Re: 6502 illegal opcodes

From: Segher Boessenkool <>
Date: Fri, 4 Nov 2011 23:32:52 +0100
Message-Id: <>
>> Opcodes $8b and $ab are not predictable in that their result  
>> depends on
>> various analog effects; their behaviour can vary on a single physical
>> chip when it heats up, even.
> these are predictable though - if you choose the right argument :)  
> not that
> ane #$00 is terribly useful =P

What is that, 8b?  Most people call that XAA afaik.

>> Various opcodes are unpredictable in the sense that their result  
>> depends
>> on what happens on the data bus while RDY is low.
> i would not call that unpredictable

If you have full control over what happens on the bus, sure.  In most
cases you do not -- it is specifically designed so you do not have to.

This also happens just for 8b and ab fwiw.  All other illegal opcodes
are more or less sane, where 10xxxx11 is the "less" and the rest is the
"more". (*)


(*) Oh and of course the STX abs,Y and STY abs,X (9e and 9c) insns,  
are naughty!

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Received on 2011-11-04 23:00:04

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