.. 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.
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.
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)
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