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Forums > C64 Coding > VIC trick hickup when wasting cycles vs. raster timing
2018-09-13 13:49
Lord Crucifier

Registered: Feb 2004
Posts: 28
VIC trick hickup when wasting cycles vs. raster timing

Hey folks, I'm running across something my brain can't figure out. Here's a reduced test case in KickAssembler:




		*= $1000

		lda #$00
		sta $3fff // Clear border garbage

		lda #$35
		sta $01
		lda #<irq
		sta $fffe
		lda #>irq
		sta $ffff

		lda #$01
		sta $d019
		sta $d01a
		lda #$7f
		sta $dc0d

		lda #$00
		sta $d012

		jmp *


		lda $d019
		and #$01
		sta $d019
		bne cycle
		jmp $ea81

		lda $d011
		ora #$08
		sta $d011

		.var loop=true

		// Simple loop, waiting until raster line $f9
		// This works!
	.if(loop) {
			ldx #$f9
			cpx $d012
			bne loop
		// Waste cycles until raster line $f9
		// Doesn't work!
	.if(!loop) {
		.var x=0
		.for (x=0;x<$f9;x++) {
			// Waste 63 cycles
		        .var y=0
		        .for (y=0;y<30;y++) {
		        bit $fe
		// Verify we're on line $f9
		lda $d012
		sta $0400

		inc $d020

		// Open bottom border, turn off screen
		lda $d011
		and #%11100111
		sta $d011

		// Give $d020 jiggle some time to show
		ldy #$10
		bne *-1

		dec $d020

		jmp $ea81


I want to reach raster line $f9, open the top/bottom borders and turn off the screen. Result: only sideborders are shown. Simple stuff. This works fine in a loop checking if $d012 has arrived at $f9.

But when I try to get to raster line $f9 by dumbly wasting cycles, it doesn't work: the screen is turned off, but the borders aren't opened.

The code above has both methods, change the .var loop=true to .var loop=false to waste cycles instead of waiting on $d012.

I check that I actually did arrive on $f9, and it is the case (when I read out $0400 where I dump the value of $d012, it's in both cases $f9). I also played with some more/less delays in case the timing depended on it, although opening top/bottom borders can require sloppy timing.

Anybody know why the delay version doesn't work?
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2018-09-13 14:45

Registered: Mar 2007
Posts: 122
Oh Christ, wait, looking further, there’s much more wrong with this. You can’t just loop over 63 * 249 cycles... never going to work. That’s ignoring bad lines, sprites, etc. Any good reason why you want to waste all your cycles there.? I assume you plan to fill that with code that does something later on..?
2018-09-13 16:16

Registered: Feb 2003
Posts: 307
Quote: Oh Christ, wait, looking further, there’s much more wrong with this. You can’t just loop over 63 * 249 cycles... never going to work. That’s ignoring bad lines, sprites, etc. Any good reason why you want to waste all your cycles there.? I assume you plan to fill that with code that does something later on..?

if screen is set off 63*249 cycles *could* work. but after "irq:" I don't think rasterline $00 has 63 cycles, since you have some instructions there. try with (x=0;x<$f8;x++) + some NOPs (but only after you have fixed what Bitbreaker said).
2018-09-13 17:10

Registered: Aug 2003
Posts: 119
and why not use a raster interrupt instead of wasting those precious cycles.
2018-09-13 18:13

Registered: Apr 2008
Posts: 280
The screen must be on during the frame when you do the first $d011 narrowing. If screen is already off, it won't show the screen. Often this is the previous frame depends on how you time the code.
2018-09-13 18:28

Registered: Feb 2003
Posts: 307
I guess Lord Crucifier is trying to have a situation with screen off and sprites on, as explained in this thread: Screen with no Badlines (just guessing, maybe his goal is completely different).
but with that trick you can't have a full screen fx, because of the raster interrupt at $f9/$fa. so he's trying to reach that rasterline by nopping till there. good guess? :) if it works I'm interested in the solution too!
2018-09-13 18:34

Registered: Jul 2009
Posts: 444
Right, first off, push your accumulator at least and secondly clear and latent IRQs from CIA. Thirdly switching off KERNAL is a bad idea if you plan to use KERNAL calls to handle the IRQ

Last, use a CIA timer (tricky) or Raster Compare (easy) to do this.
2018-09-13 19:56
Rastah Bar

Registered: Oct 2012
Posts: 160
Quoting Lord Crucifier
Yeah, like I said, I played with more/less delay a lot. Even on two-cycle levels, even though this kind of strict timing isn't needed here. But the cycle wasting method will never work.

You can open the border also on line $fa, some my first guess was BS. Sorry about that.
2018-09-13 19:56
Lord Crucifier

Registered: Feb 2004
Posts: 28
Haha, apologies to everyone who got their knickers in a knot from that code. It was just me being SUPERsüperSUBER lame, like joystick-up-my-ass lame, and thanks for all the suggestions for improvement.

Now, on to the problem at hand. The wasted cycles method didn't work because... the screen was already closed when I got to that point. All the 24/25 line toggling in the WORLD can't re-open them borders, right? Puh. It's so simple once you figured it out. Thanks ZeSmasher and Mixer for making that penny drop.

By the way, if you think THIS was lame, wait till you see what I'm bringing to X2018. Get ready to vomit your eyes out, eeeek!
2018-09-13 21:10

Registered: Dec 2001
Posts: 8900
for those who stumble about the thread looking for a solution - perhaps post the working snippet? (or... put it on codebase? :))
2018-09-14 00:30

Registered: Jan 2018
Posts: 97
I think the best way for you to achieve this effect is to do sometging like this in your irq handler
lda #8
sta $d011

;your main code here

lda #249
cmp $d012
bne -
lda #0
sta $d011

this will work no matter how much code you have between these snipets. If it doesn't work then you've got so much code that it goes beyond 249th line. This is the second irq where you have the screen already off. The first irq that your program does should trigger at line 249 and set d011 to 0. Then change irq vector to the second irq handler posted above.
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