Bootable DOS USB stick
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. This page describes how to prepare a bootable USB stick which loads DOS using tools available in Gentoo.
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:
dd if=/dev/zero of=/dev/sdb bs=1M count=COUNT
COUNT is replaced with the size of the USB stick in MiB (can be found in dmesg when plugging in the stick).
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.
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
Using ms-sys from the package sys-block/ms-sys, prepare the boot sector on the USB stick with:
ms-sys -s /dev/sdb
This writes a public domain boot sector compatible with DOSEMU which is used below.
mkfs.fat -F16 /dev/sdb1
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.
- Add the USB partition previously prepared to the list of "hdimages" in the DOSEMU configuration 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:
- Change to drive
Z:as some files are being copied from there.
- Make the USB partition a DOS boot disk with:
This should provide a working command-line 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:
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:
SWITCHES=/F DOS=UMB,HIGH dosdata=umb lastdrive=Z files=40 stacks=0 buffers=10
@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
If editing from Linux, take care to make sure that the files retain their DOS-style
pathstatement 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
- Exit DOSEMU:
Checking with QEMU
Check that the USB stick is bootable without actually rebooting the computer by using QEMU:
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.
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, then
xcopy /S /N D:\FIRMWARE F:
Here, FIRMWARE is the directory of the firmware software.
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).