Bootable DOS USB stick

This page describes how to prepare a bootable USB stick which loads DOS using tools available in Gentoo.

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, so a USB bootable DOS environment may come in useful.

Preparing the disk

Wiping

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>

<COUNT> should be replaced with the size of the USB stick in MiB (can be found in dmesg when plugging in the stick).

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

Partitioning

The stick must now be partitioned. Most any partitioning tool can be used, such as cfdisk. In this article, it is assumed 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, 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 is used below.

Formatting

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

root #mkfs.fat -F16 /dev/sdb1

Note
The maximum file system size under FAT16 is 4 GiB. It should also possible to load a DOSEMU environment from FAT32, though this author has not tried it.

Loading a DOS environment

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

Configuring DOSEMU

Add the previously prepared USB partition 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 any 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:. Start DOSEMU:

root #dosemu

Change to drive Z:, as some files are being copied from there:

C:\>Z:

Make the USB partition a DOS boot disk:

Z:\>sys F:

This should provide a working command-line environment for running the firmware software.

To have a more user-friendly and powerful DOS environment, it is helpful to copy over tools from DOSEMU into the disk:

Z:\>xcopy /S /N Z: F:

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

FILE F:\config.sys

SWITCHES=/F
DOS=UMB,HIGH
dosdata=umb
lastdrive=Z
files=40
stacks=0
buffers=10

FILE F:\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. File -> Open... and go to drive F::

Z:\>edit

If editing from Linux, take care to make sure that the files retain their DOS-style CR+LF newlines.

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

Exit DOSEMU:

Z:\>exitemu

Finishing up

Checking with QEMU

Check that the USB stick is bootable without actually rebooting the computer by using QEMU:

root #qemu -hda /dev/sdb1

Keep in mind that this is no guarantee that the BIOS will boot the computer off the USB stick successfully, only a check that the preparations are correct.

Loading firmware

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

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 (archived).