Then it might tell the alu to add the two numbers an

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Unformatted text preview: uctions, and issues signals that cause other units of the system to execute them. The Arithmetic Logic Unit (ALU) The ALU of the CPU is the place where the actual execution of the instructions takes place during the data processing operation. That is, when the control unit encounters an instruction that involves an arithmetic operation (such as, add, subtract, multiply, divide) or a logic operation (such as less than, equal to, greater than), it passes control to the ALU. As shown in Figure 7.1, the ALU has some special purpose registers (whose functions are described in a later subsection) and the necessary circuitry to carry out all the arithmetic and logic operations included in the instructions supported by the CPU. For example, the control unit might load two numbers into the registers in the ALU. Then, it might tell the ALU to add the two numbers (an arithmetic operation), or to check if the two numbers are equal (a logical operation). In case of a microcomputer, the entire CPU (both the control unit and the ALU) is contained on a single tiny silicon chip called a microprocessor. Instruction Set Every CPU has the built-in ability to execute a particular set of machine instructions, called its instruction set. Most CPUs have 200 or more instructions (such as add, subtract, and compare) in their instruction set. The machine language designed for a specific CPU is based on the list of specific instructions supported by the CPU in its instruction set. Because each processor has a unique instruction set, machine language programs written for one computer will generally not run on another computer with a different CPU. CPUs made by different manufacturers have different instruction sets. In fact, different CPU models of the same manufacturer also often have different instruction sets. However, manufacturers tend to group their CPUs into "families" that have similar instruction sets. When a new CPU is developed, it is ensured that its instruction set includes all...
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This document was uploaded on 04/07/2014.

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