Who recognizes this (UK) architecture (from 1970-1985)?

Erik Baigar erik at baigar.de
Thu Jun 16 03:23:05 CDT 2016

Hi There,

I am working on restoration, documentation of a some vintage navigation
systems from the late 1970ties, which have been designed in the UK. They
contain an archaic bit-serial computer and I'd be interested if
someone on this list recognizes the architecture and/or can confirm
my assumptions:

The bitserial computer consists of around 300 TTL chips (54xx);
it has 8 bits instructions and operates on 16 bit memory. The 3
LOWER bits of the opcode define the instruction and the HIGHER
bits the location (0-31, i.e. address) - most other architectures
I know have the instruction coded in the MSBs! Here is a list
of the basic instructions:

   1 : Load Ac from memory
   2 : Store Ac to memory
   3 : Add to Ac
   4 : Sub from Ac
   5 : IO-Instructions (32 Channels, there are some special
       channels as 0 loads AC with "0" whereas channel 31 loads
       -1 into Ac).
   6 : Shift instructions - depending on the address field,
       Ac is shifted arithmetically (preserving MSB, the sign) or
   7 : Bit test instructions - 0-15 test bits of the Ac register,
       16-31 test external digital inputs and are used for
       communication with the hardware.

And finally, put last for didactic reasons:

   0 : Here we have a bunch of special instructions depending
       on the address field, like selection of memory page,
       conditional jump, loading of data from ROM into the
       Accumulator (Ac), multiply, divide, conditional JUMP,...

What makes the architecture very unique to me is, that it has
32 bit capability, i.e. there is a "double length" flag and if this
is set, most commands operate on 32 bit (1-6, MUL, DIV).

Additionally there is a "logic flag" which causes e.g. the
instuctions ADD and SUB to switch to change their operation from
ADD/SUB to logic AND/NXOR.

Apart from this, ROM and RAM are separated (the CPU cann not exe-
cute code in RAM) and the RAM is segmented in 4 pages of 31 words.

The machine does not have got a stack, there is no subroutine call
and only just one flag used for conditional JUMPs. Via the test in-
structions (e.g. test Accu bit 3, test Ac<0) this flag is modified
and a following "Jump if Flag Set" acutally causes the conditional

As the navigation system is made by Ferranti I, already had a look
at the varouos computers made by them (Mark1, Pegasus, Atlas, and
Argus). I think given the timeline, and the word widths the Argus 600
or 700 architectures may be closely related, but the 600's command
set is quite different...


Can anyone out there confirm this? Is a instruction set listing of
the Argus 700 available somewhere?

A video on the system can be found fon YouTube although this is
not focused on the digital computer it may be of interest as
it gives a overview on the application, the projected map cockpit
display (one of the devices controlled by the system) and it shows 
my homebrew logger developed durig analysis of the box:


Interesting in this system is also the delicate mechanics and the
mix of digital computing, analog computing (platform stabilization,
compensation of cross talk errors and anisoelasticity, platform
erection, first integration from acceleration to speed) and
mechanical computing (the ingetration of turn rate happens me-
chanically within the gyrsocopes).For this reasons, these systems
are the most extraordinary masterpiece of engineering I know...

Best regards,

    Erik, Germany, Munich...

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