Cse675.02.F.ALUDesign_part2

Cse675.02.F.ALUDesign_part2 - Set Less Than (slt) Function...

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1 g. babic Presentation F 14 slt function is defined as: 000 … 001 if A < B, i.e. if A – B < 0 A slt B = 000 … 000 if A ± B, i.e. if A – B ± 0 Thus, each 1-bit ALU should have an additional input (called Less ”), that will provide results for slt function. This input has value 0 for all but 1-bit ALU for the least significant bit. For the least significant bit Less value should be sign of A – B Set Less Than (slt) Function 0 3 R e s u l t O p e r a t i o n a 1 C a r r y I n C a r r y O u t 0 1 B i n v e r t b 2 L e s s 32-bit ALU With 5 Functions 1-bit ALU1-ALU30 Carry Out S e t a 3 1 0 A L U 0 R e s u l t 0 a 0 R e s u l t 1 a 1 0 R e s u l t 2 a 2 0 O p e r a t i o n b 3 1 b 0 b 1 b 2 R e s u l t 3 1 O v e r f l o w B i n v e r t C a r r y I n L e s s C a r r y I n C a r r y O u t A L U 1 L e s s C a r r y I n C a r r y O u t A L U 2 L e s s C a r r y I n C a r r y O u t A L U 3 1 L e s s C a r r y I n slt function: Operation = 3 1-bit ALU31 the most significant bit 0 3 R e s u l t O p e r a t i o n a 1 C a r r y I n 0 1 B i n v e r t b 2 L e s s S e t O v e r f l o w d e t e c t i o n O v e r f l o w Carry Out + =0 Binvert =1
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2 g. babic Presentation F 32-bit ALU with 5 Functions and Zero Function Binvert (1 line) Operation (2 lines) and 0 00 or 0 01 add 0 10 subtract 1 10 slt 1 11 Control lines Carry Out Binvert 17 32-bit ALU with 6 Functions A nor B = A and B Figure B.5.10 (Top) Carry Out Function Ainvert Binvert Operation and 0 0 00 or 0 0 01 add 0 0 10 subtract 0 1 10 slt 0 1 11 nor 1 1 00 Figure B.5.12 + Carry Out + Binvert Binvert
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3 g. babic Presentation F 18 We have (so far) designed an ALU for most (integer) arithmetic and logic functions required by the core MIPS ISA 32-bit ALU with 6 functions omits support for: – shift instructions – XOR logic instruction – integer multiply and divide instructions. Shift instructions: – It would be possible to widen 1-bit ALU multiplexer to include 1-bit shift left and/or 1-bit shift right. – Hardware designers created the circuit called a barrel shifter, which can shift from 1 to 31 bits in less time than it takes to add two 32-bit numbers. Thus, shifting is normally done outside the ALU. Integer multiply/divide is also usually done outside the ALU.
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Cse675.02.F.ALUDesign_part2 - Set Less Than (slt) Function...

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