Raster Interrupt

From: Laze Ristoski (lazeristoski_at_veenstaete.nl)
Date: 2004-01-07 16:19:13


Someone posted a thread at CSDb, and he says
that he noticed 8 cycle variance when an
IRQ is generated. To be more precise, as
the current instruction must be finished
before jumping to the IRQ, there will be
cycle jitter. When using _legal_ opcodes,
the jitter should move in range from 0-6
(7 cycle variance). But in fact, there's
8 cycle variance (no badlines or sprites!)
This feature also happens in CCS and VICE,
so there must be an explanation. Try setting
an IRQ at rasterline $ff (or at any place
where there's no badline). In the main loop
use the following code:

inc $c000,x ;(or another address, but there should be no page-crossing)
bpl loop
bmi loop

Can anyone explain where the 8th cycle comes from?

There's also another thing: when a raster IRQ occurs,
the minimum cycle at which the first instruction executes
is 12 (counting the cycles from 1-63). The IRQ condition is
given in cycle 1, right? There's 7 cycle delay between the
IRQ occurrence and the cycle at which the first instruction
of the IRQ is processed. If we use NOPs in the main loop,
there will be minimum of 2 cycles delay + 7 cycles for the
IRQ. So cycle 1 + 2 + 7 = 10. Why does it appear to be 12?

Maybe I've done some mistake, but I'd appreciate if someone
can explain what exactly happens when a raster interrupt

Thank you!


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