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Hypnosis
Registered: Mar 2015
Posts: 36 |
Cycles to get into IRQ routine
How many cycles does it take from an IRQ trigger to the start of the IRQ routine, assuming the CPU finished the instruction when it triggers? It should be around 6-9 cycles but I'm not sure how many. |
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Flavioweb
Registered: Nov 2011
Posts: 447 |
Seven cycles are used to "setup things" and then go to irq first opcode.
Your code seems that trigger the irq on the second cycle of first nop, so the next opcode will be executed (next nop in this case).
So: irq triggered on second nop cycle, plus 2 cycles of next nop, plus seven cycles of irq "setup code": the first cycle of your irq code is executed 10 cycles after irq is triggered...
Edit: the irq is triggered on second nop cycle, probably because you are using an old cia, where irq is triggered 1 cycle "after". Maybe if you use a new cia, only first nop will be executed... |
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Hypnosis
Registered: Mar 2015
Posts: 36 |
Thanks for the input. I'll try the other CIA. This could be it. |
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Hypnosis
Registered: Mar 2015
Posts: 36 |
Yep. I get the expected behavior using the new CIA. |
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Oswald
Registered: Apr 2002
Posts: 5017 |
"Seven cycles are used to "setup things" "
from what I was told, irq is a hacked JSR inside the cpu, probably the +1 cycle is the status reg to stack. |
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Copyfault
Registered: Dec 2001
Posts: 466 |
To work around that 1-cycle CIA-irq-trigger-latency between the different CIA revision I always use the combination of raster irq + CIA timer for dejittering.
But I know there are situations in which the double timer method is a must... doesn't Codebase have some example code for CIA type detection including the corresponding code adjustments ready to use? |
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chatGPZ
Registered: Dec 2001
Posts: 11114 |
if not, there are plenty such routines in the VICE testrepo :) |
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Hypnosis
Registered: Mar 2015
Posts: 36 |
I solved it by treating it as another jitter cycle. |
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JackAsser
Registered: Jun 2002
Posts: 1989 |
Quote: I solved it by treating it as another jitter cycle.
My standard approach as well, unless you really really need that one extra cycle in your super optimised raster code. |
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Bitbreaker
Registered: Oct 2002
Posts: 500 |
And how can a demo effect profit from all this knowledge? :-D |
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lft
Registered: Jul 2007
Posts: 369 |
Typical application: 8-bit sample playback. NMI triggers a routine which must be cycle-exact, and it happens 156 times per frame so every saved cycle matters.
When using distributed jitter correction (different code variants in the 0100-08ff range), you have to take the CIA difference into account. However, you don't necessarily have to detect the CIA version, it can be enough to initialise the timer from within an NMI handler. |
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