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Forums > C64 Coding > (Ab)use of dummy accesses
2020-03-09 17:04
chatGPZ

Registered: Dec 2001
Posts: 11523
(Ab)use of dummy accesses

For the next release of my "No more Secrets" doc i am preparing a chapter related to the dummy access which happen when the CPU performs an internal operation. Once again i am looking for some examples on how to (ab)use it :) I guess everyone knows "inc $d019" - but i am sure there is more than this. And not only with RMW instructions. So if you have anything in your mind - just drop it here!

here are some related notes which i pasted together. feel free to proofread and point out mistakes :)
 
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2020-03-14 16:55
Frantic

Registered: Mar 2003
Posts: 1661
Quote: Also a good way to show the finger to shitty replacements =P

Ah.. yes! I'll remember that! :)
2020-03-14 19:24
Compyx

Registered: Jan 2005
Posts: 631
Quoting tlr
Are we counting things like inc $d016;dec $d016 in this btw? More related to BA i guess but if there weren't so many dummy cycles it wouldn't work.


I wouldn't count that under the dummy writes/reads. You're just wasting cycles with a RMW instruction to inhibit sprite 0 DMA screwing with opening the border.
2020-03-14 23:34
ChristopherJam

Registered: Aug 2004
Posts: 1424
Quoting ChristopherJam
I'm fairly sure one of the iterations in developing a stable hard restart used one cycle blips of the gate bit to allow RC to escape at known times, but it didn't turn out to be the most optimal. I'll have a rummage.


"sieve" at SID envelope rate counter phase alignment - which just reminds me I need to fix the hosting of those images and runlogs..

But yes, interesting that the readback is perfectly reliable if you're safely under 2000 cycles. Presumably one could first write to one of the undefined registers between $1d and $1f to much the same effect.
2020-03-28 18:11
chatGPZ

Registered: Dec 2001
Posts: 11523
Hey, is that all? *push* :)
2020-03-29 16:00
S.E.S.

Registered: Apr 2010
Posts: 19
If you want to have raster splits that are exactly 5 cycles wide, you can use
ldx #$ff
ldy #$05
lda #$00
sty $d021
sta $cf22,x
I don't know if anybody actually used that in an intro or a demo, though :)
2020-03-29 19:08
Oswald

Registered: Apr 2002
Posts: 5127
make em 3 cycles wide :)
2020-04-01 20:14
Hoogo

Registered: Jun 2002
Posts: 105
What about the color of the colorram in multicolor FLI?
2020-04-01 21:49
chatGPZ

Registered: Dec 2001
Posts: 11523
what about it?
2020-04-02 09:16
Hoogo

Registered: Jun 2002
Posts: 105
Is it an example for "side effect of fetching next opcode" for your purpose?
2020-04-03 15:43
chatGPZ

Registered: Dec 2001
Posts: 11523
yes and no - this particular case is already described in detail in the pdf ("Blackmail FLI") :)
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