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From: liang@cvl.UUCP (Eli Liang)
Newsgroups: net.micro.apple
Subject: Re: Undocumented 6502 Opcodes
Message-ID: <74@cvl.UUCP>
Date: Thu, 21-Feb-85 20:30:39 EST
Article-I.D.: cvl.74
Posted: Thu Feb 21 20:30:39 1985
Date-Received: Tue, 26-Feb-85 07:25:46 EST
References: <376@aicchi.UUCP>
Distribution: net
Organization: Computer Vision Lab, U. of Maryland, College Park
Lines: 45

> Does anyone out there have a listing of undocumented 6502 opcodes?  From 
> talking to several people, I have discovered:  
> 
>   - That these opcodes function differently depending upon the chip
>     manufacturer.
> 
>   - That some issue of "Compute" magazine had a listing of these at some
>     point in time.
> 
> Many thanks in advance;  I'll sleep better with one more curiousity 
> satisfied :-).
> 
> -- 
> Joe Loda
> Analysts International (Chicago Branch)
> (312) 882-4673
> ..!ihnp4!aicchi!joeloda

There is a back issue of Byte which also mentioned the undocumented opcodes.

The wierdest opcode yet though is one a friend told me about.  It may be
apochryphal so I think I will disclaim it right off.  Supposedly, the
original 6502 had an undocumented test instruction (one of the x2H opcodes,
I think it was 12H) which would tie up the cpu and run internal checks on the
processor such as run through the address/data lines lightning fast, until it
was reset or a NMI occurred.  Well, this testing had a nasty side effect....
The chips would get so hot doing this that after it was let run for a few
hours, the chip would either just fail, or every long once in a while
something catastrophic would happen and the chip would start smoking or
actually catch on fire!  It was thus descriptively named the HCF (Hold and
Catch Fire) instruction.  Supposedly, the Source or Compuserve had a 3 part
article a few years ago describing this fascinating instruction.  Perhaps
there is someone on the net who worked on the design of the 6502 and could
affirm or deny, once and for all whether the HCF instruction has ever existed
or exists.

-eli

-- 

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Eli Liang  ---
        University of Maryland Computer Vision Lab, (301) 454-4526
        ARPA: liang@cvl, eli@mit-mc, eli@mit-prep  CSNET: liang@cvl
        UUCP: {seismo,rlgvax,allegra,brl-bmd,nrl-css}!umcp-cs!cvl!liang