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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

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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2017-11-12 09:27

Registered: Apr 2002
Posts: 902
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 09:46

Registered: Apr 2002
Posts: 902
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, thenů i dunno, the people running those don't deserve a demo treat. :)
2017-11-12 13:05

Registered: Dec 2001
Posts: 216
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...
2017-11-12 13:25

Registered: Dec 2001
Posts: 216
Hmm, upon further thinking, a basic raster IRQ setup could be done even shorter.

After basic start just switch to RAM
lda #$34
sta $01
<calculations done on irq-level=0>
and place the actual init routine @$ff48 in RAM:
lda #$7f
sta $dc0d
lda #<irq
sta $fffe
lda #>irq
sta $ffff

So the CIA1-irq is misused for the init routine, which itself sets up everything as needed (and will ofcourse only be called once). It however assumes that (at least on irq-level=0) the cpu sees only RAM, i.e. that I/O is switched off there (which is how I always do it, i.e. having $01=$34 in level 0 and $01=$35 for higher irq-levels).

Or are there other traps I fail to see atm?
2017-11-12 13:29

Registered: Dec 2001
Posts: 216
must ofcourse be
!!inc $01!!
lda #$7f
sta $dc0d
2017-11-12 14:44

Registered: Apr 2002
Posts: 902
Hmm, what does that setup from timer IRQ save? I don't see the shortening (and you'd also have to ack the IRQ yourself). :)
And please explain your notion of "irq-levels". How many are there? Are interrupt handlers themselves interrupted? Are there more banking settings than all-RAM ($30/$34) for main loop and with IO ($35) for interrupt handlers? Something going via $0314/15?
2017-11-12 14:47

Registered: Dec 2001
Posts: 8275
yeah, until you dont and start having bugs, which is always the case.

i know its useless to explain, but: mindlessly doing sei/cli and not thinking about when to do it and how to enable/disable the interrupts and their sources is what creates the bugs, not omitting it and knowing what to do. as a matter of fact, that kind of nonsense gave me a lot of headache in a certain demo i linked from parts made by a coder with the same mindset - and the whole "trick" to make it work flawless and bugfree was removing this sei/cli style interrupt setups and move them where they belong - and when they happen at the right time there is simply no need for sei/cli anymore (granted, it doesnt hurt either. however still doing it equals writing 0 to $d020 at random places, just to be sure the border is still black).
2017-11-12 14:58

Registered: Apr 2002
Posts: 902
Quoting Groepaz
writing 0 to $d020 at random places, just to be sure the border is still black
Which then produces grey dots all over the place on many VICs, another pet peeve of mine! :)
2017-11-12 15:08

Registered: Apr 2002
Posts: 4126
Groppie, except your bugs werent caused by the sei/cli pair, but the irq setup wasnt timed correctly. and the bugs could have been even worse if those irq inits were interrupted by the raster irqs.
2017-11-12 16:14

Registered: Dec 2001
Posts: 8275
again, let me write it in bold for you:

IF the IRQ setup had been timed correctly THEN SEI/CLI would not have been required. and Blindlessly using SEI/CLI made it "work" WITH THE BUGS.

and sure, without SEI/CLI the broken setup might have not worked at all. i totally prefer this over halfassed buggy glitchfest however.
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