How CPU Executes Program Instructions:
Let us examine the way the central processing unit, in association with memory, executes a computer program. We will be looking at how just one instruction in the program is executed. In fact, most computers today can execute only one instruction at a time, through they execute it very quickly.
Before an instruction can be executed, program instructions and data must be placed into primary memory from an input device or a secondary storage device. Once the necessary data and instruction are in memory, the central processing unit performs the following four steps for each instruction;
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The control unit fetches (gets) the instruction from memory.
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The control unit decodes the instructions (decades what it means) and directs that the necessary data be moved from memory to the arithmetic/logic unit. These first two steps together are called instruction time, or I-time.
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The arithmetic/logic executes the arithmetic or logical instruction. that is, the ALU is given control and perform s the actual operation on the data.
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The arithmetic/logic unit stores the result of this operation in memory or in a register. Step c and d together called execution time, or E-time.
The control unit eventually directs memory to release the result to an output device or a secondary storage device. The combination of I-time and E-time is called the machine cycle.
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