rarest computers. was: RE: Xerox Alto Restoration + Emulation

From: Jim Battle <frustum_at_pacbell.net>
Date: Mon Aug 2 23:50:31 2004

Dwight K. Elvey wrote:

> Hi ben
> A good picture of it can be seen at:
> http://www.omahug.org/vcf40/v02.jpg ( the Wyse terminal is not original )
> Mine has 12Kx20 of core but they also had a 24Kx20 setup
> with an expansion chassis. ... Nicolet was one of the
> last manufactures to make core bases processors and made
> processors for use on subs because of cores resistance to
> radiation.

Speaker of core memory, I used to work for a company that was still
shipping core-based systems in 1985, when I first joined them right out
of college.

The machine was the "BTI 8000", made by BTI Computer Systems, based in
Sunnyvale, CA. I worked there for only a year, but I have some
recollection of the machine.

It was a multiprocessor 32b super minicomputer with a really odd
architecture (I wish I had kept the arch manual). BTI started in the
late 60's at a timesharing service provider, became for a time
phenominally successful reselling 16b HP 3000 machines bundled with auto
dealership software, then got big for their britches and invested
everything in developing the BTI 8000.

When I joined in 1985, they were just bringing up the first DRAM based
memory board when I joined. One of the jobs I did in my year there was
to address some of the outstanding bugs in the core memory controller.
It was a microcoded controller! Just about everything at BTI was

The cards were big -- probably 30" x 36". Warping was a big problem.

Many of the cards were wirewrapped by machine, partly because at that
time making PC boards that large with enough layers was not practical.
Some cards were layed out as much as possible with a two layer board,
then the rest was put on with wirewrap. Everything was TTL (mostly S
and F) and no PALs as they were too slow (35ns) at that time.

They had a "big system" mentality despite the volume not justifying it.
  For example, many cards had diagnostic processors. The CPU had a
diagnostic Z80 that would run self test on the CPU before allowing it to
"join" the bus. Same with the memory card. Same with the IO processor.

One to eight or so processors could be in a system. Some systems had
small backplanes (6 slots?), some had big (16 slots or so) backplanes.
The CPU had no cache and a small register file. The CPUs were
completely symmetric. At a time slice the few dozen registers would be
saved off to a process state area and the next time another CPU was
free, it would pick up the process state and continue on.

You could power off the machine with the master power button, then flip
it back on 30 seconds later and (most of the time) things kept running
like nothing had happened.

The instructions were 32b, but the address space per process was 20b
IIRC. Those extra 12 bits were used for all kinds of strange addressing
modes. For example, besides the usual pointer to memory, you could have
a pointer to a register -- or a pointer to a field of a register! Yes,
there were instructions like "increment bits [17:5] of R3". The
emphasis was on density of encoding, not speed of decoding.

The CPU was on the 5th generation by the time I got there, as the
machine was originally designed in 1977 or so. The 5th gen CPU used
eight 2910 bit slices. Earlier machines had used 74S181 slices.

The bus was synchronous and ran at 15 MHz. There were low priority and
high priority requests, but only those two levels. You could do a
single read, a single write, double word read, or double word write.
Reads were split and pipelined, but a given master could have only one
transaction outstanding at a time. The system used a very clever
distributed arbitration scheme (again, with two levels for low and high
priority requests).

The system software was almost all written in assembly language. They
had their own made up language called "dragon". They had their own
editor, own mail system, compilers (BASIC, Pascal, COBOL, RPG, but no
C). Just about everything was written in-house.

Some googling indicates the company still existed in some for up until
at least 2002, still in sunnyvale. They seem to be out of business now.
Received on Mon Aug 02 2004 - 23:50:31 BST

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