Further examining the mystery of boot sectors supposedly starting with byte value 69h, I considered the possibility that the check could have been added for MSX machines. The MSX platform ticks a lot of boxes: It wasn’t 8086 (but rather Z80), used 3.5″ floppies, used FAT format, appeared around 1985, and Microsoft was involved in it.
But looking closer, MSX is quite unlikely for one simple reason: Microsoft was clearly concerned about data exchange with PCs, and solved the problem differently. MSX floppy boot sectors actually start with the usual 8086 instructions, and if they’re booted on an MSX machine, the boot sector starts execution at offset 1Eh.
Not only that, but the MSX BIOS in fact requires that the floppy boot sector must start with byte EBh or E9h to be considered bootable at all.
The above is clearly documented in the MSX Technical Data Book (published by Sony in 1984). Extant MSX floppy images confirm the documentation—MSX floppies use 8086 opcodes in the first three bytes.
But wait, there’s more…
A reader pointed out that although the FAT driver in Windows NT does not check for byte 69h, it does check for byte 49h. The check was added sometime around 1995 and did not exist in the original NT releases.
Unfortunately the 49h check in Windows NT is just as undocumented as the 69h check in DOS. Windows NT was adding support for additional RISC platforms in that time frame (DEC Alpha AXP, IBM/Motorola PowerPC) and the added check could have been related to that. Or not.
It is notable that 69h and 49h are ASCII codes for ‘i’ and ‘I’, respectively. When considering three-byte strings, ‘IBM’ certainly comes to mind; however that is pure speculation and no boot sector with such a signature is known to exist.
I also set out to examine the relevant OS/2 driver (OS2DASD.DMD) source code in the hope that it might reveal something. But no, IBM only checks for the usual EBh and E9h bytes. Which does not answer anything, but implies that boot sectors with 49h or 69h in the first byte must have been fairly exotic.
One day, those boot sectors will hopefully turn up and then we’ll understand why the mysterious checks were added.