Manchester Mark 1 - Development and Design

Development and Design

The SSEM had been designed by the team of Frederic C. Williams, Tom Kilburn and Geoff Tootill. To develop the Mark 1 they were joined by two research students, D. B. G. Edwards and G. E. Thomas; work began in earnest in August 1948. The project soon had the dual purpose of supplying Ferranti with a working design on which they could base a commercial machine, the Ferranti Mark 1, and of building a computer that would allow researchers to gain experience of how such a machine could be used in practice. The first of the two versions of the Manchester Mark 1 – known as the Intermediary Version – was operational by April 1949. However, this first version lacked features such as the instructions necessary to programmatically transfer data between the main store and its newly developed magnetic backing store, which had to be done by halting the machine and manually initiating the transfer. These missing features were incorporated in the Final Specification version, which was fully working by October 1949. The machine contained 4,050 valves and had a power consumption of 25 kilowatts. To increase reliability, purpose-built CRTs made by GEC were used in the machine instead of the standard devices used in the SSEM.

The SSEM's 32-bit word length was increased to 40 bits. Each word could hold either one 40-bit number or two 20-bit program instructions. The main store consisted of two Williams tubes each holding an array of 32 x 40-bit words – known as a page – backed up by a magnetic drum capable of storing an additional 32 pages; the capacity was increased to 128 pages in the Final Specification version. The 12-inch (300 mm) diameter drum, initially known as a magnetic wheel, contained a series of parallel magnetic tracks around its surface, each with its own read/write head. Each track held 2,560 bits, corresponding to 2 pages (2 x 32 x 40 bits). One revolution of the drum took 30 milliseconds, during which time both pages could be transferred to the CRT main memory, although the actual data transfer time depended on the latency, the time it took for a page to arrive under the read/write head. Writing pages to the drum took about twice as long as reading. The drum's rotational speed was synchronised to the main central processor clock, which allowed for additional drums to be added. Data was recorded onto the drum using a phase modulation technique still known today as Manchester coding.

The machine's instruction set was increased from the 7 of the SSEM to 26 initially, including multiplication done in hardware. This increased to 30 instructions in the Final Specification version. Ten bits of each word were allocated to hold the instruction code. The standard instruction time was 1.8 milliseconds, but multiplication was much slower, depending on the size of the operand.

The machine's most significant innovation is generally considered to be its incorporation of index registers, commonplace on modern computers. The SSEM had included two registers, implemented as Williams tubes: the accumulator (A) and the program counter (C). As A and C had already been assigned, the tube holding the two index registers, originally known as B-lines, was given the name B. The contents of the registers could be used to modify program instructions, allowing convenient iteration through an array of numbers stored in memory. The Mark 1 also had a fourth tube, (M), to hold the multiplicand and multiplier for a multiplication operation.