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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 setupThe 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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Oswald
Registered: Apr 2002
Posts: 5017 |
"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."
basicly its not 0 jitter, just then you know how the raster and the cpu is synchronized :P cool idea! dont think anyone ever coded this for size, guess you could make it run faster if you would do this on the border on consecutive lines. bne would jump out from a loop. |
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Krill
Registered: Apr 2002
Posts: 2839 |
Frantic: True that. =)
But i wasn't so happy with the possible long wait of about one second.
Here's a fast version with 15 bytes:initstabilise ldx #10
lda $d012
lsr ; 2
rol ; 2
- dex ; (10 * 5) + 4
bpl - ; 54
cmp $d012 ; 4 = 62
bne initstabilise; 9 = 71This only considers even-numbered raster lines, so the problematic last line 311 won't terminate the loop.
Note that when replacing the X register delay stuff with some code of the same cycle count, the loop must still have 71 cycles, that is, a number of cycles that is co-prime with 63. Otherwise, not all cycles in a raster line may be reached and the loop spin endlessly. |
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Krill
Registered: Apr 2002
Posts: 2839 |
Quoting Oswaldbasicly its not 0 jitter, just then you know how the raster and the cpu is synchronized :P I meant that the loop always comes out at the same cycle on a raster line, with no jitter. =)
Quoting Oswaldguess you could make it run faster if you would do this on the border on consecutive lines. bne would jump out from a loop. One must avoid the 64-cycles line, because this would in fact produce a 1-cycle jitter with a 63-cycles check. But i think i've got this, now. :D |
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Krill
Registered: Apr 2002
Posts: 2839 |
Ah, bummer. This is the correct one with even-numbered lines only. =)initstabilise ldx #10
lda $d012
lsr ; 2
asl ; 2
- dex ; (10 * 5) + 4
bpl - ; 54
cmp $d012 ; 4 = 62
bne initstabilise; 9 = 71 |
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ChristopherJam
Registered: Aug 2004
Posts: 1378 |
13 bytes, at most a frame.
But, only works 99.9% of the time (fails to trigger DMA if it starts during line $ee), and puts about 3 and a half lines of black at the bottom of the screen for a frame.
ldx#$ee
: cpx $d012
bne :-
: dex
bmi :-
stx $d011
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Krill
Registered: Apr 2002
Posts: 2839 |
Hah, that's pretty dirty. :)
I briefly considered DMA-based methods, but yeah, they usually come with visual artefacts or VSP hazards and the like.
One could argue that- nop
lda $d012
lsr
asl
[54 cycles worth of user code not touching the accu]
cmp $d012
bne - with 11 bytes net size as proposed by Frantic is shorter, though. =D |
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ChristopherJam
Registered: Aug 2004
Posts: 1378 |
Haha well if we can pad with other code, just put something else that doesn't touch X in place of dex:bmi *-1, and we're down to 10 bytes :)
But yes, I'm not that keen on visible artefacts even for a frame. Easier to do a DMA on line $30 at the start of a blanked frame, and put all the init code somewhere that'll be overwritten by decrunched graphics or mainloop code. |
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Krill
Registered: Apr 2002
Posts: 2839 |
Quoting ChristopherJamHaha well if we can pad with other code, just put something else that doesn't touch X in place of dex:bmi *-1, and we're down to 10 bytes :) True, but it's quite hard to hit exactly, uhm, 1189 cycles. :)
Quoting ChristopherJamand put all the init code somewhere that'll be overwritten by decrunched graphics or mainloop code. In the usual size coding categories, you want the init code to be as small as possible as well, though, as the executable size counts. :) |
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ChristopherJam
Registered: Aug 2004
Posts: 1378 |
Well, I'm assuming more "just enough to pad the gap between comparison becoming true and being in the DMA enabled area", so just a couple dozen cycles should be safe.
Fair point on minimizing initcode. |
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Rastah Bar
Registered: Oct 2012
Posts: 336 |
Quoting KrillWith 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.
Funny, didn't know that. Does Vice emulate it? Hoxs doesn't. |
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