Date   : Mon, 08 Jan 2001 09:23:57 -0000
From   : "Rich Talbot-Watkins" <Richard_Watkins@...>
Subject: Re: Undoumented 6502 opcodes : BBC vs. Electron

Jonathan Graham Harston <jgh@...> wrote:

> Zalaga (or "Zalagan" as some people ;) knew it as) doesn't work on
> non-BBCs as it uses a screen lookup table with OS 1.20.  You can get it to
> work on that Masters as there is ram there (Hazel) and a small bit of code
> can be used to create the table before running Zalagan.  I've been looking
> for the relavant bit of code myself - it was published in either Acorn
> User or Micro User some time ago.

This can be created on a Master with the following code IIRC:

10FORN%=0TO31
20?(&C375+N%*2)=(N%*640)DIV256
30?(&C376+N%*2)=(N%*640)AND255
40NEXT

Run it after everything has been loaded as it corrupts workspace used by the
ADFS system.

However, the main problem with Zalaga, as previously mentioned, is that it
contains a number of routines which plot horizontally reflected sprites which
use opcodes specific to the 6502s used in the original BBC Micros, eg ASL-ORA.
In the version that I hacked about to get running on a Master OK, I had to
rewrite these routines in private RAM to use only legal instructions, which
unfortunately would've slowed everything down a little bit.  At the time, I
could hardly believe what I was seeing was true when it was apparently calling
a routine with all kinds of nonsense instructions in.

cheers,
rich