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Chapter 12 ZEBRA, a simple binary computer 201

Fig. 1. The main units of the computer.

arithmetic unit or the control. In the same way the K-bit controls the interconnection of the fast store with the arithmetic unit or the control unit. These interconnections can be seen from Fig. 1.

It will be seen that A and K can have 4 possible combinations:

Case 1. A = 0, K = 0. This is called the adding jump (Fig. 2a).

While a new instruction is coming into the control from the drum, the arithmetic unit can at the same time do an operation with the operand coming from the fast store. This is the fastest type of operation. When the following instruction is placed in the next location on the drum there is no waiting time, and 32 instructions of this type can be executed per revolution. (One revolution = 10 ins, one word time = 312 m s.)

Case 2. A = 0, K = 1. This is called the double jump (Fig. 2b).

Both stores are now used for giving information to the control, i.e., making a jump. Since the fast store is used for the control, the instruction coming in from the drum is modified by the contents of a fast register. In this way the B-line facility, as it is often called, is realised.

Case 3. A = 1, K = 0. This is called the double addition (Fig. 2c).

Both stores are now connected to the arithmetic unit. The control must take care of itself using the address counter which is stepped up by 2 at a time, thus enabling this type of instruction to reach the number lying between the two successive instructions without any waiting time. Constants in particular will always be taken from optimum places on the drum.

Case 4. A = 1, K = 1. This is called the jumping addition (Fig. 2d).

While the drum is used for the arithmetic unit the address counter is modified by a fast register. Control may thus be passed to any instruction, and not only to the next instruction.

D- and E-bits

The functional bits D and E control the direction of flow of information.

D = 0 means: read from the drum.

E = 0 means: read from the fast store.

D = 1 means: write to the drum.

E = 1 means: write to the fast store.

A few possible instructions will be given below. In the written code a drum address will always be written with 3 or more digits and the absence of the A-bit will be indicated by the letter X. (This is necessary for the input programme to recognize the be ginning of a new instruction.)

 A200.5 Add á 200ñ (the contents of address 200) and á 5ñ to the accumulator. Step the address counter by 2. X200E5 Take next instruction from 200 (= jump to 200) and store contents of accumulator in 5. X200KE5 Jump to 200 and store previous contents of address counter in 5. This amounts to placing a link instruction for return from a sub-routine X200K5 Take next instruction from 200 but modify it with á 5ñ thus making a variable instruction.

Arithmetic bits

The remainder of the function bits have arithmetic meanings. We shall only briefly indicate their different actions.

B: Do not use the A accumulator (most significant accumulator) but the B accumulator.

Fig. 2. The possible combinations of the A- and K-bits.

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