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Krill
Registered: Apr 2002
Posts: 2839 |
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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Stone
Registered: Oct 2006
Posts: 168 |
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. |
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ChristopherJam
Registered: Aug 2004
Posts: 1378 |
Quoting MagerValpFor 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 :) |
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MagerValp
Registered: Dec 2001
Posts: 1055 |
Quote: Quoting MagerValpFor 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. |
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Krill
Registered: Apr 2002
Posts: 2839 |
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. :) |
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chatGPZ
Registered: Dec 2001
Posts: 11108 |
64er mag had some example for this.... it was slooooooooooow =) |
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MagerValp
Registered: Dec 2001
Posts: 1055 |
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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Oswald
Registered: Apr 2002
Posts: 5017 |
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. |
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Bitbreaker
Registered: Oct 2002
Posts: 500 |
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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Krill
Registered: Apr 2002
Posts: 2839 |
Since the succeeding opcode is merely fetched, and not executed, then the pipeline flushed and the interrupt handled, then the opcode re-fetched and finally executed...
I can only imagine some arcane anti-cracking/debugging/reverse-engineering setup there, nothing demo-worthy. |
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TWW
Registered: Jul 2009
Posts: 541 |
Morale of the story is:
SEI/CLI your init. take care of unwanted IRG/NMI events from smegging up your shit. Let the byte optimizers optimize^^ |
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