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Unformatted text preview: an be generated from the other using an inverter. After these simplifications the control logic design is almost complete. It only remains to determine the encodings of the ALU functions. Figure 1.6 MU0 register transfer level organization. MU0 - a simple processor 11 The control logic can be presented in tabular form as shown in Table 1.2 on page 12. In this table an 'x' indicates a don't care condition. Once the ALU function select codes have been assigned the table may be implemented directly as a PLA (programmable logic array) or translated into combinatorial logic and implemented using standard gates. A quick scrutiny of Table 1.2 reveals a few easy simplifications. The program counter and instruction register clock enables (PCce and IRce) are always the same. This makes sense, since whenever a new instruction is being fetched the ALU is computing the next program counter value, and this should be latched too. Therefore these control signals may be merged into one. Similarly, whenever the accumulator is driving the data bus (ACCoe is high) the memory should perform a write operation (RnW is low), so one of these signals can be generated from the other using an inverter. After these simplifications the control logic design is almost complete. It only remains to determine the encodings of the ALU functions. Figure 1.6 MU0 register transfer level organization. 12 An Introduction to Processor Design ALU design Most of the register transfer level functions in Figure 1.6 have straightforward logic implementations (readers who are in doubt should refer to 'Appendix: Computer Logic' on page 399). The MU0 ALU is a little more complex than the simple adder described in the appendix, however. The ALU functions that are required are listed in Table 1.2. There are five of them (A + B, A -- B, B, B + 1,0), the last of which is only used while reset is active. Therefore the reset signal can control this function directly and the control logic need only generate a 2-bit function select code to choose between the other four. If th...
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This document was uploaded on 10/30/2011 for the course CSE 378 380 at SUNY Buffalo.
- Spring '09