source unknown 2014

My favorite dinosaur was the GP4B. It was designed specifically for flight
simulation. If you Google “link gp4 computer” you can find some references, but
not a lot. I started at Singer in 1978 in a group we called the “Mod Squad”. The
basic design of the GP4 was 10 years old at that point.

The machine was a drum computer. Picture a roll of paper towel with the paper
replaced by what looked like magnetic tape on the outside. That held the oxide
that your programs went onto. The drum spun around at a few thousand rpm. It had
no operating system. It had a Fram airfilter on top of the drum that you could
buy at Sears for your Dodge Dart. You basically programmed it in machine
language and the instructions were placed on quadrants on the drum. There were
20 sets of quadrants arranged vertically along the cylinder with 20 different
read/write heads. Drums only spin one way, so you couldn’t do any looping in the
conventional sense.

There was a separate memory (maybe 8 or 32K) with the first 1K addressable as
individual bits for Boolean variables. Variables were not named and were direct
addresses. It had a 25 bit signed magnitude word. Not twos complement. Signed
magnitude. The number 15 would look like 0x00000F. The number -15 would look
like 0x100000F, except we used Octal because it looked better with 24 bits.

There was one accumulator and you could see it in the chassis because there was
an 8x8 inch PCB for each bit of the accumulator. Sometimes a bit would break,
and you would replace one board out of the 25 to fix it. It didn’t have any ICs,
it was all discrete ECL logic from parts you could buy at Radio Shack.

Arithmetic was done as scaled fixed point. Everything was integer, but you had
an imaginary radix point associated with every variable. So every value was a
crude approximation. If a variable with 6 imaginary fraction bits was multiplied
by a variable with 4 imaginary fraction bits the result would have 10 imaginary
fraction bits. In that case the answer would have 14 bits to the left of the
imaginary radix point and 10 to the right. If that was then assigned to
something that had 8 imaginary fraction bits, you would right shift by 2 before
storing.

Besides that “general purpose” aspect of the box, it had other features that
were decidedly not general purpose. Buried inside was the DAFG — the Digital
Arbitrary Function Generator. This was a completely separate beast with its own
programming. Basically it was a fast hardware lookup table and interpolator. You
gave it an array of values that represented a curve, and an input variable
address and an output variable address, It would bang the input variable against
the curve and put the answer in the output variable. It would do this as fast as
possible, cycling through all the functions that you told it to do. Let’s say
fuel consumption is a non-linear curve based on engine rpm. Maybe you have 20
specific data points on the curve that have been determined from real world
measurements. The DAFG would look at the variable that you told it was engine
rpm, and it would output the lookup into the variable that you told it was fuel
consumption.

The last weird thing I remember about this computer was that it had a Morse code
generator built into it. You put in a table of the latitude/longitude pairs of
all the nearby airports and their call letters. The hardware would look at the
current lat/long and using geometry would figure out the closest airport. Then
those call letters would be sent to the Morse code generator and the pilot would
hear the call letters on his headset. If you were close to Laguardia, you would
hear LGA, LGA, LGA, in Morse code. If you were close to San Francisco, you would
hear SFO, SFO, SFO.

Supposedly the Air Force wanted a new one of these 10 years after the last one
had been built. I think Singer told them the price would be $10 million. The DoD
was pretty flush in the 1970s. Maybe they paid it. These were used for the
flight simulator of a variety of F111 models. There were simulators at Pease AFB
in New Hampshire, Clovis NM, Lakenheath England, and elsewhere.