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Ben
Account closed
Registered: Feb 2003
Posts: 163 |
Free cycles in raster line..
Is there some sort of stable, line dense raster leaving some cycles per line free which can then be filled with instructions executed outside the IRQ?
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Ben
Account closed
Registered: Feb 2003
Posts: 163 |
Quote: Putting the jump into $dc04/05 and trigger using timer B was a neat trick. Otherwise basically what I said in my last post.
Timers counting up... sorry for that. :P
It's been a really long time since I messed with this.
TLR, thanks for your advice! |
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Krill
Registered: Apr 2002
Posts: 2855 |
Also consider putting the rti instruction to $dd0c, then jmp $dd0c (the worthless serial shift register) at the end of your nmi handler. Since the cpu fetches at least 2 bytes for _every_ instruction (just that the 2nd byte is dismissed for 1-byte commands), the cpu acks the nmi by this implicit $dd0d read. Saves one (1) cycle comparing to lda $dd0d:rti :D Don't forget to do lda #$bf:and $dd0e:sta $dd0e to set the serial shift reg to input and use it as actual data storage to read and write. This technique is described in http://www.ffd2.com/fridge/chacking/c=hacking7.txt |
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Ben
Account closed
Registered: Feb 2003
Posts: 163 |
Quote: Also consider putting the rti instruction to $dd0c, then jmp $dd0c (the worthless serial shift register) at the end of your nmi handler. Since the cpu fetches at least 2 bytes for _every_ instruction (just that the 2nd byte is dismissed for 1-byte commands), the cpu acks the nmi by this implicit $dd0d read. Saves one (1) cycle comparing to lda $dd0d:rti :D Don't forget to do lda #$bf:and $dd0e:sta $dd0e to set the serial shift reg to input and use it as actual data storage to read and write. This technique is described in http://www.ffd2.com/fridge/chacking/c=hacking7.txt
Good idea!
(To be frank with you all: I am not working on something at the moment (too busy with other stuff, regretfully), but I am intrigued by the technology.. ) |
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WVL
Registered: Mar 2002
Posts: 886 |
Quote: Also consider putting the rti instruction to $dd0c, then jmp $dd0c (the worthless serial shift register) at the end of your nmi handler. Since the cpu fetches at least 2 bytes for _every_ instruction (just that the 2nd byte is dismissed for 1-byte commands), the cpu acks the nmi by this implicit $dd0d read. Saves one (1) cycle comparing to lda $dd0d:rti :D Don't forget to do lda #$bf:and $dd0e:sta $dd0e to set the serial shift reg to input and use it as actual data storage to read and write. This technique is described in http://www.ffd2.com/fridge/chacking/c=hacking7.txt
hey! I didnt know this one yet :)
have to check if ninja also used this (but i think that's very probable..) |
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Ninja
Registered: Jan 2002
Posts: 407 |
Well, I know this trick, but I didn't use it in the 2x2-routine. Simply didn't think of it at that time, hehe, you can't remember 'em all, I guess :) Wouldn't save too many cycles anyway, as in most cases the BIT $DD0D is executed before the badline. Still, I think it's worth implementing it. |
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