Manchester Mark IThe Manchester Mark I was one of the earliest electronic computers, built at the University of Manchester in 1949.
In its final specification (October 1949) it stored data in one 40-bit number (the accumulator) or two 20-bit instruction registers, and had two 20-bit address modifier registers. It could perform serial 40-bit arithmetic, with hardware add, subtract and multiply and logical instructions. It used single-address format order code with thirty function codes. Standard instruction time was 1.8 milliseconds, but multiplication was much slower.
For memory the Mark I used two Williams tubes, each storing 64 rows of 40 points, for a total of 128 words. 64 words was considered to be a single "page", so the system stored 4 pages. In addition to the tubes were a magnetic drums, which could store 32 pages each. Instructions were loaded into the machine by using paper tape.
The Mark I was developed from the Small-Scale Experimental Machine (SSEM), nicknamed Baby, developed by Frederic C. Williams and Tom Kilburn, which ran its first successful program on June 21 1948. The Manchester Mark I led to the Ferranti Mark I, the first commercially available computer.