The control unit we saw in chapter 2 see figure 21

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Unformatted text preview: rol Unit Arithmetic Logic Unit Central Processing Unit I/O DEVICES Figure 7.1. Processor and memory architecture of a computer system. THE CENTRAL PROCESSING UNIT (CPU) The CPU is the brain of a computer system. All major calculations and comparisons performed by a computer are carried out inside its CPU. The CPU is also responsible for activating and controlling the operations of other units of the computer system. Hence no other single component of a computer determines its overall performance as much as the CPU. In order to be able to quickly evaluate any computer's capabilities, it is important to know how CPUs are internally structured, how different CPUs differ from each other, and how CPU speed is evaluated. These and other related concepts about CPU are described below in subsequent subsections. The Control Unit We saw in Chapter 2 (see Figure 2.1) that the two basic components of a CPU are the control unit and the arithmetic logic unit. The control unit of the CPU selects and interprets program instructions and then sees that they are executed. As shown in Figure 7.1, it has some special purpose registers (whose functions are described in a later subsection) and a decoder to perform these activities. The special purpose registers, namely the instruction register and the program control register respectively hold the current instruction and the next instruction to be executed and in this way help the control unit in instruction selection. On the other hand, the decoder has the necessary circuitry to decode and interpret the meaning of each and every instruction supported by the CPU. Each instruction is accompanied by microcode - very basic directions that tell the CPU how to execute the instruction. Although, the control unit does not perform any actual processing on the data, it acts as a central nervous system for the other components of the computer. It manages and coordinates the entire computer system including the input and output units. It obtains instructions from the program stored in the main memory, interprets the instr...
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This document was uploaded on 04/07/2014.

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