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2020-01-20 16:20

Krill

Registered: Apr 2002
Posts: 2839

Shortest code for stable raster timer setup

While working on my ICC 2019 4K entry (now postponed to ICC 2020, but i hope it'll be worth the wait), i came up with this (14 bytes):

initstabilise   lda $d012
                ldx #10          ; 2
-               dex              ;   (10 * 5) + 4
                bpl -            ; 54
                nop              ; 2
                eor $d012 - $ff,x; 5 = 63
                bne initstabilise; 7 = 70

                [...]; timer setup

The idea is to loop until the same current raster line is read at the very beginning (first cycle) and at the very end (last cycle) of a raster line, implying 0 cycles jitter.

With 63 cycles per line on PAL, the delay between the reads must be 63 cycles (and not 62), reading $d012 at cycle 0 and cycle 63 of a video frame's last line (311), which is one cycle longer due to the vertical retrace.

The downside is that effectively only one line per video frame is attempted, so the loop may take a few frames to terminate, and the worst case is somewhere just beyond 1 second.

The upside is that it always comes out at the same X raster position AND raster line (0), plus it leaves with accu = 0 and X = $ff, which can be economically re-used for further init code.

Now, is there an even shorter approach, or at least a same-size solution without the possibly-long wait drawback?

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2020-12-02 18:18

Rastah Bar

Registered: Oct 2012
Posts: 336

Quoting Copyfault

ldx #initval ldy #<(zp_pos+1) loop: shx $ffff,y lda <($100 + zp_pos - initval),x beq loop

Nice! LDA <($100 + zp_pos - initval),X can simply be replaced with LDA zp_pos, I think.

2020-12-02 18:26

Copyfault

Registered: Dec 2001
Posts: 466

Quote: Quoting Copyfault

ldx #initval ldy #<(zp_pos+1) loop: shx $ffff,y lda <($100 + zp_pos - initval),x beq loop

Nice! LDA <($100 + zp_pos - initval),X can simply be replaced with LDA zp_pos, I think.

No, the loop'd take only 11 cycles which does not suffice, see also post #61 by Quiss. This was the reason to use the x-indexed version.

2020-12-02 18:26

Rastah Bar

Registered: Oct 2012
Posts: 336

Quote: Quoting Rastah Bar
The STA $ZP instruction (see post #44) can be made part of the init code, which reduces the timer-based stabilization approach to effectively 10 bytes:

ldy #init_value ;Init code sync: lax $dc04 sbx #51 sty ZP ;RRW instruction. Part of init code. cpx $dc04 bne sync:

STY ABS is also allowed, in combination with SBX #52.

If I'm not mistaken, this should work on PAL, NTSC, and DREAN, but the loop exit cycle may depend on the system.
But we both agree that this is not the shortest, but rather one of the shortest approaches that work without mem loc constraints, don't we?;) The code bracket with INCs is 10 bytes long, the other one with LDAs sums up to 10 + 2(for an LDY #val that effectively is also part of the init code) in total...

Now concerning the different VIC systems in your timer based approach, I wonder wether the operand of the sbx must be adjusted according to the no. of cycles per line or if this works with subtrahend 51 on all systems for some magic reason...

CJam's approach also takes 10 bytes, but maybe it requires blanking the screen? I'm not sure, but if it does, it is an extra constraint, since you do not always want that. For example, in a demo you can have some effect working on the startup screen, and then you don't want to blank it.

VIC always steals the same amount of cycles from the CPU on a badline. This is system independent.

2020-12-02 18:30

Rastah Bar

Registered: Oct 2012
Posts: 336

Quote: No, the loop'd take only 11 cycles which does not suffice, see also post #61 by Quiss. This was the reason to use the x-indexed version.

Oh yes, I forgot about that. Hard to keep track of all the subtleties :-)

2020-12-02 18:44

Copyfault

Registered: Dec 2001
Posts: 466

Quoting Rastah Bar

CJam's approach also takes 10 bytes, but maybe it requires blanking the screen? I'm not sure, but if it does, it is an extra constraint, since you do not always want that. For example, in a demo you can have some effect working on the startup screen, and then you don't want to blank it.

Ok, that's true. However screen blanking could be done inside the code bracket, but it is a constraint as it's not possible to display anything during this init.

Quoting Rastah Bar

VIC always steals the same amount of cycles from the CPU on a badline. This is system independent.

YES ofcourse!!! Thanks for the oh so pbvious yet clever point! So the drawback that remains is the demand of a running timer. With my last proposal (based on Quiss' SHX-idea), we'd have no constraints at all, but it needs 11 bytes in its current state. So definately not the shortest one, but at least something with SHX :)

2020-12-02 18:49

Rastah Bar

Registered: Oct 2012
Posts: 336

The more different approaches, the merrier :-)

It could very well be that screen blanking is not required for CJam's approach, but it needs to be analyzed. With likely multiple W instructions in the init code, that seems very difficult. And I'm too tired right now to think about it very hard. Not that I can figure it out, anyway :-)

2020-12-03 12:48

ChristopherJam

Registered: Aug 2004
Posts: 1378

Woah, this thread has been busy, nice work all :)

Um, no need to blank the screen for my bracketing method. There's never character DMA on line $0ff, which is the only one for which there are 63 cycles for which an INC $d012 will read $ff and write $00.

(assuming of course that I'm remembering correctly rumours that line $00 is only 62 cycles long - can anyone point me at documentation to confirm that? There's nothing in the venerable VIC Article [english], and I'm not spotting it anywhere on CodeBase either)

2020-12-03 13:05

Frantic

Registered: Mar 2003
Posts: 1627

cjam: See post 16 in this thread.

2020-12-03 14:21

ChristopherJam

Registered: Aug 2004
Posts: 1378

Arrgh, I'm going blind. Thank you Frantic.

2020-12-03 14:24

Rastah Bar

Registered: Oct 2012
Posts: 336

Quoting ChristopherJam

Woah, this thread has been busy, nice work all :)

Um, no need to blank the screen for my bracketing method. There's never character DMA on line $0ff, which is the only one for which there are 63 cycles for which an INC $d012 will read $ff and write $00.

(assuming of course that I'm remembering correctly rumours that line $00 is only 62 cycles long - can anyone point me at documentation to confirm that? There's nothing in the venerable VIC Article [english], and I'm not spotting it anywhere on CodeBase either)

I was just wondering of some W cycles within the init code might cause some kind of alignment on badlines, such that the first INC $d012 always appears on line $ff on a non-syncing cycle?

Btw, since the LDX #initval in Copyfault's method of post #90 is part of the init code, that one is only 9 bytes and hence the shortest unconstrained method.

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