On Thu, 6 Jun 2002, Cini, Richard wrote:
> While browsing sites for 6800 information, I came across this quote from
> <http://wombat.doc.ic.ac.uk/foldoc/foldoc.cgi?Mostec>.
> which talks about the design goals for the 650x in comparison to the 6800.
>
> >The design goal was a low-cost (smaler chip) design, realized by
> simplifying the decoder stage. There were no instructions with the value
> xxxxxx11, reducing the 1-of-4 decoder to a single NAND gate. Instructions
> with the value xxxxxx11 actually executed two instructions in paralell, some
> of them useful. <
> Now, I didn't look at an opcode map, but it seems that this is an
> interesting twist that I've never seen quoted when people discussed the
> mysterious undocumented 6502 opcodes executing what appeared to be multiple
> instructions.
> Any thoughts?
That is indeed very enlightening. I'd always known about the
"undocumented opcodes" but never ventured to use them. However, this is
pretty much what I guessed was going on (though my hankering was not as
technically astute as the reality).
Sellam Ismail Vintage Computer Festival
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Received on Thu Jun 06 2002 - 16:44:44 BST