Friday, 7 October 2016

The World's First Overclocked Computer

article from Dave's Amiga Hardware Page


The history of overclocking is as long as that of electronic computers. In fact overclocking was successfully attempted on the world's first electronic computer. Back in the early 1940s, the British designed and built a number of computers specifically designed to break German codes. Known as Colossus, the information about these computers was only declassified a few years ago, as some of the concepts are still in use today.

Colossus was made in two main versions - a single MKI and ten examples of the more capable MKII. Each Colossus contained 2,500 valves and was able to find the key needed to decode intercepted German messages in as little as a few minutes or up to 2 hours. Once found the key could be used to decode all other messages for the remainder of that day (the Germans changed the key each day). Data in the form of an intercepted message was fed into Colossus by means of punched paper teleprinter tape. This was made in the form of a loop so that the message could be processed repeatedly until the key was found. The clock signal was generated from the sprocket holes on the paper tape, which ensured that the computer acted in synchronism with the input data.

Obviously to get the key as quickly as possible, the punched tape had to be run as fast as possible through the punched tape reader. The previously used mechanical readers obviously had limitations, and could only work at 75 baud, which was clearly inadequate, so the designers of Colossus decided to build an optical tape reader. The whole system was built around a data rate of 5,000 characters/sec, corresponding to a clock frequency of 5KHz (Colossus processed it's data in parallel). To get this kind of performance, the paper tape had to run at 30 miles per hour!

Despite this, the engineers who built and used Colossus were bitten by the overclocking bug, and decided to see how fast they could get it to go. In this case it was simply a matter of increasing the speed of the punched tape. They were able to double the speed to 60 miles/hr or 10KHz before the paper tape started unravelling itself from the spools and filling the room with a sea of shredded paper! Most modern overclockers could only dream of doubling the speed of their computer!

A team in Britain is now building a full working replica of Colossus. Maybe I could repeat the overclocking experiments for them? There was also a fascinating two part series on TV called "Station X" which dealt with wartime codebreaking. Part 2 dealt with Colossus and it's overclocking. If you can, see it. I found it fascinating.

Given that the limits to overclocking for Colossus was it's data input hardware, I wonder how fast Colossus would be able to go if it's data and clock was supplied by another means that did not have the limitations of paper tape. As I have never closely examined Colossus or it's circuit schematics, it's impossible for me to have any good idea. However, given that it uses valves like the 6V6 and was mostly designed and built by Post Office engineers, I would assume it would be somewhere within the audio range - up to about 20KHz....or maybe a little more.

Is there anybody out there who has heard of any other overclocking attempts on old computers? My own experiences date back to 1975 and a calculator. I would be very interested in hearing of other stories.

First Published: October 22nd 1999
Dave's Amiga Hardware Page