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posted by janrinok on Saturday January 08, @08:10PM   Printer-friendly
from the PC-introns dept.

https://www.os2museum.com/wp/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.


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  • (Score: 2) by DrkShadow on Sunday January 09, @02:37AM

    by DrkShadow (1404) on Sunday January 09, @02:37AM (#1211177)

    :-D

    The junk in the PC DOS 1.1 boot sector isn’t the only instance of such junk. For example IBMBIO.COM also contains a different and larger junk sequence which is partly a repetition of the contents of IBMBIO.COM itself.

    It is virtually certain that the junk bytes came indirectly from development tools used for building PC DOS 1.1, namely Microsoft’s assembler/linker.

    So the junk probably comes from the linker.. which put the junk into format.com.. which dumps code and junk into the bootsector. Who'da thought, and how the hell did that not result in errors? "Works for me!"

    I recall seeing something like xbmbio.com on the Dell boot stuff - for automatically flashing the BIOS. This sort of thing persists for a long, long time. It's really amazing. Imagine how we'll be using EFI boot packages in 30 more years.

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