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Chapter 12 ZEBRA, a simple binary computer 203

Fig. 4. Control unit.

This is the most important aspect of the machine. An instruction in the address counter comes back after an A-instruction and can do something useful. To our surprise we found that in many more cases than we first suspected, the second action could be used effectively. In most other computers the time of access to the next instruction is lost because nothing can be done concurrently in the arithmetic unit.

Another example of the action of the control is the jump to a sub-routine. Suppose that we have the following piece of programme:

100 X200KE5 Jump to sub-routine starting in 200. Place return jump in 5.

102 etc. Sub-routine returns here.

The action is as follows:

By ending the sub-routine:

220 X221K5

221 - 1

we can return not two but one location further on, i.e., X221K5 takes as next instruction á 5ñ - 1 = X101. Here 5 contains the instruction and the drum modifier.

The test bits

The digits V x4 x2 x1 will not be dealt with extensively but the different combinations of these 4 digits represent different types of test. When for example V1 is attached to an instruction, this instruction will be executed when á Añ is negative, but will be skipped altogether when á Añ is positive or zero. The harmless A-instruction will then be executed instead. The test can be attached to a jump, giving a conditional jump, as well as to an A-instruction, giving a conditional addition.

The W-bit

So far the digit W has not been mentioned. When W is present in an instruction the drum address is not used. The instruction is not kept waiting but is immediately executed and the drum is completely disregarded. With the help of this digit W, jumps can be made to instructions in the fast store, e.g., XK5W takes the instruction from 5 only, and the drum does not deliver any number. The use of this type of instruction has very peculiar consequences. Let us take the following example:

100 X101KE6 á 5ñ = ARW
101 X8186K5RW á 6ñ = filled with return instruction
102 etc.

The action is as follows:

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