The CPU

The central processing unit (CPU) is the computer's brain. As its name implies, it is where the processing step takes place. Instructions are executed and data is processed. The CPU consists of three components: the control unit, the arithmetic/logic unit, and main memory. The personal computer's CPU is commonly a single chip; in larger computers, it may be many chips.

The control unit directs the step-by-step operation of the computer. Like a traffic policeman at a busy intersection, the control unit directs electrical impulses between itself, the ALU, and storage. It also controls operations between the CPU and the peripheral devices. These electrical impulses consist of instructions and the data being processed.

The arithmetic/logic unit (ALU) performs the arithmetic and logical operations. You'll recall that the arithmetic operations are addition, subtraction, multiplication, and division. The logic operations compare two pieces of data to determine if one is greater than, less than, or equal to the other.

Random access memory (RAM), also called main memory, is the third component of the computer's CPU. RAM carries out the storage step for the CPU, providing temporary storage for the programs being executed and for data as it passes through processing. RAM is short-term, volatile memory; that means everything stored there is lost when the computer's power is shut off. RAM is not a physical part of the CPU chip. Rather, it is made up of a number of additional chips electrically connected — what is termed logically connected—and in close proximity to the CPU chip. This can be seen in the rows of RAM memory chips installed beside the CPU, and the arrows indicat­ing the flow of data within the CPU and between the CPU and main memory.

There is another type of memory associated with the CPU called read only memory (ROM). It is memory chips that are used to store permanent instruc­tions that perform many routine tasks for the CPU; these instructions cannot be changed.

The motherboard, is the most important single hardware component. It contains all the circuitry for the CPU, control and routing components, and short-term storage. However, it is not the only component in the system unit. The power supply provides the proper electricity to each component. The cooling fans do just what the name implies, moving cooling air across the electronic components.