Central processing unit

The combination of the processor and memory is sometimes referred to as the CPU, although sometimes the processor itself is referred to as the CPU. All program instructions to be executed must be held within the CPU, and all the data to be processed must be loaded first into this unit.

It is convenient to consider the central processor to have three separate hardware sections:

1) an internal or main memory

2) an arithmetic and logic unit

3) a control unit.

The CPU has two functions. It must (1) obtain instructions from the memory and interpret them and (2) perform the actual operations.

The first function is executed by the control unit. This unit in its turn must perform two functions. It must (1) interpret the instruction, then, on the basis of this interpretation (2) tell the arithmetic and logic unit what to do next. The latter function is accomplished through the use of electronic signals. According to these two functions we can separate the part of the control unit that interprets or decodes the instruction called the instruction decoder from the part that generates the control signals called the control generator.

An instruction having been transmitted to the instruction decoder, where it is interpreted, the control generator senses this interpretation and then produces signals that tell the arithmetic unit which operation to perform. It also generates signals that choose the proper numbers from the memory and sends them to the arithmetic and logic unit at the proper time; and when operation has been performed, other control signals take the result from the arithmetic and logic unit back to the internal memory. After an instruction has been executed, the control generator produces signals that cause the next instruction to go from the memory to the instruction decoder. In this way the instructions are performed sequentially.

The second function of the CPU is performed by the arithmetic and logic unit which does the actual operations. This unit is capable of performing automatically addition, subtraction, multiplication, division, comparing, selecting and other mathematical and logical operations. In most computers only one word at a time can be transferred between the arithmetic/logic unit and the memory. Hence, to perform an operation involving two arguments, the first argument must be transferred from the memory to the arithmetic/logic unit and stored there temporarily while the second argument is being transferred. The special memory cell in the arithmetic/logic unit for this purpose is called the accumulator. The operation being performed, the result is formed in the accumulator before it is transmitted back to memory.

Next consider the instruction decoder that interprets instructions. It’s necessary that the instruction decoder should constantly refer to the instruction being interpreted during the time control signals are being set up. To facilitate this, while an instruction is being executed it is stored in a special memory cell called the instruction register, located in the instruction decoder.

There is another memory cell, located in the instruction decoder called the current-address register. The contents of this register is always the memory address from which the instruction being executed came. The reason for this is related to the fact that the address of the present instruction was given as part of the previous instruction.