Marconi Transistorised Automatic Computer (T.A.C.)

The Marconi Transistorized Automatic Computer was the first computer built by Britain's Marconi Company. It was designed and manufactured from around 1959.
They employed germanium transistors which by this time were sufficiently reliable with room temperatures kept below about 23 degrees C. Apart from transistor failure, other common faults included power supply capacitors 'drying out' resulting in excess ripple, and poor connections on the input/output highway. Paper tape peripherals had their own poor reliability, influenced to some degree of operator usage.
The type S3301 was a 500 kHz clocked, 20-bit word machine with two Mullard core memory stores providing 4k of 20-bit data. The internal CPU logic was synchronised to even and odd clock signals and special signals generated via microinstruction diode boards. The memory logic had slow and fast loops to speed the transfer of sequential data bursts. Its microinstructions were programmed on diode boards. A facility was provided to microstep through instructions to help with fault-finding. Processor status bits were provided with machine instructions being decoded from 6 bits in the current address memory word. Double word data had the MSB designated a sign bit coded as binary fractions (-1 to +1) for the square root, multiply and divide instructions. The instruction set had the usual functions based on three registers named A, B and D (C was the current address in memory register, M). An additional instruction assisted with checksum calculation for data transferred to and from main data stores (viz. Sperry Rand magnetic drums).
Applications included marking up radar screens with aircraft information and providing data processing for operators in a nuclear power station.
Surviving computers (ex power station) are on display at Bletchley Park National Museum of Computing (operational) and Jim Austin's collection near the University of York. Copies of original manuals and documentation exist at the Department of Computer Science, University of Manchester and the Manchester John Rylands Library. A performance summary specification is available online.
Marconi went on to develop the Myriad series of computers.