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chatGPZ
Registered: Dec 2001
Posts: 11149 |
(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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Compyx
Registered: Jan 2005
Posts: 631 |
Perhaps I'm missing something, but aren't the 'alternate' ghostbytes at $B9FF and $F9FF (when using ECM)? |
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CyberBrain
Administrator
Posts: 392 |
Yes, you're absolutely right of course. Their values unfortunately doesn't become much nicer:
$B9FF = $A0
$F9FF = $D2 |
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Compyx
Registered: Jan 2005
Posts: 631 |
Nope. But the 'trick' is very interesting though, never would have thought of it :) |
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Oswald
Registered: Apr 2002
Posts: 5031 |
nice trick, but 3fff can be set to any byte in a 8 pixel wide area with another trick. |
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Compyx
Registered: Jan 2005
Posts: 631 |
Pray tell. |
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tlr
Registered: Sep 2003
Posts: 1728 |
Quote: nice trick, but 3fff can be set to any byte in a 8 pixel wide area with another trick.
I assume you are refering to exploiting the difference in pipeline delay between changing the videomode and changing the fetched graphics data to be fetched?
That is a neat trick, but it is dependant on the type of VIC-II (old/new) and it takes more cycles per instance.
The inc $ffff trick is only 6 cycles per instance. |
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Hoogo
Registered: Jun 2002
Posts: 102 |
Dec $dc0d after a Timer A IRQ occurred would acknowledge that IRQ and stop further ones. But I don't see a good reason to do that. |
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Rastah Bar
Registered: Oct 2012
Posts: 336 |
Hoogo, perhaps when you no longer need that timer interrupt, f.e. as an exit strategy to move on to a new demo part?
Oswald, are you referring to the vertical border scrollers in Krestage? But aren't these 7 pixels wide?
CyberBrain, great idea! |
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chatGPZ
Registered: Dec 2001
Posts: 11149 |
reminds me of a subtle bug i created long ago which made me pull my hair out. somehow i thought its a good idea to ACK the timer irq by
lda $dc0d
sta $dc0d
aaaargs |
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Oswald
Registered: Apr 2002
Posts: 5031 |
Quote: Hoogo, perhaps when you no longer need that timer interrupt, f.e. as an exit strategy to move on to a new demo part?
Oswald, are you referring to the vertical border scrollers in Krestage? But aren't these 7 pixels wide?
CyberBrain, great idea!
maybe 7,dont know :)
interesting it doesnt work on old vic ? why did they change it? why is it needed? interesting details here :) |
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