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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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2017-11-09 16:27
Krill

Registered: Apr 2002
Posts: 902
Quoting MagerValp
but if something else ran before and exited to the basic prompt there may be an unacknowledged raster IRQ pending.
Hmm, if whatever ran before left unacknowledged interrupts behind, wouldn't they trigger time and again without ever being acknowledged, so the BASIC prompt would never be reached and IRQs be handled in an endless loop? :)
2017-11-09 16:44
MagerValp

Registered: Dec 2001
Posts: 891
No, they're still masked by a zero value in $d01a. Hmm, I wonder if the soft reset before the ready prompt clears it... Just checked, and afaict, nope.
2017-11-09 16:48
iAN CooG

Registered: May 2002
Posts: 1751
" if run from basic "
and what about if it's not? Why even assuming it? It's not better to be safe and put ALSO sei/cli, and clearing $D019 too in any case?
2017-11-09 16:51
MagerValp

Registered: Dec 2001
Posts: 891
Clearing $d019 is necessary, and testing a bit more it seems like it's necessary even on a freshly reset machine. SEI/CLI is still useless though, and only creates the problem of having to clear $dc0d.
2017-11-09 19:01
Stone

Registered: Oct 2006
Posts: 118
Quoting iAN CooG
" if run from basic "
and what about if it's not?
Then you either know about the calling context and set things up accordingly, OR you know nothing, in which case initial setup is the responsibility of the caller.
2017-11-10 08:56
ChristopherJam

Registered: Aug 2004
Posts: 682
Quoting MagerValp
For reference, here's a bog standard raster IRQ setup that'll work every time if run from basic.


…and if you also zero $d01a at the start of the routine, it should then behave itself if your predecessor left a raster interrupt installed :)
2017-11-10 11:16
MagerValp

Registered: Dec 2001
Posts: 891
Quote: Quoting MagerValp
For reference, here's a bog standard raster IRQ setup that'll work every time if run from basic.


…and if you also zero $d01a at the start of the routine, it should then behave itself if your predecessor left a raster interrupt installed :)


You wouldn't even be able to load stuff from disk with a raster IRQ running, so I don't think it's necessary for account for that.
2017-11-10 11:20
Krill

Registered: Apr 2002
Posts: 902
You can load fine from disk using KERNAL routines, with a raster IRQ running. You just need to do it right. And it loads even slower, of course. :)
2017-11-10 11:22
Groepaz

Registered: Dec 2001
Posts: 8275
64er mag had some example for this.... it was slooooooooooow =)
2017-11-10 11:24
MagerValp

Registered: Dec 2001
Posts: 891
It's also possible to press restore while running, and to use SD2IEC. If you set yourself up for failure, don't be surprised when you fail :)
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