Post #4176465
2026-07-28 23:28 UTC
so, a thought. a potential hack. maybe one for @foone@digipres.club
The IBM floppy controller uses a NEC 765. The NEC 765 can support four drives, but IBM didn't really use that functionality directly.
The NEC has two "Unit Select" pins intended to select a drive. For whatever reason, IBM didn't connect them, and instead added an external register called the Digital Output Register (DOR), with which you actually select the floppy drive you want to use.
Replies (2)
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@RueNahcMohr@infosec.exchange 2026-07-28 23:37
@gloriouscow@oldbytes.space @foone@digipres.club I know there are 4 drive selects on the cable and always wondered wtf
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@RueNahcMohr@infosec.exchange 2026-07-28 23:38
@gloriouscow@oldbytes.space @foone@digipres.club but of course, there is that whole mess with drive select vs motor enable and the 5000 jumper block on all 5.25" floppy drives (and some 3.5") to make a floppy drive work (if you know all the magic)