Editing Z3 (computer) (section)

== Z3 as a universal Turing machine ==
It was possible to construct loops on the Z3, but there was no [[conditional branch]] instruction. Nevertheless, the Z3 was [[Turing-complete]] – how to implement a universal [[Turing machine]] on the Z3 was shown in 1998 by [[Raúl Rojas]]. He proposed that the tape program would have to be long enough to execute every possible path through both sides of every branch. It would compute all possible answers, but the unneeded results would be canceled out (a kind of [[speculative execution]]). Rojas concludes, "We can therefore say that, from an abstract theoretical perspective, the computing model of the Z3 is equivalent to the computing model of today's computers. From a practical perspective, and in the way the Z3 was really programmed, it was not equivalent to modern computers."<ref name="Rojas_1997_Universal"/>

This seeming limitation belies the fact that the Z3 provided a ''practical'' [[instruction set]] for the typical engineering applications of the 1940s.  Mindful of the existing hardware restrictions, Zuse's main goal at the time was to have a workable device to facilitate his work as a [[civil engineer]].<ref name="Zuse_1987"/>