I have written an emulation of the Heathkit ET-3400A microcomputer trainer in Java, mainly to see if I could do it and to get my feet wet in Java. Odd question, but there's a discrepancy (one of many, I discovered) in the 6800 data sheet and the programmer's reference. The datasheet says the I flag (interrupt mask) is set at reset. The programmer's reference seems to indicate otherwise. Anybody know what it really is? Sold my ET-3400A a while back, still kicking myself for that.

If anybody's interested, I have this up on Github and it works in Windows, MacOS, and Linux.

  • 1
    Asked here as well. stackoverflow.com/questions/63872982/…
    – sparkydave
    Sep 13, 2020 at 17:31
  • This was answered in SO.
    – dave
    Sep 13, 2020 at 17:37
  • 3
    in that case please don't cross post questions. Sep 13, 2020 at 19:02
  • @Jean-FrançoisFabre I didn't know about this group, and it was suggested I post it here too. So happens I got the answer there first, but since it might benefit someone here, I posted the answer I got.
    – sparkydave
    Sep 13, 2020 at 19:10
  • Don't forget to give a green tick to the answer that helped you the most Sep 14, 2020 at 5:50

2 Answers 2


The datasheet says the I flag (interrupt mask) is set at reset

Do you mean that the I flag is set when a reset interrupt occurs? If not, please clarify what you mean. But if that's what you mean, that is correct. When any interrupt is serviced, the I flag (interrupt mask) is set, so that no additional interrupt requests will be serviced until the current interrupt has been serviced.

The programmer's reference seems to indicate otherwise.

Quoting the reference would be helpful here. I do not see any indication in the programmer's reference that the interrupt mask is cleared/reset when an interrupt request occurs.


Answered on StackOverflow. Misunderstanding of the reference manual, the I flag is set at reset.

  • 1
    Could you provide a link (for attribution) and quote (for a useful answer) please? This is currently a link-only answer without the link. (Thanks for self-answering, though.)
    – wizzwizz4
    Sep 14, 2020 at 6:21
  • @DrSheldon Actually, since the question was whether the I flag was set or cleared at reset, my own answer is fairly self-explanatory.
    – sparkydave
    Sep 15, 2020 at 0:12

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