In his autobiography, Steve Wozniak recounts[1] his difficulty getting interrupts working on the 6502 microprocessor:
The next step was to debug the 256-byte monitor program on the PROMs. I spent a couple of hours trying to get the interrupt version of it working, but I kept failing. I couldn’t write a new program into the PROMs. To do that, I’d have to go to that other building again, just to burn the program into the chip. I studied the chip’s data sheets to see what I did wrong, but to this day I never found it. As any engineer out there reading this knows, interrupts are like that. They’re great when they work, but hard to get to work.
Finally I gave up and just popped in the other two PROMs, the ones with the “polling” version of the monitor program. I typed a few keys on the keyboard and I was shocked! The letters were displayed on the screen!
Can anyone shed some light on the root cause of the problem? Were the 6502's data sheets wrong?
[1] Wozniak, Steve. iWoz. W. W. Norton, 2006.