Re: LZ: NEW Z80 INSTRUCTION FOUND!!


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Re: LZ: NEW Z80 INSTRUCTION FOUND!!



> On Thu, 26 Sep 1996 14:36:45 -0500 (CDT), you wrote:
> >There are a lot of undocumented Z80 opcodes.  At least there were.
> >They worked in all the Zilog made Z80's but I understand the TI's
> >use an Hitatchi made Z80, so who knows if they'll work in it.
> >
> >I used to have a list of them but I haven't seen it for a few
> >years. I'll look around and see if I can come up with it.
> 
> The reason they're undocumented is because when the Z80 was
> originally manufactured, certain instructions worked only a
> percentage of the time.  Since they didn't want to spend years
> refining the design, they simply didn't document the instructions
> that didn't work all the time!
> 
not totally correct: teh circuitry that implemented these 
instructions was at the edges of teh chip, and when the z80 was 
origionally made, the chip manufacturing proceess couldn't guarantee
the integruty of teh chip edges, so these instruction were'nt
documented. with teh advent of modern chip production techniques, the
integrity of these circuits is guaranteed, so the instructions are
perfectly safe to use, it's just that zilog didn't want to change the
chip specifications.


Rob Taylor MAIL - mailto:rtaylor@ticalc.org 
           WEB  - http://www.u-net.com/~rtaylor/    


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