The basic components of a computer system, the input, the output, the memory, and the processor operate only in response to commands from the control unit. The control unit operates by reading one instruction at a time from memory and taking the action called for by each instruction. In this way it controls the flow between main storage and the arithmetic-logical unit.
A control unit has the following components:
a)A counter that selects the instructions, one at a time, from memory.
b)A register that temporarily holds the instruction read from memory while it is being executed.
c)A decoder that takes the coded instruction and breaks it down into the individual commands necessary to carry it out.
d)A clock, which, while not a clock in the sense of a time-keeping device, does produce marks at regular intervals. These timing marks are electronic and very rapid.
Binary arithmetic (the kind of arithmetic the computer uses), the logical operations and some special functions are performed by the arithmetic-logical unit. The primary components of the ALU are banks of bi-stable devices, which are called registers. Their purpose is to hold the numbers involved in the calculation and to hold the results temporarily until they can be transferred to memory. At the core of the arithmetic-logical unit is a very high-speed binary adder, which is used to carry out at least the four basic arithmetic functions (addition, subtraction, multiplication, and division).
Typical modern computers can perform as many as one hundred thousand additions of pairs of thirty-two-bit binary numbers within a second. The logical unit consists of electronic circuitry which compares information and makes decisions based upon the results of the comparison. The decisions that can be made are whether a number is greater than (>), equal to (=), or less than (<) another number
