Central Processing Unit


The central processing unit (CPU) is the heart of the computer system. It is composed of three parts that function together as a unit. These components are the control unit, the arithmetic/logic unit, and memory. Although memory is considered to be part of the CPU, it is normally located on separate circuit boards which are then connected to the CPU. In PC's these boards are often refered to as memory modules (also called RAM). Each part of the CPU performs its own unique functions (see Figure below).

Computer System Components

 

While the CPU incorporates all three components, the control unit and the arithmetic/logic unit are often referred to collectively as the processor. A processor may incorporate one or more circuit elements, or "chips." In a large computer, the processor may be built on several circuit boards in boxlike structures or frames, hence the term mainframe. Processors in microcomputers have been reduced in size to fit onto a single plug-in chip and are referred to as microprocessors.

When data and programs enter the CPU, they are held in memory. Since memory is volatile, or nonpermanent, data or programs will be erased when the electric power to the computer is turned off or disrupted in any other way. When any changes or results are to be preserved, they must be saved on an external form of storage, for example, on disks or magnetic tapes.

To begin work, programs and data to be manipulated are written into memory. What happens to the contents of memory depends on the processor. The processor, as stated earlier, consists of two processing units: the control unit and the arithmetic/ logic unit. The control unit maintains order and controls activity in the CPU. It does not process or store data; it directs the sequence of operations. The control unit interprets the instructions of a program in memory and produces signals that "command" circuits to execute the instructions. Other functions of the control unit include communicating with an input device in order to begin the transfer of instructions and data into memory and, similarly, communicating with an output device to initiate the transfer of results to storage.

The manipulation of the data occurs in the arithmetic/logic unit (ALU) of the CPU. The ALU performs arithmetic computations and logical operations. Arithmetic computations include addition, subtraction, multiplication, and division. Logical comparisons include six combinations of equality: equal to, not equal to, greater than, less than, equal to or greater than, and equal to or less than. Since the bulk of internal processing involves calculations or comparisons, computer capabilities often depend upon the design and capabilities of the ALU.

Memory (also known as primary memory, internal memory, or main memory) holds instructions, data, and intermediate and final results of processing. At the start of processing, data is transferred from some form of input media by an input device to memory, where it is kept until needed for processing. Data being processed and intermediate results of ALU calculations are also stored here. After all computations and manipulations are completed, the final results remain in memory until the control unit causes them to be transferred to an output device. See Concept Summary below for a review of the central processing unit.

 

 

Last Updated Jan.6/99