As I mentioned in yesteday's log, I finally got the computer to run some code from ROM, but that is was unreliable and always seemed to wander off into oblivion after some time (a few tens-of-thousands of microseconds).
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From doing a lot of poking and prodding, and running trace after trace with the analyzer, I thought this was related to timing. My ROMs should be fast enough (just) to run with DTACK grounded, but clearly something was off.
For testing, I jury-rigged a DTACK generator using a 74LS93 binary counter in 3-bit mode. It looks like this: