As it is known the two functional units of the CPU are the control unit (CU) and the arithmetic-logical unit (ALU). The control unit manages and coordinates the entire computer sys­tem. It obtains instructions from the program stored in main memory, interprets the instructions, and issues signals that cause other units of the system to execute them.

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 the main storage and the arithmetic-logical unit.

The control unit has the following components: a counter that selects the instructions, one at a time, from memory; a reg­ister that temporarily holds the instructions read from memory while it is being executed; a decoder that takes the coded instruc­tion and breaks it down into individual commands necessary to carry it out; a clock, which produces marks at regular intervals. These timing marks are electronic and very rapid.

The sequence of control unit operations is as follows. The next instruction to be executed is read out from primary stor-

Английский язык. Основы компьютерной грамотности 104

age into the storage register. The instruction is passed from the storage register to the instruction register. Then the operation part of the instruction is decoded so that the proper arithmetic or logical operation can be performed. The address of the op­erand is sent from the instruction register to the address regis­ter. At last the instruction counter register provides the address register with the address of the next instruction to be executed.

The arithmetic-logical unit (AL U) executes the processing op­erations called for by the instructions brought from main mem­ory by the control unit. Binary arithmetic, the logical operations and some special functions are performed by the arithmetical-logical unit.

Data enter the ALU and return to main storage through the storage register. The accumulator serving as a register holds the results of processing operations. The results of arithmetic op­erations are returned to the accumulator for transfer to main storage through the storage register. The comparer performs log­ical comparisons of the contents of the storage register and the accumulator. Typically, the comparer tests for conditions such as "less than", "equal to", or "greater than".

So as you see the primary components of the arithmetic-log­ical unit are banks of bistable devices, which are called regis­ters. Their purpose is to hold the numbers involved in the cal­culation and hold the results temporarily until they can be tranferred to memory. At the core of the ALU is a very high-

105 Unit 8. Central Processing Unit

speed binary adder, which is used to carry out at least the four basic arithmetic functions (addition, subtraction, multiplication and division). The logical unit consists of electronic circuitry which compares information and makes decisions based upon the results of the comparison.

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