On Mon, 12 Mar 2018 01:16:16 -0500, Michael J. Mahon wrote:

> As one of the designers of the successors to the B2500/B3500, and one of
> the “variable microcode” implementors of the B1700/B1800/B1900 line of
> machines, I can tell you that you are thinking of the B1700 (small
> systems) line, not the B2500 (medium systems) line.
>
Fair enough, and thanks for providing such a good summary of the
Burroughs range.

The ICL 1900 range were all fairly simple beasts that used 24 bit words,
split into 4 6-bit ISO-code characters and did not implement a stack.
There were only two hardware registers, which held datum and limit for
the executing program. Since all program addressing was relative to
datum, and the first few words of a program contained the 8 program-
accessible registers PC and instruction result bit flags, programs could
be easily swapped in and out of RAM without regard for where they were
reloaded. They also used 'extracodes' - each of these was an opcode that
was implemented by an OS module. Some instructions, e.g. for i/o were
implemented as extracode on all 190 models, while others, e.g. floating
point operations were hardware operations on the bigger machines and
extracodes on lesser systems. The effect was that any 1900 program would
run on any system with enough memory and the appropriate peripherals.

> Burroughs was a great environment for studying comparative computer
> architecture, because they had so many of them! ;-)
>
:-)


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Martin    | martin at
Gregorie  | gregorie dot org

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