Newsgroups: alt.folklore.computers
Subject: Re: TX-0 code
From: cchd@lucifer.latrobe.edu.au (Huw Davies)
Date: Mon, 28 Dec 1992 00:24:02 GMT
Organization: La Trobe University

In article <BztJsM.Mvy@sdf.lonestar.org> df@sdf.lonestar.org (daniel finster) writes:
>
>  Reading in _Hackers_ a while back, i remember seeing that the TX-0 had 18-bit words,
>the first two bits of which were instructions.  Now, that comes out to
>4 instructions (assuming Levy got it right); I've been racking my brain
>off and on over the past year or so trying to figure out WHICH 4 instructions
>that the TX-0 included 

Quoting from the alt.folklore.computer's DEC bible ("Computer
Engineering - A DEC view of hardware systems design") we have the
following information about TX-0:

"...the TX-0 was designed as a test device. It was designed to test
transistor circuitry, to verify that a 256x256 (64K-word) core memory
could be built."

"The original TX-0 had a number of I/O devices. After it was moved to
M.I.T., the largest device was a 12-inch point-plotting cathode ray tube
and light-pen console, giving the TX-0 some physical resemblance to
Whirlwind. In addition to the cathode ray tube, there was a high speed
(300 characters per second) Ferranti paper tape reader and a Friden
Flexowriter that was used as both a typewriter and paper tape punch.
There was also a large bank of toggle switches, some of which formed the
two program accessible registers and some of which formed the first 16
memory locations, permitting direct entry of variables."

"The two program accessible registers were called the Accumulator and
the Live Register. The Accumulator was used for logic functions and the
Live Register was used for controlling and buffering transfers to
various I/O equipment. The initial version of the TX-0 had only four
instructions encoded in two bits, leaving sixteen bits to access the
large, 64K-word memory. Three of the instructions accessed memory,
"store in location", "add from location," and "transfer if Accumulator
is negative to location." The fourth instuction, "operate," was for
program controlled I/O transfers and included commands that could be
combined to produce a large number of instructions. The combining
process was called "microprogramming" because bits in the instruction
specified particular register transfer operations and could be
programmed. Among the instructions that could be created were "clear the
right half of the Accumulator," "cycle the Accumulator right one
position," "start the paper tape reader." The operations encoded in the
instruction could occur at any one of six possible times during the
instruction; thus, a multi-function instruction could be formed, such as
one to display a point on the screen and to generate a new pseudo-random
point."

I hope that this is of some interest.

I'd really like to hear from people who have had more direct experience
with TX-0....

-- 
  Huw Davies           | Huw.Davies@latrobe.edu.au
  Computing Services   | Phone: +61 3 479 1500   Fax: +61 3 479 1999
  La Trobe University  | I own an Alfa to keep me poor in a monetary
  Melbourne Australia  | sense, but rich in so many other ways
Newsgroups: alt.folklore.computers