[BBC-Micro] 65C02 instructions

Jonathan Graham Harston jgh at arcade.demon.co.uk
Thu Jun 25 01:03:33 BST 2009


> Message-ID: <4A3E7297.1010307 at rs432.net>
 
Rick Murray wrote:
> > No, your emulator should execute them as NOPs or as the combined
> > effects that they perform on actual hardware.
> 
> The problem, which isn't a great one, is that everybody says the 65C02
> undefined instructions are NOPs. They aren't. They are instructions that
> have no effect. Subtle difference, but one which allows for the
> instruction to be between 1 and 3 bytes in length...
 
Yes, you can work out how long they are by what addressing range
they are in the opcode map. eg some are NOP abs, some are NOP (zp),Y
etc. Some do odd things like ASL and then ORA from (zp),Y. The
lists at http://mdfs.net/Docs/Comp/6502 show this, specifically
http://mdfs.net/Docs/Comp/6502/OpList
 
-- 
J.G.Harston - jgh at arcade.demon.co.uk - mdfs.net/User/JGH
A Review of Sheffield City Council's Members' Allowances Scheme
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