29 Nov
2022
29 Nov
'22
9:54 p.m.
In 1982, I returned to BRL (having spent a year working for Martin
Marietta on project involving a dual process RSX-11M system. I did
spend some time setting up the PWB UNIX tools with the QA guys while
there).
BRL had commissioned the Denver Electronics Corporation (i.e., Denelcor)
to build a MIMD system called the HEP, ostensibly to do fluid dynamics
problems. It was looking like the manufacturer wasn’t going to have a
useful OS for it so Mike Muss did his usual thing and suggested he could
put UNIX on it (he had done this twice previously with orphaned
machines: some 11/34 graphics stations that were supposed to be
connected to the CDC7600 and converting the University of Illinois
Arpanet Terminal Server (ANTS) to a host that could support long
leaders.
I was part of Mike’s team. We split up various aspects of the project.
Mike ported PCC. I ported ld, and F77 (as we figured the
“scientists” would probably want that). Mike did the core of the
kernel. I handled all the I/O system because I was the “device driver”
guy at JHU and continued on with this at BRL. Our initial work was
done by traveling out to Denelcor’s facility where our machine was
assembled on their floor. They had started their operating system work
(not UNIX), so we took the machine after hours. Amusingly, at BRL in
the room the HEP was going in, the cabinet layout was taped out on the
floor around a central pillar. The tape marked with the component that
went there. Out in the Denver machine room, the equipment was there
but there was a square taped out that said “BRL post” so they knew not
to put/route anything there.
The software being ported was 4BSD. I believe we started with the
George Gobels mods of the dual VAX (BRL already had built one of these).
In working on that I found there was a rather serious portability
problem. The kernel did what I refer to as “conversion by union”
where they stuck a pointer of one type into one element of the union and
retrieved it by a different pointer type union element. Well, such is
all well and good on the VAX, but the HEP encoded the partial word size
into the low order bits of the pointer. This meant that you ended up
with pointers that would access the wrong size operand. I spent a few
days chasing down all these things and fixing them.
The HEP had an 11/34 as a control processor that allowed you to
configure the switch system that interconnected the memory in the
processors. It did this via a special feature called (aptly) “The Low
Speed Bus” (more on that later). But once running, everything was self
contained on the HEP hardware itself. Other amusing things about the
HEP is it had a lot of registers. It also had this stuff called “table
memory” which essentially were read-only registers. You could but a
variety of constants there (we had all the integers from 0-255 and then
powers of two after that). The rest of the memory had an extra bit
per 64-bit word called the “full empty” bit. While the thing was
capable of running the equivalent of 32 UNIX processes simultaneously,
it also had the ability within each of those to hardware schedule
threads. Many instructions had features that used the extra bit like
“WAIT FULL, READ, SET EMPTY” or “WRITE, SET FULL”.
The whole thing was built out of Motorola 10800 ECL. All hand
assembled. Quite impressive.
After we got the system up, and Mike tried to port his ray tracing
software to it, we found the I/O system was beastly slow. The I/O
system was a cute device. It had a bunch traditional memory (no
full/empty bits) connected to the main memory via the switch but also to
32 individual UNIBUSes. The only problem was that in order to whack
the registers on the UNIBUS you had to do it through the “Low Speed
Bus.” This was the performance bottleneck. The original designer
of this thing, Burton Smith and I sat down at the Golden Corral
Steakhouse in Aberdeen and worked on a new plan. The key was to allow
the kernel tasks to talk directly to the UNIBUS registers via the
regular memory switch. Fortunately, we had an extra switch node in
spare parts. We did however need software and another PDP-11 to
control the thing. Fortunately, I had spares from my building of the
early internet router: BRL-GATEWAY. I also used the little OS on it
(LOS) to write the control software. In a couple of days, we had
rebuilt the I/O system out of essentially spare parts and it flew.
Amusingly, this was the first of two UNIX ports I worked on that didn’t
use the standard ISR thing that most of the kernels those days did.
After the device driver on the HEP had started the IO up, that thread
just ended. When the UNIBUS interrupt came in, a new thread was
created to service it. I later did a similar strategy with the
message passing coprocessor on the Multibus II.
This was a fun waste of about a year. By the time we got this ready
for prime time, the Army had decided they didn’t have any supercomputers
(other than HEP) and immediately used defense priority to snag the next
Cray X/MP off the assembly line (which had been slated for Apple).
Mike got one run of his raytracer to make a movie: The artillery
shell’s view of collision with a tank. The HEP could generate the
frames about 60:1 (took a minute to make one second of movie time)
faster than anything else we had in the day. Of course, it pales
compared to your modern video games. After that, the HEP was
dismantled (Denelcor had gone out of business) to make room for the
Cray. I recently just found pictures of us all sitting on the “seats”
of the Cray and one of me peering out from the core of the machine.
The Army went on for plans for multiple Cray 2 purchases. After a lot
of amusing “systems engineering” work that originally slated for all of
them to go in Oak Ridge Tennessee, and then all of them to go at BRL,
they decided to spread them around the country (what I told them they
should do in the first place). My last months at BRL, I signed my
name to the $25 million dollar Cray 2 procurement, but it got delivered
after I left (I visited it a few years later).
Somewhere I still have the name plates for the Denelcor HEP H1000.
They, in fact, were never actually installed on the machine,