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Forums > C64 Coding > TIL: The instruction after SEI can be executed before a pending IRQ is handled

2017-11-07 15:56

Krill

Registered: Apr 2002
Posts: 2850

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 12:26

Oswald

Registered: Apr 2002
Posts: 5022

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 13:23

chatGPZ

Registered: Dec 2001
Posts: 11129

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 16:23

Krill

Registered: Apr 2002
Posts: 2850

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 16:29

chatGPZ

Registered: Dec 2001
Posts: 11129

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 =)

2017-11-11 21:05

Frantic

Registered: Mar 2003
Posts: 1627

Okay. I'll always try to make sure to remember to use SEI/CLI then.

2017-11-11 21:06

Frantic

Registered: Mar 2003
Posts: 1627

Ha ha

2017-11-12 03:49

Oswald

Registered: Apr 2002
Posts: 5022

"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.

2017-11-12 08:27

Krill

Registered: Apr 2002
Posts: 2850

Quoting Oswald

hatred towards doing it the >right< way, having seen his overly pedantic sources.

"Hatred" is too strong a word in this context, and the flip side of pedantry is that unnecessary bytes also cause that certain itch. :)
And as for "right", again, either you come from BASIC, in which case there only is CIA1 interrupt to worry about, or you don't, in which case all bets are off and you're doing whatever specific stuff needs doing, with or without SEI/CLI.

2017-11-12 08:46

Krill

Registered: Apr 2002
Posts: 2850

Oh, and when assuming that some people run demos from a BASIC environment with all kinds of interrupts enabled, you can still disable all of them without SEI/CLI.
Unless its a truly hostile environment with interrupt handlers destroying register contents and setting up other interrupts, etc., as you describe, then i dunno, the people running those don't deserve a demo treat. :)

2017-11-12 12:05

Copyfault

Registered: Dec 2001
Posts: 466

I think Oswald is basically right here as he's just applying Murphy's Law to the coding of IRQs and their setups.

Then again, once you start optimizing (which is what C64 demo coding is all, no?) it's absolutely right to track down every unnecessary byte or cycle - even if it "feels odd" at first. Have also been using SEI/CLI ever since but I'm really happy and thankful to be enlightened by this discussion.

Just my 2c...