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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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JackAsser
Registered: Jun 2002
Posts: 2014 |
Quote: 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?
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. |
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Shadow
Account closed
Registered: Apr 2002
Posts: 355 |
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). |
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JackAsser
Registered: Jun 2002
Posts: 2014 |
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).
ZP is just memory and not related to any I/O chips at all in any way except through kernel calls.
Although when adding music and IRQ-loaders you must know what ZP-locations they use so that you doesn't accidentally overwrite those. F.e. I normally use GoatTracker and Krill's IRQ-loader, both with customizable ZP-locations so it's really not any problems at all.
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HCL
Registered: Feb 2003
Posts: 727 |
IRQ-loaders don't use ZeroPage, only decrunchers. That's a rule. Plus music of course. |
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Oswald
Registered: Apr 2002
Posts: 5076 |
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: 699 |
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: 1069 |
you might want to skip using $90 (I/O status) and $ba (device number). some loaders will set/use those. |
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