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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.

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    If I understand the story correctly, he tried to get the interrupt version of the monitor to work and failed, using a fallback version in the end. Without a listing of the erroneous program it would be pure speculation to say what was the problem. I found a listing of wozmon at this site sbprojects.net/projects/apple1/wozmon.php which has no interrupt service routines. – Peter B. Nov 7 at 0:29
  • Thanks for reminding me of iWoz - i bought a copy a few years ago, never got around to reading it, and forgot all about it until now! – LAK Nov 7 at 1:01
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    Above statement reads as if the issue wasn't the interrupt, but his attempt to write/change a PROM with the interrupt version. The source code comments do show hints of interrupt usage. Likewise do the Apple 1 schematics. As @PeterB. says, it's all speculation as not even the issue is described in consistent form. – Raffzahn Nov 7 at 6:44
  • SO has the close reason, "This question was caused by a problem that can no longer be reproduced or a simple typographical error." which I think fits here? – Wilson Nov 7 at 16:24
  • @Wilson If you can prove that the interrupt-based monitor program no longer exists, I think it would be better as an answer than as a close reason. – snips-n-snails Nov 7 at 16:29

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