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Table Of Contents
Modern Computer Architecture and Organization - Third Edition
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Processor registers temporarily store data values and serve as input to, and output destinations for, operations such as data movement to and from memory, as well as arithmetic and bit manipulation. Registers within a processor are written and read in parallel, meaning all the bits in the register are written or read simultaneously on individual signal lines under the control of a clock edge. The examples in this section use 4-bit registers for simplicity, but it is straightforward to extend them to 8-, 16-, 32-, or 64-bit registers.
Figure 2.14 shows a simple 4-bit register with parallel input and output. This is a synchronous circuit where data bits on inputs D0–D3 are loaded into the flip-flops on the rising edge of the CLK signal. The data bits appear immediately at the Q0–Q3 outputs and retain their values until new data values are loaded on a subsequent rising clock edge.

Figure 2.14: 4-bit register circuit
To perform useful functions beyond simply storing...