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Konrad Zuse's Z3 was the first working programmable, fully automatic machine; whose attributes, with the addition of conditional branching, have often been the ones used as criteria in defining a computer. The Z3 was built with 2,000 relays, had a clock frequency of ~5–10 Hz, and a word length of 22 bits.^[1] Calculations on the computer were performed in full binary floating point arithmetic.

The machine was completed in 1941. On May 12 1941 it was successfully presented to an audience of scientists of the DVL (Deutsche Versuchsanstalt für Luftfahrt, i.e. German Experimentation-Institution for Aviation), in Berlin.^[2] The original Z3 was destroyed in 1944 during an Allied bombardment of Berlin. A fully functioning replica was built in the 1960s by the originator's company Zuse KG and is on permanent display in the Deutsches Museum. In 1998 the Z3 was proven to be Turing-complete. The Z3 was used for designing aircraft.

Unlike the first non-programmable computer built by Wilhelm Schickard in 1623, the Z3 of 1941 was program-controlled.

The success of Zuse's Z3 is often attributed to its use of the simple binary system. This was invented roughly three centuries earlier by Gottfried Leibniz; Boole later used it to develop his Boolean algebra. In 1937, Claude Shannon of MIT introduced the idea of mapping Boolean algebra onto electronic relays in a seminal work on digital circuit design (see also Z1). Nevertheless, Zuse (who did not know Shannon's work) was the one who put the ideas together and made it work on the program-controlled Z3.

The first design of a program-controlled computer was Charles Babbage's Analytical Engine in the 1830s.

The ENIAC was completed 5 years after the Z3. ENIAC used vacuum tubes to implement switches, Z3 used relays (a request for funding for an electronic successor was denied as "strategically unimportant"). ENIAC was decimal, Z3 was binary. Until 1948, to program ENIAC actually meant to rewire it; while the Z3 read programs off a tape (actually a punched film). Today's computers are based on transistors instead of tubes or relays.

Z3 needed an external tape to store its program. The Manchester Baby of 1948 and the EDSAC of 1949 were the world's first computers with internally stored programs, implementing a concept frequently attributed to a 1945 paper of John von Neumann and colleagues. A patent application of Konrad Zuse, however, mentioned this concept almost a decade earlier in 1936, although the patent was rejected.

It was possible to construct loops on the Z3, but there was no conditional jump instruction (although it would have been rather straightforward to insert one). Nevertheless, there is a way of implementing a universal Turing machine on a Z3 (assuming unlimited storage and zero crashing probability), as was shown in 1998.^[3][4]

From a pragmatic point of view, however, it is much more relevant that the Z3 provided a quite practical instruction set for the typical engineering applications of the 1940s—Zuse was a civil engineer who only started to build his computers to facilitate his work in his main profession.

• Manchester Small-Scale Experimental Machine
• Manchester Mark I
• Manchester Mark II
• IBM SSEC

• Z3 page at the Technical University of Berlin
• Was Zuse's Z3 the First Programmable Computer? – Slashdot item, 7 June 2004, with several links and comments
• The Life and Work of Konrad Zuse
• Konrad Zuse?s Legacy: The Architecture of the Z1 and Z3 (PDF)
• How to Make Zuse's Z3 a Universal Computer Raúl Rojas
• The Zuse Computers Raúl Rojas