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MC
Account closed
Registered: Jul 2009
Posts: 71 |
Is this a proper stable raster?
I was searching for stable raster interrupts and only found ugly looking code tbh. I came up with this little trick and wonder if it will be stable under all conditions...
It is on VICE but I haven't had the chance to test this interrupt on a real machine yet... This is what I do in the IRQ:
raster1: lda #rast1
br1: cmp $d012
bne br1
nop
nop
nop
nop
nop
lda #$00
sta $d020
sta $d021
(rest of interrupt stuff goes here)
The code simply compares the value in $d012 with the value stored in there by the last IRQ call and waits until the raster makes the switch, then a built in delay of 5 x nop makes sure we are in between the left and right border flip. (4 x nop and 6x nop also seem to work so I chose the safe value of 5 nops).
It SEEMS to work properly but I forgot how I used to do stuff before my 20 year lunchbreak...
I managed to open up the border with my second attempt at an all dancing & singing piece of raster interrupts with music-assembler tunes playing. Next up is a bouncy sprite scroller in the lower (and side) border area.
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Mr. SID
Registered: Jan 2003
Posts: 424 |
Have a look at Fungus' double irq stable raster, it's pretty simple:
http://codebase64.org/doku.php?id=base:double_irq |
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MC
Account closed
Registered: Jul 2009
Posts: 71 |
Thanks. I dropped the idea of having a stable raster running as I'm now using two interrupts (experimenting with the NMI to handle sfx like vibrato, pulse modulation and really fast arps ) while the raster IRQ handles the music tracking.
Experiments experiments I do love em...
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Testa
Account closed
Registered: Oct 2004
Posts: 197 |
I always do this:
Init interrupt:
Disable timer interrupts
Enable raster interrupt
Interrupt vector $ffffe/$ffff
$01 = $35
$d012 = $2c
ack interrupt before clear the interrupt flag
Stable1:
inc $d012
inc $d019
cli
stable2:
nop
nop
nop
nop
nop
nop
nop
nop
jmp stable2
irq
ldx #$2c
cpx $d012
beq stable1
dec $d012
inc $d019
ldy #5
dey
bne *-1
nop
inx
cpx $d012
beq stable3
stable3
raster is stable here
don't know if it is sloppy code, but it seems to work...
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