Add bootable DOS support to your USB stick

Building on the previous post where we added SystemRescueCD capabilities to our USB stick (please see that post for details and conventions that pertain to this one as well) while still being able to use it for transferring files between various OS, we will now add bootable DOS support so that we can use it for BIOS updates as well as other DOS based tools such as Intel's SSD firmware updater.

I was frustrated by other methods I found when researching. One needed my whole USB stick, which was undesirable and another using a pre-built image with no control over the config.sys, etc. left me in a pseudo floppy mode (A:\ drive) and also crashed on a regular basis. There had to be a better way. I wanted the whole nine yards - one USB stick for system rescue, BIOS/firmware upgrades, and file transfer between Windows and 'nix systems. A veritable digital swiss army knife! This way you get full control over the config.sys and autoexec.bat files. New embedded systems without floppy and CD/DVD drives really helped make the case for solving this issue without purchasing new hardware to drag around.

This post will have less hand-holding than the previous one. Hopefully the detail provided previously will get you through this post without problems.

For enhanced flexibility we'll use a separate "/boot" partition. This will allow for easily updating the SystemRescueCD and other partitions with new/updated data without disturbing the installed GRUB files.

We'll need two more products: FreeDOS (download the fdfullcd.iso) and install makebootfat (your distro most likely has this packaged).

Create a working directory for the necessary FreeDOS files and loop mount the FreeDOS iso.
Example:

• mkdir fdwrk
• mkdir fdboot
• mount -o loop /path/to/fdfullcd.iso /mnt/loop

Copy (as a minimum) the files:

• kernel.sys
• himem.exe
• command.com
• more.exe

 from /mnt/loop/freedos/setup/odin to your fdwrk directory.

Create or copy a config.sys file (necessary to load the command interpreter) to your fdwrk directory:
Example config.sys file:
===============================================
DEVICE=HIMEM.EXE
LASTDRIVE=Z

BUFFERS=20     

FILES=40

DOS=HIGH,UMB

DOSDATA=UMB

SHELLHIGH=command.com /P

===============================================

Create or copy an autoexec.bat file (the simple one below will avoid the date/time prompt) to your fdwrk directory:

Example autoexec.bat file:

===============================================

CLS

===============================================

Make sure your config.sys and batch files use DOS/Windows line termination - CR+LF and not just LF as is standard with 'nix systems.

Copy the file:

•  /source/ukernel/boot/fat16.bin

 from inside the /mnt/loop/freedos/packages/src_base/kernels.zip file to the fdboot directory.

Instead of just two partitions as in the previous post we will make 4. Basically the same instructions but partitions 2 & 3 can be pretty small. I made them both 128M which really is overkill but I don't need the space that bad. I used:

1. 6500M type "c" FAT32LBA (for file sharing - must be first partition, sde1/hd4,0)
2. 128M type "e" FAT16 (the DOS partition, sde2/hd4,1)
3. 128M type "83" Linux (the /boot partition - where grub resides, sde3/hd4,2)
4. 800M (or remaining space) type "83" Linux (the SystemRescueCd partition, sde4/hd4,3)

Format partition 1 as previously, format partitions 3 & 4 as we did partition 2 previously. Format partition 2 as fat16:

• mkfs.vfat -F16 /dev/sde2

Mount the linux partitions on available mountpoints:

• mount -t ext4 /dev/sde3 /media/usbstickgrub
• mount -t ext4 /dev/sde4 /media/usbsticksysrescue

Copy the SystemRescueCD files to /media/usbsticksysrescue:

• cp -a /where.your.System.RescueCD.files.are/* /media/usbsticksysrescue/*

Prepare /media/usbstickgrub:

• mkdir -p /media/usbstickgrub/boot/grub
• cp -a /lib/grub/<distro-specific>/* /media/usbstickgrub/boot/grub

Add the FreeDOS entry to the grub's menu.lst file we created in the previous post (/media/usbstickgrub/boot/grub/menu.lst):
===============================================
timeout 10

title SystemRescueCD 32bit
kernel (hd0,3)/isolinux/rescuecd
initrd (hd0,3)/isolinux/initram.igz

title SystemRescueCD 64bit
kernel (hd0,3)/isolinux/rescue64
initrd (hd0,3)/isolinux/initram.igz

title FreeDOS
rootnoverify (hd0,1)
makeactive
chainloader +1
===============================================

Install grub:

• grub
•   grub> find /boot/grub/menu.lst (will help identify the root partition)
•   grub> root (hd4,2)    (specify our new /boot partition)
•   grub> setup (hd4)     (install GRUB in the USB stick's MBR)
•   grub> quit

Prepare our bootable DOS partition:

• makebootfat -P -o /dev/sde2 -b /path/to/fdboot/fat16.bin /path/to/fdwrk

When this is complete you can mount the DOS partition and add any other files you want. You could have also placed these in fdwrk and they would have been added by the makebootfat process.

The nice thing about this method is that when booting into DOS the partition is seen as the C: drive, however, many BIOS/firmware upgrade batch (and config.sys) files may assume you're booting from a floppy and have some hardcoded A:\ paths. Simply edit these files as needed and remove the A:\.

The FreeDOS partition may have to exist under some limit (8GB, 16GB ?). As my USB stick is only 8GB I didn't run into any issues. If you have a larger USB stick you may not have success if your FreeDOS (2nd) partition is beyond such limit.

Please comment on whether you find this information useful or what might be done to improve it. Thank you!