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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: 898
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 10:48
ChristopherJam

Registered: Aug 2004
Posts: 682
Oswald, 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.

And if you do know about it, you can disable it, in which case an SEI before you disable it will create extra work for you, or an SEI after you disable it will do nothing.

Either way all you are doing is adding extra bytes to your init code with zero improvements to reliability.

It may even cause your raster interrupt to have it's first occurrence on the wrong line altogether, which can occasionally produce a frame of garbage at the start of your routine.
2017-11-11 11:28
Groepaz

Registered: Dec 2001
Posts: 8271
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: 898
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: 4124
"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: 898
Now you're grasping at straws.
2017-11-11 13:06
Oswald

Registered: Apr 2002
Posts: 4124
Quote: Now you're grasping at straws.

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

Registered: Apr 2002
Posts: 4124
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: 8271
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: 898
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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