I'm having some issues getting VBlank and HBlank interrupts to fire on a simple 68K program I have wrote specifically for the SEGA Megadrive.

It runs without exception, I can and have debugged through it many times and each time it follows the path I expect and ends up in the infinite loop I expect. But for reasons I cannot yet fathom it never fires the VBlank or HBlank interrupts.

I've not quite exhausted all avenues of solving the problem myself yet but am preparing for the fact that I may not be able to get to the bottom of it without assistance. Even if I do it might be a good self-answered question.

What would be the recommended way I could pose a 68K programming question (specific to the SEGA Megadrive) that will be well received? (or as likely to be as is ever possible before submitting?)

My main fear is that my question might end up quite code heavy (even sticking to the relative parts only, it'll be fairly short but could well contain an obvious error so needs included) along with the problem description then as soon as people see it they will be triggered and hit back with 'we are not a debugging service' response. I'm aiming to avoid that by making it clear that I'm not looking for debugging help, I'm looking to find out what initialization I'm not doing or am doing incorrectly.

The type of answer I would expect would be along the lines of:

Although you are fooing the bar, the Megadrive also needs you to bar the foo in order for the 68K to acknowledge and process the interrupts.

Related: Are x86 programming questions on-topic?

Related: Should we have retro-coding questions on Retrocomputing?

  • 2
    The question is never going to be raised. I got to the bottom of it and it turned out I'd reversed the logic for turning the interrupts on and off, so I was turning them off when I thought I was turning them on. A very mundane error and not one that would be useful having on the site. Jul 24, 2019 at 22:14
  • Thanks for telling us what the reason was - I'd have gone nuts if you'd never have asked or never said how you fixed it!
    – knol
    Jan 11, 2021 at 22:34
  • 1
    @knol - I never want to cause anyone the pain of xkcd.com/979 :) Jan 12, 2021 at 9:47

1 Answer 1


As per Should we have retro-coding questions on Retrocomputing? and Are x86 programming questions on-topic?, this question would be fine. It requires specific domain knowledge to answer.

  • Thanks for the confirmation wizzwizz4, I figured it was on-topic but that isn't always enough to make it well received. Any suggestions on common pitfalls to avoid? Jul 23, 2019 at 12:51
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    @RyanfaeScotland Focus on the actual retro part, and make sure it's clear that you aren't asking for help with the code, but it's just there so people can see what you're doing wrong. Try to produce an MCVE,
    – wizzwizz4 Mod
    Jul 23, 2019 at 13:05
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    The Megadrive is 68k - So, I'm sure even if asking for code that's controlling retro hardware (especially this example) is still on-topic
    – tofro
    Sep 12, 2019 at 13:59
  • @tofro If it's actually about controlling the hardware, sure. If it's asking about, say, why the modern assembler isn't picking up on the data section of the assembly file (even if that file's dealing with controlling the hardware) it wouldn't be on-topic.
    – wizzwizz4 Mod
    Sep 12, 2019 at 17:02

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