ZEBRA

ZEBRA and Van der Poel

Van der Poel posing with ZEBRA.

After the completion of the PTERA Kosten and Van der Poel started designing a commercial computer at the PTT that was going to be called ZEBRA (acronym for "Zeer Eenvoudig Binair Reken-Aparaat", "Very Simple Binary Computing Machine"). Even though they just finished the PTERA they looked back one machine further to the ZERO for inspiration in the design. This inspiration came in the form of the 'functional bits' that were used in the ZERO. These functional bits were essentially a form of micro-programming, in the sense that each bit directly controlled some lower level operation. The ZEBRA design got these as well, only this time with 15 instead of 4 functional bits. These 15 functional bits were included in the instruction words which were 33 bits. The functional bits allowed the programmer to control all kinds of aspects of the operation of the machine directly. For example the 'A' bit was used to select where the information stored on the drum was send to, the arithmetic unit or the control unit. The 'K' bit controlled where the registers where connected to, the drum or the control unit. And so there were other bits that controlled if it was a read or write operation.

The functional bits made the ZEBRA structurally very simple, and extremely flexible. The structural simplicity came from the fact that it had only addition and subtraction built in which made that it required far less components, making it smaller, requiring less maintenance and therefore cheaper. The flexibility comes from the functional bits, they made it possible to have potentially very powerful instructions and do more things in one step like performing an addition while the new instruction was being loaded from the drum.

ZEBRA

ZEBRA with its cabinets and the console desk.

Commercial production

As this was to be a commercial computer they only made the logic design and the company building it would make the construction details of the machine based on this. At first Kosten and Van der Poel asked Philips to build the machine, but because Philips had an agreement with IBM not to enter the computer market and remain a component producer it declined this. Therefore they went overseas to England and found Standard Telephones and Cables to build the machine for them. At first the machines were made out of vacuum tubes, but over the years it was built it eventually became a fully transistorized machine that was still based on the same design when production was stopped in 1964.

ZEBRA programming

Among the programmers of the ZEBRA a subculture developed regarding their programming style. They made things they called 'underwater programming' which means they made seemingly very simple program's that when run suddenly became very complex by repeating and even adapting itself to eventually perform big complex operations. They called it underwater programming as you could only see the surface, the few lines of instructions which could be only 7 to 10 lines but couldn't see what under the surface happened to produce the complex result. All these tricks that the ZEBRA programmers subculture developed and exchanged were called 'trickology' by Van der Poel[1].

Two ZEBRA simulators for PC are currently available at the FTP server of the University of Manchester. One of them is written by Van der Poel himself. See the ZEBRA simulators directory.

Properties

  • Design completed in 1956
  • Built in England by Standard Telephones and Cables as Philips refused
  • First machines delivered in 1958
  • First version build with vacuum tubes, later versions with transistors
  • Production was stopped in 1964
  • 55 were sold world-wide
  • Used 15 functional bits
  • Instruction words of 33 bits
  • Unique design due to its functional bits and their incorporation in the instruction words
  • A subculture developed around the ZEBRA among the hundreds of programmers working on it
  • Programming often done with smart programming tricks which Van der Poel called 'trickology'

References

  1. Kranakis, E. , "Early Computing in the Netherlands", CWI Quarterly, vol. 1, issue 4, pp. 61-84, 12/1988.
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