Zuse Z4 (developed 1942-1945; rebuilt 1950)

.. under construction ..

Another particularly interesting feature of the Z4, was the incorporation of a lookahead feature in the control and memory systems. Three basic features were introduced. Firstly, 2 instructions could be swapped around, if it would not affect the operation, and would make the intermediate results of either instruction more readily available. Secondly, if memory instructions were detected in the near future, the loading of the information would be started early, to help reduce the impact of the slow cycle time of the memory. Thirdly, the results of an instruction could be stored locally to the arithmetic and control units if it was required in one of the next two instructions.

4. Look-Ahead: The program is read two instructions in advance, and it is tested whether memory instructions can be performed ahead of time.
5. Pseudo-memory: In case the look-ahead mechanism finds that a number that is to be stored is needed again within the next two instructions, the number is placed into a register of mechanical contacts where it is available with no access time. For this purpose, the memory has two registers of reading contacts.

See Michael R. Williams, A History of Computing Technology, Prentice Hall, 1985, pp. 218-219. Z4 read the program tape two steps ahead of current instruction.

• next two instructions could be executed in reverse order if they were not dependent and if reversing them would make intermediate results available more quickly
• two memory operations could be executed ahead of time [my note: cf. Stretch!]
• control unit could cache a copy of a value it was about to store back into memory if either of the next two instructions needed it

An architectural sketch of the Z4 is presented in Blaauw and Brooks, Computer Architecture: Concepts and Evolution, section 10.3.

The mechanical memory did not allow a Load to be followed immediately by a Store. Furthermore, a stored number could only be loaded again after at least three intervening instructions. The instruction Space is intended to be used for proper timing.

Z4 Links

The Z4 Computer and the Zuse Apparatebau in Berlin (1940-1945), part 6 of The Life and Work of Konrad Zuse, by Dr. Horst Zuse

Z4 page of Dr. Horst Zuse

Deutsches Museum page on the Z3 and Z4 (in German, with picture of Z4)

[History page] [Mark's homepage] [CPSC homepage] [Clemson Univ. homepage]

mark@cs.clemson.edu