DOS-friendly bootable ISOs with mkisofs


I’ve used WinImage and WinISO in a Windows 2000 virtual machine for years to create and modify ISO images, but I decided in the spirit of the occasion to learn how to do the same thing on FreeBSD. The experience also taught me a few things about how bootable CDs actually work.

My biggest surprise was realising that ISO 9660 makes no provision for bootable media. This makes sense given earlier machines couldn’t boot from CDs, but I had long assumed this was only a BIOS limitation. Bootable CD support came with the El-Torito extension, so named for the restaurant where the developers first conceived of its design. It can even include multiple boot records for various architectures.

Jörg Schilling’s excellent cdrecord tools includes mkisofs, which can be used to create bootable ISOs that support this. The FreeBSD Handbook’s storage chapter includes a section on creating CDs with it, but I wanted to be able to boot on legacy x86 machines like 486s and early Pentiums. I scoured the mkisofs manpage(8) and came up with these options:

mkisofs                             \
    -eltorito-boot "BOOTFLOPPY.IMG" \
    -input-charset "cp437"          \
    -iso-level 1                    \
    -joliet                         \
    -output "OUTPUT.ISO"            \
    -rational-rock                  \
    -volume-set "DISKNAME"          \
    "$_source file"

The options are:

  • -eltorito-boot is the floppy image you want to boot from
  • -input-charset "cp437" for DOS-based systems
  • -iso-level 1 enforces DOS-style 8.3 filenames
  • -joliet isn’t ISO standard, but is useful for 95/NT
  • -output is the exported ISO image
  • -rational-rock adds Rock Ridge extensions
  • -volume-set is the CD volume name the OS displays

Rock Ridge extensions aren’t used on DOS or 16-bit Windows as far as I can tell, but doesn’t impact its use and is valuate when attached to other systems, so I still include anyway.

From my experience with WinISO, I knew that bootable CDs for early machines used floppy disk images, which would be emulated by the optical drive on boot. One common trick was to embed a Windows 98 boot disk with CD-ROM drivers into an otherwise-unbootable CD, like those for Windows 95 or earlier OS/2. In an age where blank CDs are easier to find than blank floppy disks, I’ve even been know to create bootable DOS installers.

I’ve wrapped this in a shell script on my lunchbox if you want to use.

Author bio and support


Ruben Schade is a technical writer and infrastructure architect in Sydney, Australia who refers to himself in the third person. Hi!

The site is powered by Hugo, FreeBSD, and OpenZFS on OrionVM, everyone’s favourite bespoke cloud infrastructure provider.

If you found this post helpful or entertaining, you can shout me a coffee or send a comment. Thanks ☺️.