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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: 952
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 15:48
iAN CooG

Registered: May 2002
Posts: 1920
" 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 15:51
MagerValp

Registered: Dec 2001
Posts: 917
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 18:01
Stone

Registered: Oct 2006
Posts: 124
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 07:56
ChristopherJam

Registered: Aug 2004
Posts: 788
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 10:16
MagerValp

Registered: Dec 2001
Posts: 917
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 10:20
Krill

Registered: Apr 2002
Posts: 952
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 10:22
Groepaz

Registered: Dec 2001
Posts: 8543
64er mag had some example for this.... it was slooooooooooow =)
2017-11-10 10:24
MagerValp

Registered: Dec 2001
Posts: 917
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 :)
2017-11-10 10:30
Oswald

Registered: Apr 2002
Posts: 4239
Quote: 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.

relying on things that are not under your control is usually a bad idea. (machine is in an after reset state). this is much more of a cargo cult, than using SEI/CLI.
2017-11-10 10:55
Bitbreaker

Registered: Oct 2002
Posts: 432
After all, why bother about those few wasted bytes, where's the highly optimized demo parts? :-D How can this behaviour finally be misused? By having the upcoming instruction of the SEI on a register being read or not?
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