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Forums > C64 Coding > Shortest code for stable raster timer setup
2020-01-20 16:20
Krill

Registered: Apr 2002
Posts: 2844		 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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2023-05-11 19:00
Quiss

Registered: Nov 2016
Posts: 37		 Sometimes, you want to set up a timer that loops once per frame, not per line, and on a stable cycle.
The shx code snippets posted so far give us a constant x position, but on a random y, so won't work for that.

But you can do:

        ldy #$01
loop:
        ldx #243  (or any other badline)
        cpx $d012
        bne *-3
        shy $ffff, x
        lda $ff, x
        beq loop

This always drops us out on line 243 cycle 62, and takes up to seven frames to do so.
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