Michal Necasek investigates a few unexplained bytes in the boot sector and also some bytes in IBMBIO.COM…
Unidentified PC DOS 1.1 Boot Sector Junk Identified
The first person to make this discovery (as far as we can tell) was Daniel B. Sedory aka The Starman, whose illustrated PC DOS 1.1 boot sector page is much nicer than anything I could put together.
Perhaps see also Sedory’s page
The Mystery Bytes of the Windows 95 MBR (paraphrased)
Perhaps see also
PC DOS 1.1 reassembled from sources
Low-level Explorations of Commodore’s Soft-sectored Floppies
via Yellow Plastic