Re: [stella] "Illegal" opcodes

Subject: Re: [stella] "Illegal" opcodes
From: "Matthew W. Miller" <mattm@xxxxxxxxxxx>
Date: Sat, 17 Jun 2000 12:50:40 -0400 (EDT)
On Sat, 17 Jun 2000, Ruffin Bailey wrote:

>> Can anyone modify DASM so that it accepts illegal opcodes or recommend
>> a PC assembler that accepts them?
>if these instructions not only work, but in this case do something useful
>_and_ more efficiently, why are they called "illegal" opcodes?  

Because they aren't officially defined.  That they work at all is a side
effect of the way the 65xx's microcode is lashed together.  Also, if you
use a 65xx-descendant such as the 65816 (or is it the 65c816?) in the
Nintendo Super Famicom, you will find that some or all of the 'illegal'
opcodespace is taken up by new official opcodes which aren't compatible
with the 65xx's unofficial ones.

Matthew W. Miller -- mattm@xxxxxxxxxxx

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