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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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2017-11-08 14:26
dano

Registered: Jul 2004
Posts: 112
Quote: I think you're reading the trace wrong. The next instruction following an SEI can be fetched, but not executed if an IRQ occurs during the SEI. The instruction will be re-fetched and executed following the RTI.

And yes, protecting IRQ setup with SEI/CLI is still wrong, but not for this reason :)


Sorry for being offtopic there, but why is putting the IRQ setup into SEI/CLI wrong? Doesn't this ensure that no IRQ is happening while setting up the shizzle? I am happy to be enlighted on how to do this correctly.
2017-11-08 15:50
TWW

Registered: Jul 2009
Posts: 409
Quote: Sorry for being offtopic there, but why is putting the IRQ setup into SEI/CLI wrong? Doesn't this ensure that no IRQ is happening while setting up the shizzle? I am happy to be enlighted on how to do this correctly.

What he said
2017-11-08 16:01
ChristopherJam

Registered: Aug 2004
Posts: 682
Quoting MagerValp
I think you're reading the trace wrong. The next instruction following an SEI can be fetched, but not executed if an IRQ occurs during the SEI. The instruction will be re-fetched and executed following the RTI.


Ok, that makes a lot more sense.
2017-11-08 16:07
MagerValp

Registered: Dec 2001
Posts: 891
SEI doesn't prevent IRQs from happening, it just stops the CPU from handling them. Odds are good that you have a pending IRQ when you CLI, which means you have to have extra setup code to clear that. So for your generic $080d setup code, simply do
lda #$7f
sta $dc0d
before setting up a raster IRQ instead of using SEI.
2017-11-08 16:14
lft

Registered: Jul 2007
Posts: 314
Quoting MagerValp
I think you're reading the trace wrong. The next instruction following an SEI can be fetched, but not executed if an IRQ occurs during the SEI. The instruction will be re-fetched and executed following the RTI.


That makes more sense indeed. The status register will still be pushed with I set, but the interrupt handler is called between the SEI and the following instruction. Phew!
2017-11-08 19:09
Oswald

Registered: Apr 2002
Posts: 4124
Quote: SEI doesn't prevent IRQs from happening, it just stops the CPU from handling them. Odds are good that you have a pending IRQ when you CLI, which means you have to have extra setup code to clear that. So for your generic $080d setup code, simply do
lda #$7f
sta $dc0d
before setting up a raster IRQ instead of using SEI.


you also need lda $dc0d to clear the pending irq, lda #$7f.. just stops more to happen. afaik.

lda #$7f
sta $dc0d
sta $dd0d

lda $dc0d
lda $dd0d

to be on the safe side.
2017-11-08 20:47
lft

Registered: Jul 2007
Posts: 314
Yes, but the point is that if you do
   lda #$7f
   sta $dc0d

before turning off interrupts, then you don't have to clear the flag. If there is an interrupt pending, it will be invoked right now, once. Then it will be off.

On a side note, there is no need to disable CIA2 interrupts (NMI) at the beginning of a program that is launched from BASIC. They aren't active in that environment.
2017-11-08 20:48
Krill

Registered: Apr 2002
Posts: 902
Quoting MagerValp
I think you're reading the trace wrong. The next instruction following an SEI can be fetched, but not executed if an IRQ occurs during the SEI. The instruction will be re-fetched and executed following the RTI.
You're probably right, that makes a lot more sense. The text on visual6502.org is a bit ambiguous.

Oswald: I guess it's safe to assume that previously installed interrupt handlers would acknowledge their IRQs until you disable them. So don't exchange one cargo cult for another. :)

But anyhow, SEI/CLI is required when you install interrupt hooks on top of pre-existing handlers, especially if you have no knowledge of which interrupts are currently active and shall remain so.
2017-11-08 21:12
Oswald

Registered: Apr 2002
Posts: 4124
ah right, my eye didnt catch the instead :)thanks.
2017-11-08 21:17
Krill

Registered: Apr 2002
Posts: 902
Quoting lft
On a side note, there is no need to disable CIA2 interrupts (NMI) at the beginning of a program that is launched from BASIC.
You do, however, want to trigger one NMI that you never acknowledge, so the pesky restore key won't disrupt your meticulously hand-crafted timing. :)
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