Re: LZ: NEW Z80 INSTRUCTION FOUND!!


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Re: LZ: NEW Z80 INSTRUCTION FOUND!!



On Fri, 27 Sep 1996, Mel Tsai wrote:


> 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!
 
Those undocumented opcodes were used by thousands of CP/M
programmers for years and I never heard of any problems.
Most professionals in most programs didn't use them, of
course, and maybe the non-professionals didnt want to
complain or mention it.  But I do think I would have heard
about it.  I havent used them myself so I can't tell you from
experience.  I only did a small amount of Z80 coding from
time to time.


I've heard another reason they don't want to document them is
that they don't expect to support them in future revisions or
upgrades because they don't feel theyre needed.


Barry


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