Konrad Zuse -- The Zuse KG

The Life and Work of Konrad Zuse (by Horst Zuse)

Part 7 (continued): The Zuse KG

Konrad Zuse referred to programs as Rechenplans (calculating plans). The Z4 made use of a unit called a Planfertigungsteil (program construction unit),⁽⁸⁾ which was used to produce punch tapes containing instructions for the Z4 in a very easy way. For this reason, it was possible to learn the programming of the Z4 in as little as three hours.		⁽⁸⁾Konrad Zuse also planned a Planfertigungsgerät (program translation unit), which was to be an extended version of the Z4s Planfertigungsteil (program construction unit). Konrad Zuses idea was to write programs in a symbolic way, like the notation in mathematics, and to then compile these high-level programs into the Z4s machine language.

The Planfertigungsteil was already a part of the Z4 in 1945. We can see, that Konrad Zuse tried to make it as easy as possible to write programs for his machine. On the one hand the Planfertigungsteil allowed the programmer to use symbolic memory cells, instructions, and symbolic arithmetic operations for the creation of a program, and it also was possible to copy programs and to make corrections. On the other hand the instructions and data on a punch tape could easily be shown by lamps.



Fig.71 (Left). The Z4s Planfertigungsteil (program construction unit), which was used to create punch tapes for the Z4 in an easy way. Fig 72 (Right). One of the Z4s punch tapes with instructions on how to make it. It was also possible to store intermediate results on a punch tape and to then use this tape as input data.

The Z4s Instruction Set The Z4 had a large instruction set in order to calculate complicated scientific programs. Almost all of the Z4s instructions were created in 1942, although some extensions were added in 1949 as was discussed earlier. The Z4 could perform 1000 instructions per hour. The arithmetic processor was a very powerful binary floating processor. The list of instructions is as follows:

	Instruction A n: For example A 17. This reads the contents of memory cell 17 into the Register R1. If Register R1 is occupied, then the contents are loaded into Register R2.

	Instruction S n: For example S 18. This writes the contents of Register R1 into the memory cell 18.

	The Registers R1 and R2 were used as arguments for the arithmetic operations. For dyadic operations the contents of both Registers R1 and R2 are used and the result is then stored in Register R1 (the contents of Register R2 are deleted). For monadic operations the contents of Register R1 are used and the result is stored in Register R1.

	Dyadic operations are: +, -, x, /, MAX, and MIN.

	Monadic operations are: x², SQR(x), 1/x, \| x \| , sign(x), x1/2, x2, x(-1), x10, x3, x1/3, x1/5, x1/7, xPi, x1/Pi.

	Instructions for comparison (x = 0, x >= 0, \| x \| = infinity ) test the value in Register R1 and set Register R1 to +1 if the condition is fulfilled, if not, then the contents of Register R1 are set to 1. This instruction was already planned in 1942, but lack of material made the realization impossible at that time.

	The conditional branch SPR was a special requirement by the ETH. The instruction SPR skips the punch tape to the instruction ST, if Register R1 contains +1 (if Register R1 contains 1 then there is no impact).

	Instruction UP: The Z4 had two punch tape readers. In the original version up to six such readers were planned (this was referred to as the subprogram technique). The instruction UP switches from the main punch tape reader (A0, see Fig.73 below) 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 binary numbers to be converted into their decimal equivalents and the results to be displayed with lamps, on the MERCEDES typewriter as floating or fixed point numbers, or on the punch tape.

	Instructions for Input: <-, At1, etc.: These allow numbers to be read from the punch tape.

Arithmetic Exception Handling
Like the Z3, the Z4 supported powerful arithmetic exception handling. If there are numbers outside the range of approximately 10^-20 < x < 10²⁰ then the machine returns the range where the result is, for example:

Very big + very big = very big

Very big very big = undefined

0/0 = undefined

If a sign of undefined is combined with another sign of undefined, then the result is undefined. This method could be used to prevent the Z4 from calculating incorrect numbers when it was working without supervision (this was mostly the case at the ETH).

The Z4s two punch tape readers
Fig.73. The Z4s two
punch tape readers.

Restoring the Z4 cost the Zuse KG about 60,000 DM. The ETH Zürich paid an amount of 40,000 Swiss Francs (around 100,000 DM at that time) in advance for the Z4. With this money it was possible to found the Zuse KG and restore the Z4. (It is worth mentioning that the average income at this time was about 180 DM per month.) The Z4 was a great success for both the ETH and the Zuse KG.

The Z4s Computing Times
The Z4 had the following computing times in seconds:

Addition	0.5
Multiplication	3.0
Division	6.0
Square-Root	6.0
Memory Access (Mostly overlapped with operations)	0.5
Overall Performance	2000 instructions, or 1000 arithmetic operations per hour

In [SPEI98] Speiser writes:

All these specifications of the Z4, as seen in 1949, were very convincing for Stiefel,Rutishauser and Speiser. It must be borne in mind that at this time there were hardly a dozen program-controlled computers in operation, all of them in US. Less than a handful were in use for research in numerical mathematics, the others performed routine calculations. There were no doubts that Z4 could be used for serious mathematical research.

We now want to cite Speiser [SPEI98] again, when he writes about the scientific work done with the Z4 and the reactions in Germany at this time:

When the Z4 machine was installed, significant work started almost immediately. Within a few years Zürich rose to be one of the foremost centers in numerical analysis. ... The creative spirit that was ever-present, the continuous expression and evaluation of new ideas, the thoroughly based academic knowledge and the sound scientific judgment were daily realities, I am almost tempted to say: This was the air that we were breathing. I can hardly believe, that Stiefel, when he decided to acquire the Z4, would have dared to hope for success of this degree!

The Z4 was also extensively used in education. As early in 1951, we offered to students a course in computer programming with practical exercises on the machine. We believe we were the first on the European continent to do so. This should be taken in consideration by those who often criticize that Swiss Universities were late in recognizing the importance of informatics.

The situation in Germany in the early 50s is worth being commented. While the Z4 was installed and operated in Zürich, there were three computer projects under way in West Germany: Darmstadt, München, and Göttingen. Their interest in the Z4 was only moderate, to put it mildy. To understand this attitude one must take in account the fundamental difference in priorities: Stiefel had the desire to have computing power at his disposal as fast as possible, even if it was modest. The three German groups on the other hand had the ambition to build electronic computers with advanced technology; they were under less pressure from the side of a computer-hungry mathematician. But when the scientific results started to flow out of Zürich in 1951 and 1952, there was some criticism voiced in Germany saying the Z4 should have been kept at home rather than let it go abroad.

To me, and now in the light of retrospect, the explanation of what happened is quite clear: Universities in 1950 were still suffering from the effects of the war. Buildings were badly damaged, equipment was almost non-existent, and, accordingly, the limited funds had to be spent for the urgent needs of the day in order to keep university life on an acceptable level. The sum of 50,000 DM we paid for the Z4 was clearly outside the reach of any universities. Funds in this amount could only have come from the federal ministry, possibly a state ministry, and from the Marshall Plan whose influence was highly beneficial. But in these circles the opinion was crystal-clear: The future belonged to electronics, it would be a big mistake to divert the limited funds to a relay machine.

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