Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!cornell!biar!trebor
From: trebor@biar.UUCP (Robert J Woodhead)
Newsgroups: comp.arch
Subject: Re: 8086 design goals (and Pascal)
Message-ID: <572@biar.UUCP>
Date: 19 May 89 13:21:49 GMT
References: <912@aber-cs.UUCP> <3312@bd.sei.cmu.edu> <362@verdix.verdix.com> <15766@vail.ICO.ISC.COM> <370@telesoft.UUCP> <13822@steinmetz.ge.com>
Reply-To: trebor@biar.UUCP (Robert J Woodhead)
Organization: Biar Games, Inc.
Lines: 17

In article <13822@steinmetz.ge.com> davidsen@crdos1.UUCP (bill davidsen) writes:
>  There were also (as I recall) 12 undocumented instructions in the Z80,
>at least the Zylog version, of which only a few were useful to a sane
>person. Some of the others were really neat, but not useful.

The 6502 (at least some versions) had many undocumented instructions.  I
believe that they were written up in an ancient issue of Byte.  In particular,
there was a Store Immediate counterpart to Load Immediate, that stuck the
register in the byte following the instruction and skipped it.  And there was
the aptly named HCF ``Halt and Catch Fire'' instruction.  This instruction
locked the cpu, made it insensitive to anything but power down, and rapidly
cycled the address lines.

-- 
Robert J Woodhead, Biar Games, Inc.  !uunet!biar!trebor | trebor@biar.UUCP
"The lamb will lie down with the lion, but the lamb won't get much sleep."
     -- Woody Allen.