Classic race condition... when I read about it being intermittent my first thought was something timing-related - interrupts are always a good guess.
The only remaining mystery is why nobody has ever seen the issue on a real BBC Master.
I think it would be possible on real hardware, but the probability of getting the exact conditions required to trigger it is so tiny that someone might've seen it once at most, and ignored it as a random glitch that never happened again. It's just by pure luck that the emulator's timing quite consistently matched the trigger conditions.
At least this type of race condition is still essentially within the digital abstraction of the machine and can be debugged at that level - those related to analogue timing are even more aggravating; here's one interesting example of that:
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[ 3.0 ms ] story [ 13.1 ms ] threadThe only remaining mystery is why nobody has ever seen the issue on a real BBC Master.
I think it would be possible on real hardware, but the probability of getting the exact conditions required to trigger it is so tiny that someone might've seen it once at most, and ignored it as a random glitch that never happened again. It's just by pure luck that the emulator's timing quite consistently matched the trigger conditions.
At least this type of race condition is still essentially within the digital abstraction of the machine and can be debugged at that level - those related to analogue timing are even more aggravating; here's one interesting example of that:
http://www.linusakesson.net/scene/safevsp/