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2012-01-05 11:35

Flavioweb

Registered: Nov 2011
Posts: 463

Raster sync question.

i'm writing a -reverse timer- raster sync routine.
In my routine the kernal is switched off and i use $fffe/f as irq vector.
Also all cia irq are switched off.
The main loop is an endless self jmp to avoid opcode cycles problem.
All routines work 'in irq' so i can omit the registers saving part. Every routine start just after the irq trigger.
The cia timer setup is in the initial sei/cli routine but the timer is started using a first raster irq call where the first 2 instructions are lda/sta $d019.
Timer is set up to cycle counter with $07 as value.
The stable irq part is stabilized with $dc04 eor #$07 and the result used to branch into a lda #$a9 part as in most stable irq routines.
To test i use an inc $d021 just to see where/if stable code work.
And it work... But not always at same point!
Sometimes the inc start under border and some others start after it!
The position change if routine is restarted. When run, the position never change.
How can i fix this making inc been executed always at same position?

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2012-01-08 19:59

ChristopherJam

Registered: Aug 2004
Posts: 1409

@Flavioweb, this doesn't directly answer your question, but you should be able to adapt it to your needs; I've just done a post on Stablising raster IRQ with DMA

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