True Story No20


What would you do in four minutes?

In the fifties Britain probably led the world in real-time computers.

Computers were gigantic and those on the drawing board even bigger.

Silicon devices weren't yet around, at least in commercial quantities, and certainly not at economic prices.

Integrated circuits hadn't surfaced and people designed their mammoth processing equipments with germanium transistors.

Before even those were available in quantity, and more importantly in a reliable form, neon tube devices were in use.

Early electronic computers developed in WWII had used valves and later included gas-filled things called thyratrons. 12SN7 tubes, which were double triodes had been popular and could be used to make single bit memory cells and eight of these, for example, were used to make an elementary storage device called a register.

To reduce size and power consumption a bank of neon devices could also be used to make a rudimentary register but these were very unpredictable. An early computer prototype, using these devices at a certain Government establishment worked OK until someone opened the door of the rack. Then the change to ambient conditions from room lighting affected the triggering of the neons and we had what was probably the first computer crash (predating Windows by over 40 years).

Using experience gained on this early model the first germanium transistor based computer prototypes were developed.

The first machines being designed and built in a purpose built factory in the North West of England, under the vague guise of telephone equipment, and later installed near London to form the heart of the system designed to provide the UK with a 4-minute warning of impending Soviet attack.

The initial computers called XL2, later supplemented by the XL4, took their names from the building in which they were born (Exchange Laboratories) and were manufactured in the early 60s.

No more than 25 of these gigantic data processors were installed in a large building which, whether by plan or accident, or for some other reason, was located in a housing estate next to a main railway line!

I use the term "gigantic" because, including their interfacing hardware, the 25 machines filled some 1000 racks, each standing taller than a man.

Interestingly, during the years of the hardware production, silicon transistors could have been used.

New computers were of course being developed and these, not only used discrete silicon devices but also very early integrated circuits.

"The powers that be", were unsure of the new technology and, it is said, because of the unknown reliability of such things vetoed their introduction except in a very minor role.

In fact as experience was gained, reliability of silicon transistors proved to be far better than that of their germanium counterparts and integrated circuits even better than this. Not only would reliability have been improved but device speed would have been dramatically increased and power requirements much reduced. But that's the way things were.

Being made from germanium transistors the computers were much more complex than equivalent designs based on silicon.

Early transistors suffered from thermal runaway, so biasing of the devices was critical, and the extra circuitry demanded required more current.

Enormous power supplies, providing many hundreds of amps, were built, and these, backed up by lead acid accumulators, provided the several different voltages which were distributed by large-section copper bus bars to minimise resistive losses. When one considers a building of maybe 300 feet in length and perhaps half of this in width, the complexity of the power distribution problem can be imagined.

Cabling of the computers was also a mammoth task, demanding a complete floor of the building specifically for this purpose.

Cooling, another huge undertaking, calling for trunking to each and every rack so that the germanium devices did not cook.

Apart from the real time data processing for providing signals to radar consoles there was also raw radar processing which occupied another mammoth system in an adjacent room of similar proportions and the cabling of signals between this area and the radar console floor above holding scores of giant displays, was in keeping with the scale of the system.

Probably the biggest challenge though, was not so much the hardware, because once designed, could be mass produced, and although reliability was finite, this could be calculated and provision made for failures.

The biggest challenge was software.

In parallel with the hardware development of the first real-time solid state computers was the specification and design of the means of programming the machines.

At the lowest level one used machine code through which the hardware could be directed with precision. Above this was assembler level and above this a high level language which provided a general way of communicating with the hardware for which a programmer did not need to understand the intricate operation of the computer.

A new real-time language, loosely based on the contemporary "CORAL" language, called "MINI-CORAL" was written for the XL computers and provided the first insight into the difficulties of getting computers to perform their tasks.

Because huge teams of people had to be involved there was clearly a need for intercommunication and this demanded tons of documentation and a carefully structured organisation.

Translating performance specifications into computer tasks using systems analysis was embryonic and nearly all the staff involved would have had no previous experience and had to learn on-the-job.

Anyone that's worked in Industry must be aware that everything that's ever been written down is subject to errors. These include misunderstandings, incompetence, plain mistakes and failure to advise or communicate with other people involved with associated software routines to which a particular program may interface. On top of this add typographical errors and compilation problems due to the conversion of the high level or assembler level code into machine code.

All these things made life for the first systems programmers next to impossible and the science of Configuration Mangement, or the rigorous control of changes, which commenced following the first Apollo shots was unknown in the UK.

Input to computers was via paper tape using crude electro-mechanical machines and involving huge rolls of paper tape which had to manually respooled after loading.

Correcting errors usually required a second reel of tape, which contained program patches, to be loaded. This was fraught with problems as it was necessary to distribute the patches to other teams so that everyone was working with the same up to date information.

Sometimes of course patches were unofficial, and didn't work, and merely one of several attempts to overcome a problem.

Periodically patches were assimilated into re-compilations when everyone resynchronised their program build. This recompilation would often generate more problems than had been solved.

All in all for an undertaking of this magnitude it was virtually impossible to complete the job before the whole system became obsolete.

Programmers would move on to better things, losing the project valuable experience, and those remaining would be overloaded and unable to cope and of course new recruits would need to be trained and this would dilute the efforts of the others.

Any major computer system will suffer from these problems, and the faster does computer hardware evolve, the more the difficulty in completing to a customer's satisfaction, the task of getting everything to work properly.

If one is philosophical, one could say that to a large extent, a major defence contract like the one described above, need never be completed as long as it is never actually required to be used for its prime purpose.

If it added to our overall deterrent, and I'm sure it did, then even if it never worked properly, or had to be scaled down to a shadow of its original promise, it had at least been useful in that role.

My memories are fading fast but I do remember the huge screen showing really useful information with the illuminated legend above, fortunately not counting down, with those chilling words "MINUTES TO IMPACT... 4".

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