Konrad Zuse developed the world's first mechanical and electromechanical computers called the Z1, Z2, and Z3. Written by Konrad's eldest son, Horst, this article features many hitherto unpublished photographs from Horst's private collection. In this installment we take a detailed look at the architecture of the Z1

The Architecture of the World's First Mechanical Computer – the Z1
In 1936, my father finished the logical plan for his first computer, the V1 (he later changed the name to Z1 in order to avoid any connection with the V1 rocket). He had studied the available mechanical calculating machines of that time, all of which were based on the decimal number system. However, he never planned to build a modified or extended decimal machine, because he wanted to build a new type of computer for universal scientific applications.

From 1936 to 1938, my father constructed the Z1, which worked on the principles discussed in Part 3. In many ways the Z1 was a remarkable machine. In addition to a 64-word memory (where each word contained 22 bits), this machine had all of the components we previously discussed in Part 3. Thus, the Z1 truly was the first freely programmable, binary-based machine in the world!

Figure 4-1. The Z1 computer in the living room of Konrad Zuse's parents in 1936.

The Z1 did not use relays, but instead consisted completely of thin metal sheets, which he and his friends produced using a jigsaw. The only one electrical unit was an electrical engine, which was used to provide a clock frequency of one Hertz.

Figure 4-2. The building blocks of the Z1 were thin metal sheets.

The Z1 was programmed via a punch tape and a punch tape reader. There was a clear separation between the punch tape reader, the control unit (which supervised the whole machine and the execution of the instructions), the arithmetic unit (with registers R1 and R2), the memory, and the input/output devices.

In 1986, Konrad Zuse decided to rebuild the Z1 (Figure 4-3), because the architecture of the Z1 was almost identical to that of his Z3 computer (discussed below), which was unfortunately destroyed in the Second World War. Thus, the saying "War is the father of everything," is not true in the case of the invention of the computer.

Figure 4-3. The rebuilt Z1 seen from a "birds-eye" view (source: Deutsches Technik Museum, Berlin).

A high-level block diagram of the Z1 is shown in Figure 4-4. Observe how the elements in this diagram map onto the corresponding areas of the physical Z1 illustrated in Figure 4-3.

Figure 4-4. High-level block diagram of the Z1.

Most of the components are self-explanatory. The memory, which consisted of 64 words, each containing 22 bits, was formed from three blocks. The first block contained 64 words for the exponents and signs (8 bits for each word). The other two blocks each contained 32 words for the mantissa (14 bits for each word). The selection unit interpreted the address for the memory by the control unit. The arithmetic unit was an adder, and all of the operations were reduced to additions or subtractions (adding and subtracting are very similar operations).

The registers R1 and R2 were two words, each containing 22 bits. The two circles on the left-hand side (on the clock generator block) are cranks for executing a clock cycle manually, where the upper one is for the memory and the lower one for the control and arithmetic unit.

The original Z1 was privately financed by Konrad Zuse, his parents, his sister Lieselotte (1908-1953), and friends. Konrad Zuse rebuilt the Z1 in his atelier in Hünfeld between 1987 and 1989. Rebuilding the Z1 was a difficult task for my father because he was 77 years old. It was also very expensive (financially) to reconstruct all the pieces of the machine. In fact the cost of rebuilding the Z1 was around 800,000 DM. The Siemens AG coordinated a consortium of about five companies and paid the major part of the costs.

Figure 4-5. On the brown-colored drawing board, Konrad Zuse designed the Z1 a second time.

Working in his atelier, Konrad recreated thousands of engineering drawings of every piece of the machine, because the original plans were destroyed during the war. At the end of 1987 the reconstruction of the Z1 was interrupted when my father suffered a heart attack. However, after 6-months, the construction of the machine proceeded.

Figure 4-6. Another view in the atelier of Konrad Zuse (1986).

Approximately 30,000 components for this machine – drawn by Konrad Zuse – were then constructed by Siemens AG in Bad Hersfeld using modern numerical machines. These components were then assembled by Konrad Zuse, an employee of Siemens AG, and two students Schweier and Saupe [SCHW89] from Köln.

Figure 4-7. The rebuilt Z1 in the atelier of Konrad Zuse
(this picture was taken on December 25, 1988 by Horst Zuse)

The recreated Z1, which was completed in 1989, was moved from Hünfeld to Berlin. Rebuilding the Z1 was a tremendous achievement, which allowed my father to show the world the first freely programmable computer based on binary floating point numbers and a binary switching system.

As we previously discussed, the Z1 had a 64-word memory, where each word contained 22 bits. Every word was directly addressable by the punch tape and the punch tape reader together with the control unit, and data could be read from and written to each word. It is worth noting that this mechanical memory was no slower than a memory constructed using relays, but the mechanical memory required much less space.

Figure 4-8. One bit of Konrad Zuse's mechanical memory.

It is undisputed that this type of memory is unique, and Konrad Zuse obtained a patent for this memory in 1936 [ZUSE36]. The metal sheet illustrated in Figure 4-8 stores one bit. In this illustration, a binary 1 (left position) is stored (the right position equates to a binary 0). The metal sheets a and b are moveable, and are directed by the control unit to store a bit (at the clock frequency generated by the electrical engine).

As was noted earlier, the Z1’s programs (Rechenplans) were stored on punch tapes using an 8-bit code. Storing the programs on tape rather than "hard-wiring" them into the mechanism was what made the Z1 a freely programmable machine. The instruction set of the Z1 was as follows:

Pr z Read the contents of the memory cell z into Registers R1 or R2.
Ps z Write the contents of Register R1 to the memory cell z.
Ls1 Add the two floating-point numbers in the Registers R1 and R2.
Ls2 Subtract the two floating-point numbers in the Registers R1 and R2.
Lm Multiply the two floating-point numbers in the Registers R1 and R2.
Li   Divide the two floating point numbers in the Registers R1 and R2.
Lu To call the input device for decimal numbers.
Ld To call the output device for decimal numbers.

The punch tape (using 35mm standard movie film) and the punch tape reader of the Z1 are shown in Figure 4-9.

Figure 4-9. The punch tape reader of the Z1.

The full Z1 is shown in Figure4-10. In addition to the crank (in the foreground) for manually cycling the machine, there was also an electric motor, which was used to generate a clock frequency of one Hertz (one cycle per second). To the rear of this picture are the three blocks of memory, while the binary floating point arithmetic unit is on the right.

Figure 4-10. The full Z1 (in the foreground is the manual crank for driving the clock by hand).

If you write two floating-point numbers on a piece of paper using the binary number system, and you try to develop the algorithms to perform the basic arithmetic operations on these numbers, then you will understand just how much effort was necessary to build the Z1’s arithmetic unit, which comprised thousands of metal plates as illustrated in  Figure 4-11.

Figure 4-11. The Z1's binary floating point arithmetic unit.

My father’s design for the Z1’s arithmetic binary floating point unit was ingenious. However, the technical realization using the thin metal sheets was too complex. The arithmetic unit in the original Z1 was not very reliable (similarly with the rebuilt Z1).

In Figure 4-12 we see the arithmetic unit on the left separated from the memory of the right. When these units were connected together (as shown in Figure 4-17), then in today's terms there was a parallel bus between them.

Figure 4-12. The interface between the arithmetic unit (left) and the memory (right).

The Z1 had decimal input- and output devices as illustrated in Figures 4-13, 4-14, 4-15, and 4-16. The numbers were presented to the machine in a decimal form with an exponent. The Z1 then converted the decimal numbers to a binary normalized floating point representation. Similarly, the output device converted the binary floating point number in Register R1 into a decimal number with an exponent.

Figure 4-13. The Z1's input (bottom right) and output devices (middle of picture).

Figure 4-14. Konrad Zuse enters a decimal number into the input device
(above the keys the exponent could be set from +8 to -8).

Figure 4-15. The output device was realized as an annunciator (he exponent is marked
in red at the bottom).

Figure 4-16. Adjusting the metal sheets and the pins.

Thus, as early as 1936-1938, the Z1 exhibited almost all of the facilities of the so-called von Neumann machine [NEUM45], [BURK46]. In fact the only feature that was not implemented was loading the program into the Z1’s memory. This was because building a large memory was a very expensive task at that time. The calculation of a determinant (third grade) requires about 50 instructions and 15 words of memory for the variables and the intermediate results. This simple example shows that storing the program in memory would block the idea of a freely programmable machine.

Figure 4-17. Konrad Zuse with the rebuilt Z1 in the Deutsche Technik Museum in Berlin in September 1989.

In Part 5 we will take a detailed look at the architecture of the Z2 and Z3 computers (see also the Main Index for a quick and easy way to navigate the entire article).