Examine Figures A.5.10 and A.5.11 closely. (a) Why is"Less" directly connected to the "Operation" multiplexor?(b) For input lines a and b, list all potential operationsthatthis 1-bit ALU performs. Give details. (c) How can "Set"and "Overflow" be used to determine the "set on lessthan" operation?
Examine Figures A 5 10 And A 5 11 Closely A Why Is Less Directly Connected To The Operation Multiplexor B For 1 (70.83 KiB) Viewed 51 times
Examine Figures A 5 10 And A 5 11 Closely A Why Is Less Directly Connected To The Operation Multiplexor B For 2 (92.85 KiB) Viewed 51 times
CU b Less Ainvert 0 1 Binvert 0 0 1 + Overflow detection Operation Carryin 0 1 2 3 Result Set Overflow FIGURE A.5.10 (Top) A 1-bit ALU that performs AND, OR, and addition on a and b or b, and (bottom) a 1-bit ALU for the most significant bit. The top drawing includes a direct input that is connected to perform the set on less than operation (see Figure A.5.11); the bottom has a direct output from the adder for the less than comparison called Set. (See Exercise A.24 at the end of this appendix to see how to calculate overflow with fewer inputs.)
Binvert Ainvert a0 bo a1 b1 0 a2 b2 0 a63- b63- 0 Carryln Carryln ALUO Less CarryOut Carryin ALU1 Less CarryOut Carryln ALU2 Less CarryOut Carryin Carryin ALU31 Less Operation → Result0 → Result1 → Result2 Set Result63 → Overflow FIGURE A.5.11 A 64-bit ALU constructed from the 63 copies of the 1-bit ALU in the top of Figure A.5.10 and one 1-bit ALU in the bottom of that figure. The Less inputs are connected to 0 except for the least significant bit, which is connected to the Set output of the most significant bit. If the ALU performs a - b and we select the input 3 in the multiplexor in Figure A.5.10, then Result = 0... 001 if a <b, and Result = 0... 000 otherwise.
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