Re: Integer BASIC & Applesoft BASIC Do Not Work

[dempson@actrix.gen.nz (David Empson)]

Bryan Parkoff <bryan.nospam.parkoff@nospam.com> wrote:

>     I did an experiment to switch from ROM to RAM at $D000 through $FFFF.  I
> mean that $D000 through $FFFF are set to read/write instead of read-only
> mode.  I can be able to modify monitor routines and autostart routines.  For
> example, I change from $A5 + $22 (LDA $22) to $60 (RTS) at $FC58 (HOME) by
> typing "*FC58: 60 (CR)" using Apple II+'s keyboard.  You type, "CALL - 936"
> or "HOME" to clear the screen, but it will never clear the screen.
>     I start to test Integer BASIC and Applesoft BASIC by typing, "*E003G
> (CR)".  "> 65535 " appears for Integer BASIC and Applesoft BASIC crashed.

Not surprising. You can't start Applesoft or Integer BASIC from this
entry point (warm start, Ctrl-C command from the montior) unless you
have previously done a cold start of the same BASIC (E000G, Ctrl-B
command from monitor).  Note that if you're using DOS, you shouldn't use
either of these entry points: use 3D0G instead.

The cold start routine is responsible for setting up the zero page
variables required by BASIC.  If you bypass this step, critical zero
page variables might contain arbitrary data which results in strange
side effects if you attempt to enter BASIC by doing a warm start.

For example, Applesoft BASIC has a self-modifying function on zero page
which fetches the next token from a program.  This function is placed on
zero page by the cold start routine.  If it is missing, Applesoft will
crash as soon as you try to execute a command.

Another likely problem is the memory limit variables (HIMEM and LOMEM),
as they will not be set correctly, possibly resulting in BASIC trying to
overwrite memory in the zero page, stack, text screen, DOS (if present),
I/O space and the language card.

>     I did discover that $F000 through $F022 routine after I typed "*E000G
> (CR)" is used to modify from $00 to $FF each page ($0000 through $FFFF).  I
> suspect that this routine is used to test RAM available like 4K, 8K, 16K
> through 48K.  If it is only 8K on RAM, it will stop scanning until it
> reaches $1FFF.  If it is only 48K on RAM, it will stop scanning until it
> reaches $BFFF.  If it is only 64K on RAM, it will stop scanning until it
> reaches $FFFF.

There probably is a routine which does a memory size test, as part of
the cold start function.  It is designed to test memory starting at 0800
and stopping at BFFF.  I don't remember how this interacts with
RAM-based versions of Applesoft (loaded from tape into low memory) or
with DOS/BASIC.SYSTEM.

-- 
David Empson
dempson@actrix.gen.nz