The Z4, the second general purpose computer, was almost completed in 1945. The Z4
was an customer order by the Henschel Aircraft company in 1942. However, it was not
possible for Konrad Zuse to get the Z4 operative before his escape from Berlin to
Hinterstein March 16, 1945.
At March 16, 1945, Konrad Zuse, his wife and some employees of his small company
Zuse-Apparatebau left Berlin with the Z4 packaged in 20 boxes.
The Z4 was reassembled in the years following 1945.
From July 11, 1950, this configuration was used for five years at the Institute of
Applied Mathematics at ETH Zurich. In 1950, the Z4 was the only operational computer
In 1955, the Z4 was transferred to the Institut Franco-Allemand des Recherches de
St. Louis in France, where it was in use until 1959. Today, the Z4 is on display
in the Deutsche Museum in Munich.
Z4 - Planfertigungsgerät (Program making support feature)
The Z4 had a Planfertigungsgerät. It was a unit in order to produce punch tapes as
a program with instructions for the Z4 in a very easy way. For this reason it was
possible to learn the programming of the Z4 in at least three hours. The Planfertigungsgerät
already was a part of the Z4 in 1945.
It was possible to use symbolic memory cells, instructions and symbolic arithmetic
operations for the creation of a program. It also was possible to copy programs and
to make corrections.
Performance of the Z4
From 1950-1955 the Z4 processed approx. 100 different projects. The Z4 executed approx.
100,000 instructions. For extern projects the costs were one Rappen (0.01 CHF) per
Instruction Set of the Z4
The Z4 had a large instruction set in order to calculate complicated scientific programs.
The instruction set of the Z4 was formulated in 1942. The Z4 had a performance of
1000 instructions per hour.
* Instruction A n: For example A 17. It reads the contents of memory cell 17 into
Register R1. If Register R1 is occupied, the contents is loaded into Register R2.
* Instruction S n: For example S 18. It writes the contents of Registers R1 into
the memory cell 18.
* As arguments for the arithmetic operations the Registers R1 and R2 were used. For
monadic operations the Register R1 is used and the result occurs in Register R1,
too. For dyadic operations both Registers R1 and R2 are used and the result occurs
in Register R1. The contents of Register R2 is deleted.
* Monadic operations are: + - × / MAJ (Maximum) Min.
* Instructions for comparison (x=0, x>=0, IxI = infinity) are testing the number
in Register R1 and set Register R1 to +1 if the condition is fulfilled, if not, then
the contents of Register R1 is -1. This instruction was planned in 1942 but not realized
because of the war.
* The conditional branch SPR was a special requirement by the ETH. The instruction
SPR can be used after a test instruction. It skips the punch tape to the instruction
ST, if the contents of Register R1 is +1. If the contents of the register is -1 then
there is no impact.
* Instruction UP: The Z4 had two punch tape readers. In the original version up to
6 such readers were planned. The instruction UP switches from the main punch tape
reader (A0) to the sub punch tape reader (At1). The instruction FIN causes a switch
back to A0.
* Instructions for Output: -> D L etc.: These instructions cause the conversion of
binary numbers to decimal numbers with lights, MERCEDES typewriter of the punchtape.
* Instructions for Input: <- At1 etc.: It allows to read numbers from the punch tape.
Like the Z3, the Z4 has an arithmetic exception handling. If there are numbers outside
the range of 10^-20 < x < 10^20 then the machine gives the range where the result
is, for example:
* Very big + very big = very big.
* Very big - very big = undefined.
* 0/0 = undefined.
If an undefined value is combined with a number, the result is again undefined. Using
this method it can be avoided, that the Z4 calculates wrong numbers when it is working
without attendance. This was often the case at the ETH.