Subj : Re: zero page allocation for Forth
To   : Harry Potter
From : chitselb
Date : Thu Aug 10 2017 18:40:23

On Thursday, August 10, 2017 at 8:08:57 PM UTC-4, Harry Potter wrote:
> On Thursday, August 10, 2017 at 3:48:55 PM UTC-4, chitselb wrote:
> > If you needed 141 bytes of zero page on a C-64, what addresses would you 
use?  BASIC is switched out, Kernal is still switched in, so anything BASIC 
uses can be reallocated.  Having it all be contiguous so I can go in and out of 
BASIC is important 
too.
> > 
> > On a PET (4.0 ROM) I'm just using $00-$8C (everything below jiffy clock) 
and there's a little ML routine ('aloha') to swap the half page in and out, but 
I want to port PETTIL to the C=64
> 
> On a C64, the BASIC ROM uses ZP addresses $02-$8F, but you can use kernal ZP 
locations you don't need in your program.  For example, $83-$8B and $AE-$B6 are 
used for tape and RS232 and $F7-$FF are for RS232 and free space.  Does this 
help?

It looks like what they did is put the memory mapping I/O port at $00-$01 and 
moved everything up by two?  Does it still have the problem of occasionally 
pooping on address $10 ? There are a couple of places in the PET ROM where 
writes to $10 occur e.g. 
B3D8 STA $10 , even though Raeto's "Programming the PET/CBM" book says no, it 
is unused

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