Subject: Re: AppleSoft memory usage
Newsgroups: comp.sys.apple2.programmer
From: dempson@actrix.gen.nz (David Empson)
Date: Thu, 29 Jun 2000 23:30:37 +1200
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<bobryan9@my-deja.com> wrote:

> $67 $68   load location

> Or, what is required to RUN another program that is IN MEMORY, without
> erasing variables?  I do not want to use CHAIN.

There was a reasonably common technique used by some programs (Taipan
comes to mind) to deal with a large BASIC program that needs to use the
hires screen.

Taipan was an Applesoft file, loaded at $0801, as normal, but was
actually a saved binary image of memory containing several pieces that
went something like this:

$0801 to about $083F: stub Applesoft program
about $0840 to $17FF: lots of little machine code routines (sound, etc.)
$1800 to $1BFF: hi-res character generator
$1C00 to $1FFF: character sets for the HRCG (two of them).
$2000 to $3FFF: title screen
$4000 to whereever: actual Applesoft program.

I don't recall whether it also used hires page 2.  If so, then $4000 to
$5FFF was unused in the loaded file, and the actual Applesoft program
started at $6000.

The stub program consisted of a single line, which did three POKEs and a
RUN to start the second part of the program running.

Two of the POKEs simply set the "run location" zero page variables to
the new location.  Given your details above, I expect this was $67 and
$68.  The third POKE stuck a zero byte before the start of the actual
Applesoft program (this is a requirement of Applesoft).

i.e. it was probably:

10 POKE 103,1: POKE 104,64: POKE 16384,0: RUN

If I remember right, this technique doesn't work with GOTO - you have to
use RUN.

-- 
David Empson
dempson@actrix.gen.nz
Snail mail: P O Box 27-103, Wellington, New Zealand