Unidentified PC DOS 1.1 boot sector junk identified

Anyone trying to disassemble the PC DOS 1.1 boot sector soon notices that at offsets 1A3h through 1BEh there is a byte sequence that just does not belong. It appears to be a fragment of code, but it has no purpose in the boot sector and is never executed. So why is the sequence of junk bytes there, and where did it come from?

The immediate answer is “it came from FORMAT.COM”. The junk is copied verbatim from FORMAT.COM to the boot sector. But those junk bytes are not part of FORMAT.COM, either. So the question merely shifts to “why are the junk bytes in FORMAT.COM, and where did they come from?”

It is not known if anyone answered the question in the past, but the answer has been found now, almost 40 years later—twice independently.

This kind of digital archeology is deeply fascinating.


  1. 2022-01-08 2:52 am
    • 2022-01-08 12:27 pm
  2. 2022-01-08 12:40 pm
    • 2022-01-12 6:32 am
  3. 2022-01-08 1:50 pm
  4. 2022-01-09 9:33 am