The Architecture of Konrad Zuse’s Z4 Computer

This paper describes the programming architecture of Konrad Zuse’s Z4 computer. The machine’s logic was implemented with telephone relays while the memory was a mechanical attachment. It was the successor to the Z3 machine completed in 1941. The Z4 was assembled over the next four years, until German capitulation. In its first embodiment, the machine featured 64 words of mechanical memory, two CPU registers, one punched tape reader and one tape puncher. The keyboard accepted decimal input, but the internal numerical representation was fully binary, based on a particular floating-point format. The computer had a relatively large instruction set of arithmetical operations. In 1949, the Swiss Federal Institute of Technology in Zürich decided to lease-buy the Z4 from Zuse’s fledgling computer company. An additional punched tape unit for reading auxiliary programs or number tables was added, and the instructions necessary for calling subprograms, as well as the conditional jump, were included in the instruction set.