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Forums > C64 Coding > TIL: The instruction after SEI can be executed before a pending IRQ is handled
2017-11-07 16:56
Krill

Registered: Apr 2002
Posts: 902
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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2017-11-11 11:28
Groepaz

Registered: Dec 2001
Posts: 8275
Quote:
Yes, you'd have to be running code stored in read-sensitive IO registers, at the precise cycle an IRQ is expected. I find it hard to imagine any kind of scenario where that would save you any cycles or memory.

it could be a fun riddle for obfuscation though :)
2017-11-11 11:40
ChristopherJam

Registered: Aug 2004
Posts: 682
(actually scratch that "it may even" para from the end of my last comment, I didn't quite think that one through - if a CIA interrupt goes off during setup unacknowledged, the consequences are going be considerably worse than a single frame...)
2017-11-11 11:45
Krill

Registered: Apr 2002
Posts: 902
Quoting Oswald
this is a religious argument
This may be an academic discussion (unless you need the size, 4K and all), but it's certainly nothing to do with religion. Well, unless you are a cargo cultist. :)
2017-11-11 11:56
Oswald

Registered: Apr 2002
Posts: 4126
"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.
2017-11-11 12:34
Krill

Registered: Apr 2002
Posts: 902
Now you're grasping at straws.
2017-11-11 13:06
Oswald

Registered: Apr 2002
Posts: 4126
Quote: Now you're grasping at straws.

Now you're trolling.
2017-11-11 13:26
Oswald

Registered: Apr 2002
Posts: 4126
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.
2017-11-11 14:23
Groepaz

Registered: Dec 2001
Posts: 8275
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
2017-11-11 17:23
Krill

Registered: Apr 2002
Posts: 902
And we are discussing interrupt setup when run from a plain BASIC environment. So disabling CIA1 interrupts is enough, no SEI/CLI required. Period. :)
2017-11-11 17:29
Groepaz

Registered: Dec 2001
Posts: 8275
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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