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Konrad Zuses Z3 in Detail

Konrad Zuse built the machine Z3 from 1939 to 1941 in the Methfesselstraße 7 in Berlin-Kreuzberg with some friends and a small support by the government. With the Z3 Konrad Zuse wanted to show, that it is possible to build a reliable working machine for very complicated arithmetic calculations, which is freely programmable and is based on a binary floating point number and switching system. For reliability reasons he used relays for the entire machine.

Konrad Zuse rebuilt the Z3 in 1960/61 in his Zuse KG in order to show the patent justice and the world the performance of this machine and to demonstrate this machine for advertising purposes. Today, it is undisputed, that the Z3 was the first freely programmable, based on a binary floating number and switching system, really working computer of the world. The Z3 did not store the program in the memory, but it contained in 1941 almost all the components of a modern computer as required by John von Neumann et al. in 1946 /BURK46/. One exception is the ability to store the program in the memory together with the data. Konrad Zuse did not implement this feature in the Z3 because the memory of 64 words was too small for this. He wanted to calculate thousands of instructions in a meaningful order. The machine he constructed should be freely programmable. For this reason, he only used the memory for values or numbers. The block structure of the Z3 is very similar to a modern computer. The Z3 disposed of separate units, like punch tape reader, control unit, floating point arithmetic unit and input- / output devices. In as much as the completed device will be a general-purpose computing machine it should contain main organs relating to arithmetic, memory-storage, control and connection with the human operator. It is intended that the machine be fully automatic in character, i.e. independent of the human operator after the computation starts. It is important to notice, that the Z3 contained organs like an arithmetic unit, memory-storage, control and connection with the human operator. The machine Z3 contained a special operating modus. With the instruction Lu the input device was activated and the program was stopped. The human operator could check, among others, the Registers R1 and R2 of the arithmetic unit and he could make intermediate calculations using the Registers R1 and R2. Then he could proceed the program. This is a very early interactive communication with the machine.

Let us consider the discussion of floating point number. It would therefore seem to us not at all clear whether the modest advantages of a floating binary point offset the loss of memory capacity and the increased complexity of the arithmetic and control circuits. It is very important to notice, that floating point numbers were discussed as late as in 1946 by Bmany scientificst and they were not really convinced that this is a good concept. Konrad Zuse described in 1934 the concept of a floating processor and implemented in 1936 such a floating point processor. This is ten years before Burks et al. postulated it as a concept for modern computers (not a built machine). As shown above and will be shown below, Konrad Zuse also implemented a powerful arithmetic exception unit. Let us consider the logical concepts of the Z3 in detail.

Slav Petrov 2001-07-02