In article ,
Cliff wrote:
[
Undocumented 8088/8086/80286/80386 Instruction Set

[ ... ]

These instruction vary from machine to machine, but all
computers have a certain set of them in common. As a
service to humanity, I am here revealing these common
instructions for the first time.

ARG Agree to Run Garbage

One, which really *was* on the Motorola 6800 CPU was called:

HCF Hang and Catch Fire

And what it really did was to tri-state the data bus (no reads or
writes) and sequentially produce every possible address, rolling over
from 65535 (FFFF) back to zero (0000).

As far as I know, it was intended as a quick test of
functionality of the CPU, probably run before it was even mounted in a
chip package.

What *I* used it for was as a debugging aid when building
wire-wrapped computers. Since each address line was either twice or
half the frequency square wave as the next one up or down, it was easy
to identify which line you were looking at with a scope, and pretty easy
to determine which address lines were contributing to a decoded address
as well.

So -- I burned an EPROM which had the reset vector pointed to
an instance of that instruction, with the address selected to require
only the bare minimum of functioning address lines.

Enjoy,
DoN.
--
Email: | Voice (all times): (703) 938-4564
(too) near Washington D.C. | http://www.d-and-d.com/dnichols/DoN.html
--- Black Holes are where God is dividing by zero ---