The History of TOPS
                                      or
                             Life in the Fast ACs

                               by Peter J Hurley
            for the Spring 1984 DECUS Symposium in Cincinnati, Ohio
                          transcribed by Jack Stevens

                                   Abstract

     Peter Hurley, Manager of 10/20 Software Engineering at Digital,  gave
     a  brief  history, complete with war stories and the recollections of
     many of those involved, of DEC's 36-bit product line on the  occasion
     of the twentieth anniversary of the PDP-6.

The main speaker, Peter Hurley, is the Manager of TOPS-10/20 Software
Engineering at DEC, where he has spent the last 16 years.  He had, in fact,
been working with DEC's 36-bit computers for two years before that.  His first
programming job (which he got by strategically losing a squash match with his
boss) was on a PDP-1 at M.I.T.'s Lab for Nuclear Science.  The next year they
upgraded to a PDP-6, running version 1.6 of the monitor, and a TTY 33 as a line
printer (at least until they tried to do a listing of the Fortran compiler on
it).  Peter followed his boss to DEC, as the latter still needed someone to win
against at squash.

The PDP-6 was first shipped in June of 1964.  It was followed by the KA-10 in
1967, about 1.5 times the power of the PDP-6.  The KI-10, about 1.8 times the
KA-10, was released in 1972.  The KL-10 was introduced in 1975 for TOPS-10,
about 2.5 times the KI.  In 1972, the first TOPS-20 KL-10 was released.  In
1978, the 2060, 2020, and 1091 came out.  The letters in the processor names
did, in fact, mean something.  The "K" stood for Komputer ("C" had already been
used for something important, like a card reader);  "A" was the first letter of
the alphabet.  By the time they got to the KI, however, marketing had gotten
involved, so "I" stood for integrated circuits.  To help sell the idea of the
KL to management, "L" stood for low-cost system.  The "S" in KS probably stood
for small.

The PDP-6 could be purchased with optional fast accumulators.  They took up an
entire 19-inch rack (that's 16 words in one rack).  One megaword, therefore,
would take up 98,958 cabinet feet of floor space (or, approximately 18.7
miles).

The development team of the PDP-6 included Gordon Bell (who designed the system
architecture, then started to bail out the documentation group by writing half
the hardware manual, then moved to the software group to try to help bail them
out).  Alan Kotok, who was hired in 1961 as a PDP-4 Fortran compiler writer,
turned into a PDP-6 architect as assistant logic designer.  Russ Donne, a
circuit designer, was pulled into the project late (just about every engineer
at DEC was pulled into the project) and given three weeks to do the main PDP-6
module layout and design.  One of the modules, the biggest one he had ever
done, had 108 transistors on it (whatever they are).  Leo Gussell wrote the
diagnostics for the -6, the basic diagnostics A through H, that are still in
use today.  Harris Hyman was the author of Macro and a little absent minded.
One day he managed to lose all the sources to Macro.  Peter Sampson, who wrote
Fortran II, put all of one comment in it.  That comment, which commemorated the
numerological identity of the octal equivalent of 1000 and the year of Johann
Sebastian Bach's death, read "JSB RIP".  Tom Hastings, hired in 1961 as the
first software engineer at DEC, had a few strange habits.  One of them was that
when he was tired he would lay a listing on the floor and take a nap on it.
One could not be sure, coming across him in that state, whether he was still
alive.  Dave Gross, brilliant but absent minded, slept through all his DEC
employment interviews except one.  He was hired in his last one of the day,
with the documentation group, because that did not take place until 3 PM, and
he had awakened by then.  Tom Eggers, DEC's first school dropout to become a
programmer, developed DDT while at school.  DEC paid him $500 for it.  He used
to crawl into the office of Harlan Anderson, VP of Engineering, to sleep on the
couch there.  Harlan would discover him the next morning (this happened once a
week) and kick him out.  Ed Yourdin, of structured programming course fame,
wrote Loader.  Loader has since passed out of use;  perhaps it was not
sufficiently structured.  :-)

The PDP-6 project started in early 1963, as a 24-bit machine.  It grew to 36
bits for LISP, a design goal.  The IBM 7090 was also 36 bits, so that was okay.
The PDP-6 did go the 7090 one better, in that it had an 18-bit address rather
than the 7090's 15.  The theory was that 256K was clearly enough to last the
entire life of the product.  After all, that much memory wouldn't even fit in a
room.  The design engineers really didn't even know how to call subroutines, so
they designed in all the ways they could think of:  JSR, JSA, JSP, PUSHJ, and
UUO (JFFO [and JSYS were] added later).  They couldn't decide which Boolean
instructions to have, so they did them all.  The reason that the bits were
numbered from left to right was that IBM had done it that way (as Alan Kotok
put it, "We hadn't invented 'Not Invented Here', yet.").  NIH did arrive by the
time the PDP-11 was developed.  The PDP-6, having only two or three thousand
gates, didn't have any error recovery.  In fact, it didn't have any error
checking.  Memory parity was an add-on box that sat between memory and the CPU.

A lot of the sites that bought PDP-6's were involved with physics research.
The others included artificial intelligence research organizations and
timesharing utilities.  Apparently proving DEC's masochistic tendencies, the
first PDP-6 sold went about as far from Maynard as one could possibly go, to
the University of Western Australia, in Perth.  The second went to Brookhaven
National Labs, in an air conditioned trailer in which it was to spend its days,
the intention being to drive it between experiments (in fact, it was never
moved).  With this experience in shipping computers via truck, DEC started to
ship all its products by truck.  Twelve foot trucks.  DEC learned a lot more at
a well-known bridge on Route 62 in Hudson, Massachusetts.  An eleven foot
bridge.  (This is where DEC made its first drop shipment.)  The PDP-6 that made
this unfortunate journey was already some months late for the University of
Pennsylvania.  DEC not having its own van, had rented some space in a moving
van filled with household goods.  The PDP-6 was in the back of the van, and it
appears that the furniture successfully cushioned the impact for the computer.
They did have to shovel the remains out of the truck afterwards, however.
(The PDP-6 was able to be repaired in a couple of more months).

Two philosophies were applied to the design of the PDP-6.  The first was
expressed by John McCarthy of M.I.T., who helped design it.  That was "to
provide each user with the illusion of having his own large computer."  The
other was "gentleman's timesharing", which was the only way one could exist
with no protection and with manual sharing of all the peripherals, core memory,
system DECtapes, etc.

The early developers of the PDP-6 software used a cross assembler running on
the PDP-4.  The -4 was in another building, which required a considerable trip
to transfer software (via paper tape).  Tom Eggers debugged DDT before the
hardware was working fully.  This required that the instructions that did not
work be simulated by instructions that did work so that they could debug the
ones that were not working (for example, left shift did work, but right shift
didn't).

The first successful timesharing test on the PDP-6 consisted of two "JRST ."
(branch to current location) jobs.  The lights showed that the scheduler was,
in fact, switching between the jobs.  Immediately following that demo they
invented Control-C, because they had no way of stopping the test jobs.  Version
numbers were also developed during this period, as Harris Hyman had a habit of
labeling the Dectape of each new version of Macro as "Latest".  After six
versions of "Latest" were accumulated, they started numbering them.

P)