[CCTECH] Interesting tidbit on 6502

From: Sean 'Captain Napalm' Conner <spc_at_conman.org>
Date: Thu Jun 6 18:49:23 2002

It was thus said that the Great Cini, Richard once stated:
>
> 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?

  I've actually heard that some of the undocumented 6502 opcodes appeared to
do two operations, but I've never programmed a 6502 (a 6809 yes, and I found
out that encoding a TRF 8b,16b would set the high byte of the 16b register
to $FF---I never did see what the undocumented opcodes did though; I tended
to stay away from the undocumented stuff as a programmer).

  -spc (I find that interesting though ... )
Received on Thu Jun 06 2002 - 18:49:23 BST

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