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Shadow
Account closed
Registered: Apr 2002
Posts: 355 |
Safe zeropage addresses
Which zeropage addresses are safe for use?
I've checked through a C64-memory map, and it seems like most zeropage locations with the exception of $00 and $01 are mostly for BASIC stuff, meaning that they should be OK when coding demos that doesn't use ase any BASIC or Kernel stuff?
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Oswald
Registered: Apr 2002
Posts: 5094 |
I prefer stacking the zp pointers used by the music (the better players use only one) and then forget about it. |
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Martin Piper
Registered: Nov 2007
Posts: 722 |
Quote: Thanks, good to know. I was a bit unsure of things like $a0-$a2 and $c5-$c6 for example (some kind of timer thing, and keyboard).
$c5-$c6 are updated by the keyboard routines quite late on after $ea31 is called.
$a0-$a2 are updated by the real time clock rountine which is the first thing called by $ea31.
So as long as you're not using $ea31 to exit your interrupt then you're fine.
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Skate
Registered: Jul 2003
Posts: 494 |
in other words as long as you are not using kernel... |
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Soren
Registered: Dec 2001
Posts: 547 |
@Oswald: afaik one can just do some selfmodding code instead of using the stack... throw the zp values into somewhere else in the music-player.... that's the "trick" I use nowadays, but ofcourse I used the stack in my earlier players :-)
2 zeropage adresses is usually what's needed for a musicplayers, I'd say. |
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Mantiz
Account closed
Registered: Apr 2006
Posts: 36 |
$00 is okay too, as long as you remember to set it accordingly if you plan to change $01. |
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Burglar
Registered: Dec 2004
Posts: 1101 |
you might want to skip using $90 (I/O status) and $ba (device number). some loaders will set/use those. |
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Cruzer
Registered: Dec 2001
Posts: 1048 |
Quote: You said it yourself. If your demo doesn't use any BASIC or Kernel stuff, all addresses except $00 and $01 are safe. It's really simple as that.
And if we could just have had these two as well, everything would be perfect. I'll never forgive the C64 designers for this. DAMN YOU RUSSELL AND YANNES!! ;)
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Oswald
Registered: Apr 2002
Posts: 5094 |
yeah, also chuck peddle should suffer for not adding atleast one more damned index register ;) |
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Skate
Registered: Jul 2003
Posts: 494 |
@Oswald: registers are just like girls. you can load them as much as you want but you will still miss another one ;) tax, tay, tya, txa are all lesbians... |
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HCL
Registered: Feb 2003
Posts: 728 |
Which register is most teh ghay?
..and @Oswald: a bit naive to judge a music player as better/worse by the number of zp-adresses it uses.. lda zp,pha is not teh quality. |
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