Date   : Wed, 17 Mar 2004 15:48:24 +0000
From   : Richard_Talbot-Watkins@...
Subject: Behaviour of 6502 opcode 9C

Hi,

Just wondered if anyone has ever checked the exact behaviour of opcode 9C
on a real BBC B.  It's one of the unspecified 6502 opcodes, but there seems
to be division on what it actually does.

Some Beeb-related docs describe it as STZ abs, meaning ?abs = 0, just like
its 65C12 equivalent.

Most other docs describe it as SAY abs,X, meaning ?(abs+X) = Y AND ((abs
DIV 256) + 1), which is rather less useful.

I was looking through the code for Zalaga ages ago, and remembered how its
whole protection system was based on obfuscating its code by executing a
piece of readable ASCII text, which turned out to contain all sorts of
invalid opcodes, and decrypting its tape loader with it.  Clever chap, that
Orlando - he must've figured it all out himself, as I suspect there was no
documentation out there back in 1983.  Zalaga used all sorts of
undocumented opcode tricks in its sprite routines, and its protection
system.  Anyway, I'm sure I've seen opcode 9C used in a few games (perhaps
Repton or Repton 2), so who knows for sure what it does?

Rich



**********************************************************************
This email and any files transmitted with it are confidential and
intended solely for the use of the individual or entity to whom they
are addressed. If you have received this email in error please notify
postmaster@...

This footnote also confirms that this email message has been checked
for all known viruses.

**********************************************************************
 SCEE 2004