Path: utzoo!attcan!uunet!mcvax!ukc!mucs!trevor
From: trevor@ux.cs.man.ac.uk (Trevor Hopkins)
Newsgroups: comp.arch
Subject: Re: TISC processor (not a joke, but not real either)
Keywords: One Instruction Computers
Message-ID: <6266@ux.cs.man.ac.uk>
Date: 29 May 89 08:02:37 GMT
References: <18777@cup.portal.com>
Reply-To: trevor@mucs.UUCP (Trevor Hopkins)
Organization: Computer Science, University of Manchester, UK
Lines: 45

This topic seems to come up (in comp.arch and elsewhere) about one a year,
but I'll bite again anyway.....

In article <18777@cup.portal.com> mmm@cup.portal.com (Mark Robert Thorson) writes:
>The recent joke postings on SISC (Single Instruction Set Computers) remind
   They're no joke!
>me that I once heard the minimum instruction set of a computer is two:
   No, the minimum number really is *one*.

The instruction is `Reverse Subtract, Skip if Borrow' (RSSB), and the
operand is given by a single memory address (a one-address format).

Roughly, the operation says: `take the value from the memory at the address
given, subtract it from the accumulator, putting the result both in the
accumulator *and* back in the addressed location, and skip the next
instruction if a borrow took place.'

The code, of course, consists of a sequence of addresses, upon which to
perform the RSSB instruction.

This topic came up on USENET several years ago, and I was interested enough
to suggest a final year (undergraduate) project to actually construct such
a thing (called an OIC -- One Instruction Computer).  The hardware worked
well, using TTL technology and wire-wrap, with a clock (and instruction)
rate of 16 MHz and 16-bit addresses and operands.  I have the hardware in
front of me as I type this message.

The student also constructed a (macro) assembler, which took input in a
vaguely PDP11-style (16-bit instructions only) and generated a sequence of
RSSB instructions.  This compination managed to execute `PDP11' type
instructions at about 0.5 MIPS -- not too bad for a TTL single-board
computer.

A subsequent student started producing a Pascal compiler for the OIC, but
with less success (he was a less able student :-).

If anyone's interested, I can provide further information.

Trevor.
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