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Dale - Computer Science Illuminated 153

Dale - Computer Science Illuminated 153 - tion that asks a...

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126 Chapter 5 Computing Components Figure 5.3 The fetch-execute cycle Control unit (2) Decode instruction FETCH CYCLE (1) Fetch Instruction ALU (3) Get data EXECUTION CYCLE (4) Execute the instruction Main Memory Registers to add the contents of a memory location to a register, the control unit must get the contents of the memory location. Execute the Instruction Once an instruction has been decoded and any operands (data) fetched, the control unit is ready to execute the instruction. Execution involves sending signals to the arithmetic/logic unit to carry out the processing. In the case of adding a number to a register, the operand is sent to the ALU and added to the contents of the register. When the execution is complete, the cycle begins again. If the last instruction was to add a value to the contents of a register, the next instruction probably says to store the results into a place in memory. However, the next instruction might be a control instruction: an instruc-
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Unformatted text preview: tion that asks a question about the result of the last instruction and perhaps changes the contents of the program counter. Figure 5.3 summarizes the fetch-execute cycle. Hardware has changed dramatically in the last half-century, yet the von Neumann machine is still the basis of most computers today. As Alan Perlis, a well-known computer scientist, said in 1981, “Sometimes I think the only universal in the computing field is the fetch-execute cycle.” 2 This statement is still true today, 20 years later. RAM and ROM We said previously that RAM stands for R andom A ccess M emory. RAM is memory in which each cell (usually byte) can be directly accessed. Inherent in the idea of being able to access each location is the ability to change the contents of each location. That is, storing something else into that place can change the bit pattern in each cell....
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