Manchester Mark I

The 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.

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