Path: utzoo!utgpu!watmath!clyde!att!osu-cis!tut.cis.ohio-state.edu!unmvax!ncar!ames!lll-tis!oodis01!uplherc!esunix!bpendlet
From: bpendlet@esunix.UUCP (Bob Pendleton)
Newsgroups: comp.arch
Subject: Re: OISC was Re: ZISC computers
Message-ID: <1120@esunix.UUCP>
Date: 29 Nov 88 15:21:52 GMT
Organization: Evans & Sutherland, Salt Lake City, Utah
Lines: 45

Several years ago I also came up with the one instruction computer
that everyone else has also invented. Since I went to a school that
Ivan Sutherland taught at I must assume I picked up the idea as it
floated past, but I really thought I invented it, sigh.

My variation of the one instruction was the Conditional Move From Here
To There And Clear Flags or CMFHTTACF instruction. The idea being that
if any flags are set the instruction is skipped, in all cases the
flags register is cleared. All instructions are two addresses, a
source and a destination. The PC, the flag register, the flags mask
register, and all ALU registers are mapped to memory locations. And,
of course, some operations have the side effect of setting bits in the
flags register.

Using this machine as a basis for argument I "showed" that there were
good economic reasons, the cost of RAM and such, to try to reduce the
length of the addresses in the CMFHTTACF instruction. If your address
space is 2^32 then the CMFHTTACF instruction is 64 bits long.

The easiest way I could think of to reduce the length of the
instruction was to use two address spaces. One very small address
space that only had enough addresses to map the ALU registers, the PC,
and such.  One large address space for memory. And, to add a one bit
direction flag. To tell if the move was from the large address space
to the small or from the small to the large.

This looked suspiciously like the opcode, register index, and address
type of instructions we see in typical computers. Hmmm.

Well, repeated applications of the cheapness principle rather rapidly
resulted in a multilevel machine that looked an awful lot like a
a sort of RISC with microcoded coprocessors. 

This exercise lead me to the conclusion that all modern computers are
economically motivated variations of the CMFHTTACF machine. :-)

The odd things that go through the minds of grad. students while
trying to write a thesis.

		Bob P.

-- 
              Bob Pendleton, speaking only for myself.
UUCP Address:  decwrl!esunix!bpendlet or utah-cs!esunix!bpendlet

		Reality is what you make of it.