The Z1: Architecture and Algorithms of Konrad Zuse's First Computer

This paper provides the first comprehensive description of the Z1, the mechanical computer built by the German inventor Konrad Zuse in Berlin from 1936 to 1938. The paper describes the main structural elements of the machine, the high-level architecture, and the dataflow between components. The computer could perform the four basic arithmetic operations using floating-point numbers. Instructions were read from punched tape. A program consisted of a sequence of arithmetical operations, intermixed with memory store and load instructions, interrupted possibly by input and output operations. Numbers were stored in a mechanical memory. The machine did not include conditional branching in the instruction set. While the architecture of the Z1 is similar to the relay computer Zuse finished in 1941 (the Z3) there are some significant differences. The Z1 implements operations as sequences of microinstructions, as in the Z3, but does not use rotary switches as microsteppers. The Z1 uses a digital incrementer and a set of conditions which are translated into microinstructions for the exponent and mantissa units, as well as for the memory blocks. Microinstructions select one out of 12 layers in a machine with a 3D mechanical structure of binary mechanical elements. The exception circuits for mantissa zero, necessary for normalized floating-point, were lacking; they were first implemented in the Z3. The information for this article was extracted from careful study of the blueprints drawn by Zuse for the reconstruction of the Z1 for the German Technology Museum in Berlin, from some letters, and from sketches in notebooks. Although the machine has been in exhibition since 1989 (non-operational), no detailed high-level description of the machine’s architecture had been available. This paper fills that gap. 1 Konrad Zuse and the Z1 The German inventor Konrad Zuse (1910-1995) built his first computing machine from 1936 to 1938 (from 1934 to 1935 he experimented with small mechanical circuits). In Germany, Zuse has always been considered the father of the computer although the machines he built during WWII became known only after the conflagration. Zuse studied civil engineering at the Technische Hochschule Charlottenburg (today’s Technical University of Berlin). His first employer was the company Henschel, who had just started building military airplanes in Berlin in 1933 [1]. The duty of the 25 years old was to carry out the long chains of structural calculations needed for the manufacturing process of aircraft components. As a student, Zuse had already started thinking about ways of mechanizing computation [2]. Therefore, after working just several months for the Henschel Flugzeugwerke, he decided to quit, build a mechanical computer, and start his own business, in fact, the first computer company in the world. During the period 1936-1945, Konrad Zuse was unstoppable, even after two short-lived calls to the front. He could manage to be recalled to Berlin to work part-time for the Henschel Flugzeugwerke, and part-time for his own company. In those nine years, he built the six computers known today as the Z1, Z2, Z3 and Z4, as well as the specialized S1 and S2 machines. The last four were built after WWII had already started. The Z4 was finished during the closing months of the war. Zuse’s original abbreviations for the machines’ names were V1, V2, V3 and V4 (meaning “Versuchsmodell”, or prototype). After the war, he changed the V for a Z for obvious reasons. The V1 (Z1 in what follows) 1 The precise chronology of his line of computing machines was provided by K. Zuse in a small handwritten note from March 1946. There, the V1 is dated as having been built in the years 1936-1938.