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The CPU
At the heart of a computer is its central processing unit, or CPU. The CPU’s job is to fetch instructions, follow the instructions, and produce some result. Internally, the central processing unit consists of two parts: the control unit and the arithmetic and logic unit (ALU). The control unit coordinates all of the computer’s operations. It is responsible for determining where to get the next instruction and regulating the other major components of the computer with control signals. The arithmetic and logic unit, as its name suggests, is designed to perform mathematical operations.
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A program is a sequence of instructions stored in the computer’s memory. When a computer is running a program, the CPU is engaged in a process known formally as the fetch/ decode/execute cycle. The steps in the fetch/decode/execute cycle are as follows:
Fetch: The CPU’s control unit fetches, from main memory, the next instruction in the sequence of program instructions.
Decode: The instruction is encoded in the form of a number. The control unit decodes the instruction and generates an electronic signal.
Execute: The signal is routed to the appropriate component of the computer (such as the ALU, a disk drive, or some other device). The signal causes the component to perform an operation. These steps are repeated as long as there are instructions to perform.
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