[fpc-devel] [Slightly off-topic]Mark Morgan Lloyd: Burroughs emulator

[Mark Morgan Lloyd]

Reinier Olislagers wrote:
> Hi Mark (& all),
> 
> Just wanted to know how you're doing with that conversion of the
> Javascript Burroughs B5500 emulator that you said you were looking at...
> 
> Wonder if you have interesting stories about performance to tell us ;)

Noting obviously that I'm not the prime mover of this, and that I'm 
mainly transcribing code from Javascript to Pascal to run on unix.

As background: the B5x00 was an early-60s design using transistors and 
diodes rather than valves, assembled as modules and plugged into a 
wire-wrapped backplane (which, incidentally, is why I was digging for 
info on the Lawrence Livermore S-1 computer a couple of weeks ago, hence 
the Stallman comment I posted elsewhere). The B5000 was drum based, the 
B5500 used discs (/big/ discs), and the B5700 could cluster up to 4x 
systems around shared disc. Later machines had 2x CPUs, with one mainly 
handling the operating system and the other devoted to user code. These 
three machines were not code-compatible with the later B6xxx or B7xxx 
range, or with the B5900.

The operating system was written in an ALGOL derivative, with fairly 
heavy use of embedded assembler. The frequently-repeated statement that 
these machines were programmed entirely in ALGOL and that there was no 
assembler is a simplification. Dijkstra liked them, Knuth and Wirth used 
them on occasion; Gary Kildall was involved in writing an APL 
interpreter at IIRC Washington.

I'm starting off with a command-line driven variant, i.e. there's no GUI 
and controls are simulated by typed-in commands using a parser I wrote 
for something else. I've transcribed the CPU, central control and IOPs, 
and am currently working on I/O devices starting off with the SPO 
(Supervisory Printer Output) and card readers/punch. Importantly, as I 
go I'm adding a fairly extensive set of test/debug commands.

I'm hoping to later put together a GUI using Lazarus, and to allow 
symbolic debugging of the operating system etc. by transferring compiler 
output listings from the emulated environment to the host. But I'm a 
long way from that stage.

I've got to the point where I can enter a 48-bit I/O descriptor at which 
point the first free IOP's thread spins once to e.g. read a card into 
memory. I'm currently tidying up the code but at this time of year I've 
got a lot of other demands on my time which are causing problems.

I'm not sure whether to tackle more I/O devices next (tape or disc), or 
to start debugging the CPUs mow that I can read a loader deck into memory.

I'm hoping to be able eventually to get a cluster running, i.e. 4x 
twin-CPU systems sharing common disc. Good old mid-60s technology ;-)

I'm also hoping to be able to support e.g. non-standard tape formats by 
loading shared libraries, rather than hardcoding them. There's a number 
of projects that emulate mainframes from the 7-track tape era, but like 
the original systems they're very poor at reading each others media.

In the longer term, it would be nice to get a Pascal compiler running 
(there's a listing on Bitsavers). However that would probably need a lot 
of typing, which takes me back to a large-scale collaborative project 
that's something else I'm trying to work on. And which is, as they say, 
yet another story.

-- 
Mark Morgan Lloyd
markMLl .AT. telemetry.co .DOT. uk

[Opinions above are the author's, not those of his employers or colleagues]