Two components handle processing in a computer: the central processing unit, or CPU, and the memory. They are located on the computer’s motherboard, the circuit board that connects the CPU to the other hardware devices.
The CPU, or processor, is the place where data is manipulated. In a personal computer, the processor usually consists of one or more microprocessors (sometimes called “chips”) which are slivers of silicon or other material with many tiny electronic circuits. The CPU has two basic parts: the control unit and the arithmetic logic unit.
The Control Unit
All the computer’s resources are managed from the control unit. The control unit directs the flow of data through the CPU, and to and from other devices. The CPU’s instructions for carrying out commands are built into the control unit. The instructions, or instruction set, list all the operations that the CPU can perform. Each instruction in the instruction set is expressed
in microcode – a series of basic directions that tells the CPU how to execute more complex operations. When the control unit encounters an instruction that involves arithmetic or logic, it passes that instruction to the second component of the CPU, the arithmetic logic unit, or ALU.
The Arithmetic Logic Unit
Because all computer data is stored as numbers, the processing involves comparing numbers or carrying out mathematical operations. The computer performs two types of operations: arithmetic operations and logical operations. Arithmetic operations include addition, subtraction, multiplication, and division. Logical operations include the following ones: “equal to”, “not equal to”, “greater than”, or “less than”. The ALU includes a group of registers – high-speed memory locations in the CPU that are used to hold the data currently being processed. For example, the control unit may load two numbers from memory into the registers in the ALU. Then it may tell the ALU to divide the two numbers (an arithmetic operation) or to see whether the numbers are equal (a logical operation). Each time the CPU executes an instruction, it takes a series of steps that is called a machine cycle. A machine cycle can be broken down into two smaller cycles: the instruction cycle and the execution cycle. During the instruction cycle the CPU takes two steps: fetching (the control unit retrieves, or “fetches”, a com-mand or data from the computer’s memory) and decoding (the control unit breaks down, or decodes, the command into instructions that correspond to those in the CPU’s instruction set.
At this point, the CPU is ready to begin the execution cycle. When the command is executed, the CPU carries out the instructions in order by converting them into microcode. The CPU may be required to store the results of an instruction in memory (but this condition is not always required).