9 Mar
2007
9 Mar
'07
12:16 p.m.
Erik, well done with the reverse engineering.
I read your description of this computer with a strange feeling. I
did not recognise it having 12 bits but everything else seemed very
reminiscent of the Elliott 920 series.
Then I looked at the pictures, and saw it was made by my old
employers, Marconi Avionics who took over Elliott Brothers, and
continued to make 920 series machines. I think there were 12 bit
versions made by one of the divisions, though I am surprised they
were still making them in the 1980s.
One part of Elliott Brothers became GEC Computers (based in
Borehamwood).
One of your labels is from Airborne Displays Division based at the
same Rochester site as I worked. This was also part of Marconi
Avionics, and the company later changed its name to GEC Avionics. I
worked on compilers, linkers and other utility software for the 18
bit 920s before moving on to the Zilog Z8001.
Erik, could you tell me if the instruction code is anything like this:
0 Load B (indexing) register and the Q register (shift extension)
1 Add to accumulator
2 Negate and add to accumulator
3 Store the Q register
4 Load accumulator
5 Store accumulator
6 And to accumulator
7 Jump if zero
8 Jump
9 Jump if negative
10 Increment
11 Store program counter (for function return)
12 Multiply
13 Divide
14 Shift
15 Input/Output and special (like interrupt return)
16 to 31, as above but indexed by B register.
In early versions of the 920, the B and Q registers were the same
register and it and the program counter were held in memory. There
were four levels of interrupt and a set of these registers for each,
held in location 0 to 7. The high end of memory held a paper tape
bootstrap, in later versions, this was just copied into core when the
machine was initialised.
If this indeed a military machine, you can be sure the memory was
erase by flipping every bit backwards and forwards several hundred
times to remove any trace magnetism before it was released from the RAF.