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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: 2804
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 19:48
Krill

Registered: Apr 2002
Posts: 2804
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 20:12
Oswald

Registered: Apr 2002
Posts: 5007
ah right, my eye didnt catch the instead :)thanks.
2017-11-08 20:17
Krill

Registered: Apr 2002
Posts: 2804
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. :)
2017-11-08 20:38
Oswald

Registered: Apr 2002
Posts: 5007
I dont care about restore, if the user wants to crash the demo let it be. its his own fault :)
2017-11-08 20:46
lft

Registered: Jul 2007
Posts: 369
The enemy (nmi?) cannot push RESTORE if you disable his hand.
2017-11-08 21:05
chatGPZ

Registered: Dec 2001
Posts: 11088
Quote:
I dont care about restore

i've switched to just pointing the NMI vectors to some silly routine that punishes the bastard who pressed RESTORE accordingly :)
2017-11-09 14:43
TWW

Registered: Jul 2009
Posts: 541
Ground the fucker ^^
2017-11-09 15:18
MagerValp

Registered: Dec 2001
Posts: 1055
For reference, here's a bog standard raster IRQ setup that'll work every time if run from basic:
	lda #$7f
	sta $dc0d

	lda #$35
	sta $01

	lda #$1b
	sta $d011
	lda #$fa
	sta $d012

	lda #<irq
	sta $fffe
	lda #>irq
	sta $ffff

	lda #1
	sta $d019
	sta $d01a
NB: Clearing pending IRQs with $d019 is not strictly necessary if you run from a freshly reset machine (the kernal writes $0f to $d019 on reset), but if something else ran before and exited to the basic prompt there may be an unacknowledged raster IRQ pending.
2017-11-09 15:27
Krill

Registered: Apr 2002
Posts: 2804
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 15:44
MagerValp

Registered: Dec 2001
Posts: 1055
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.
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