctly.

Loading your firmware software

If your kernel supports the FAT filesystem, just mount the USB stick as you would normally, and copy over the necessary software. Otherwise, this can be done within DOSEMU, where D: is your home directory. Place the firmware software in your home directory, then

C:\>xcopy /S /N D:\FIRMWARE F:

Here, FIRMWARE is the directory of the firmware software.

Possible improvements

To have a bootable DOS partition as well as a bootable Linux partition (e.g., SystemRescueCD) on the same disk, a bootloader setup more sophisticated than mbr-sys is required. This can be done with GRUB, as described in another HOWTO.

See also