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Krill
Registered: Apr 2002
Posts: 2980 |
TIL: The instruction after SEI can be executed before a pending IRQ is handled
As described here: http://visual6502.org/wiki/index.php?title=6502_Timing_of_Inter..
I never knew this, after all those years, and thought i'd share this as a heads-up.
Thanks to Bubis for pointing it out to me! |
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Oswald
Registered: Apr 2002
Posts: 5094 |
"Either way all you are doing is adding extra bytes to your init code with zero improvements to reliability."
An IRQ may destroy A while you try to lda #$7f sta $dc0d. another irq from the other CIA may re-enable $dc0d. You may have interrupt sources from VICII too, fucking you up in all sorts of ways. (rewriting fffe/f after you've set them up for example).
"if there's a source you don't know about to disable, you're going to have bugs regardless of whether you SEI/CLI or not, as it's going to hit while your code isn't expecting it."
you may just forget about your super size effective irq setup and it will backfire when you least expect it. |
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Krill
Registered: Apr 2002
Posts: 2980 |
Now you're grasping at straws. |
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Oswald
Registered: Apr 2002
Posts: 5094 |
Quote: Now you're grasping at straws.
Now you're trolling. |
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Oswald
Registered: Apr 2002
Posts: 5094 |
anyhow I have real machine experience when I've been setting up raster irq without acking timer interrupt for ages, and then at a case it refused working, and I wondered how could I forget to set it up correctly. Since then I religiously ack it. |
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chatGPZ
Registered: Dec 2001
Posts: 11386 |
Quote:An IRQ may destroy A while you try to lda #$7f sta $dc0d
in that case you have much bigger problems than anything discussed here |
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Krill
Registered: Apr 2002
Posts: 2980 |
And we are discussing interrupt setup when run from a plain BASIC environment. So disabling CIA1 interrupts is enough, no SEI/CLI required. Period. :) |
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chatGPZ
Registered: Dec 2001
Posts: 11386 |
even in most other scenarios... eg in a trackmo you pretty much want to always do it at a very defined point to avoid glitches - and then you exactly know what happens anyway, and dont need sei/cli at all. you DO need them only when you do brute force AEG style linking =) |
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Frantic
Registered: Mar 2003
Posts: 1648 |
Okay. I'll always try to make sure to remember to use SEI/CLI then. |
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Frantic
Registered: Mar 2003
Posts: 1648 |
Ha ha |
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Oswald
Registered: Apr 2002
Posts: 5094 |
"and then you exactly know what happens anyway"
yeah, until you dont and start having bugs, which is always the case.
Suprised by Gunnar's hatred towards doing it the >right< way, having seen his overly pedantic sources. As a rule of thumb you dont want to rely on things being in a certain state when programming. |
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