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From: gene@cooper.cooper.EDU (Gene (the Spook) )
Newsgroups: comp.arch
Subject: Re: Zilog's mnemonics; a boon to programmers
Message-ID: <3073@cooper.cooper.EDU>
Date: 17 Nov 90 02:07:14 GMT
References: <1990Nov16.050844.12539@hayes.ims.alaska.edu>
Organization: The Cooper Union (NY, NY)
Lines: 35

in article <1990Nov16.050844.12539@hayes.ims.alaska.edu>, floyd@hayes.ims.alaska.edu (Floyd Davidson) says:
> It is also true that some of those instructions that don't exist (in
> the books, anyway) do some strange things, some of which were very
> useful.  It has been too long for me to remember exactly what they
> are, but I think 8 bit manipulation of the index registers were the
> useful ones.

Yup.  The ALE editor/assembler for old TRS-80s even supported those
instructions!  You could do things such as setting the high- or low-
byte of the IX and IY registers by specifying the instruction as
	<op>	{HX|LX|HY|LY}
when HX is the high byte of the IX register, LX is the low byte of the
IX register, HY is the high byte of the IY register, and LY is the low
byte of the IY register.

Also, you could easily get 2x+1 from a register containing value x by
using the SLS (shift left and set) pseudo-instruction.  This grew from
the fact that there was a shift right arithmetic, shift left arithmetic,
and shift right logical, but no shift left logical.  The Z80 would
shift the quantity x left by one (2x), and set the LSB (+1).  Nifty
for doing "trick" multiplies.

According to every source for the Z80 (as far as I know), all these
pseudo-instructions worked just fine, but were neither tested-for
nor guaranteed.

Also, if I'm not mistaken, the Z[12]80 support similar operations.

Play with them and have fun!

(BTW, try to get hold of an old 6502 and use the HCF (halt and catch
fire) instruction.  Loads of laughs.  Impress your friends.)

						Spookfully yours,
						Gene