Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Posting-Version: version B 2.10.2 9/18/84; site digi-g.UUCP
Path: utzoo!watmath!clyde!burl!ulysses!mhuxr!ihnp4!stolaf!umn-cs!digi-g!jel
From: jel@digi-g.UUCP (John Lind)
Newsgroups: net.micro
Subject: Re: HCF
Message-ID: <516@digi-g.UUCP>
Date: Mon, 25-Mar-85 09:56:10 EST
Article-I.D.: digi-g.516
Posted: Mon Mar 25 09:56:10 1985
Date-Received: Tue, 26-Mar-85 06:27:40 EST
References: <796@ecsvax.UUCP> <644@mako.UUCP>
Reply-To: jel@digi-g.UUCP (jel)
Organization: Digigraphic Systems Corp., Mpls, MN
Lines: 35


The HCF instruction is not merely left-over microcode.  It was built into
the chip ON PURPOSE for a first-line rejection test while the chips are
still on the wafer.  The instruction causes the chip to increment the
address bus forever.  This is a simple way of seeing if the chip is alive
at all.  If it doesn't respond, it is marked dead on the wafer (the test
device has to supply proper power, ground, and reset signals first, of
course).  The name comes from two things -- 1) since the chip only
responds to interrupts at instruction COMPLETION, and this instruction
never completes, it is like a halt, and 2) if you have a front panel
with LED's on it, it resembles something burning, if you have a good
imagination.  The inspiration for the name comes from a long humorous
list of imaginary opcodes that I first saw in Sperry (then Univac) land
that included such goodies as the following:

    EDPMB	Execute Data Processing Manager and Branch
    RST		Rewind and Stretch Tape
    PRI		Print in Red Ink (this was funny at the time -- now
		we do it regularly)
    SSD		Seek and Scratch Disk
    SSP		Seek Spindle
    HCF		Halt and Catch Fire

In fact, the 6800 gets no hotter when executing this instruction than
it does with any other.  Dissapating 2W from a 40 pin chip, it gets
uncomfortably warm during normal operation.  My authority is experience
and information from Motorola.  It has been awhile since I did it, but
one of the TWO codes that used to cause that was, if I remember correctly,
D9.  I seldom had to know, as I could usually achieve the desired effect
by simply jumping to random memory -- my chances were good of hitting one
of these -- maybe it was a data item near the start of a ROM, I don't know.
------
John Lind, Starfire Consulting Services
E-mail: ihnp4!umn-cs!digi-g!jel
USnail: PO Box 13001, Mpls MN  55414