Date   : Fri, 10 Jul 1992 08:56:11 GMT
From   : comp.vuw.ac.nz!actrix!naos!ewen@uunet.uu.net (Ewen McNeill)
Subject: Re: undocumented Z80 Ops

u8515682@cc.uow.edu.au (Wayne Jefferson Doust) writes:

> Why did Zilog produce a chip with codes that were not documented?
> I don't understand.
>
Because they didn't work properly. Eg, the shift instruction puts a 1
into the new gap instead of a 0.  This isn't a major problem if you know
about it, but it is non-standard (compared to the other shift codes).
The extra index register instructions were probably more due to the way
the instructions are decoded than anything else; I think they were
always intended to be index registers (pointers) rather than extra 8 bit
registers - they are slower than the normal 8 bit registers.

> Wayne

--
Ewen McNeill, ewen@naos.actrix.gen.nz (or ewen@actrix.gen.nz)