lect18-datapath

lect18-datapath - Lecture 18: Datapath Functional Units 18:...

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Unformatted text preview: Lecture 18: Datapath Functional Units 18: Datapath Functional Units 2 CMOS VLSI Design CMOS VLSI Design 4th Ed. Outline Comparators Shifters Multi-input Adders Multipliers 18: Datapath Functional Units 3 CMOS VLSI Design CMOS VLSI Design 4th Ed. Comparators 0s detector: A = 00000 1s detector: A = 11111 Equality comparator: A = B Magnitude comparator: A < B 18: Datapath Functional Units 4 CMOS VLSI Design CMOS VLSI Design 4th Ed. 1s & 0s Detectors 1s detector: N-input AND gate 0s detector: NOTs + 1s detector (N-input NOR) A A 1 A 2 A 3 A 4 A 5 A 6 A 7 allones A A 1 A 2 A 3 allzeros allones A 1 A 2 A 3 A 4 A 5 A 6 A 7 A 18: Datapath Functional Units 5 CMOS VLSI Design CMOS VLSI Design 4th Ed. Equality Comparator Check if each bit is equal (XNOR, aka equality gate) 1s detect on bitwise equality A[0] B[0] A = B A[1] B[1] A[2] B[2] A[3] B[3] 18: Datapath Functional Units 6 CMOS VLSI Design CMOS VLSI Design 4th Ed. Magnitude Comparator Compute B A and look at sign B A = B + ~A + 1 For unsigned numbers, carry out is sign bit A B A 1 B 1 A 2 B 2 A 3 B 3 A = B Z C A B N A B 18: Datapath Functional Units 7 CMOS VLSI Design CMOS VLSI Design 4th Ed. Signed vs. Unsigned For signed numbers, comparison is harder C: carry out Z: zero (all bits of A B are 0) N: negative (MSB of result) V: overflow (inputs had different signs, output sign B) S: N xor V (sign of result) 18: Datapath Functional Units 8 CMOS VLSI Design CMOS VLSI Design 4th Ed. Shifters Logical Shift: Shifts number left or right and fills with 0s 1011 LSR 1 = 0101 1011 LSL1 = 0110 Arithmetic Shift: Shifts number left or right. Rt shift sign extends 1011 ASR1 = 1101 1011 ASL1 = 0110 Rotate: Shifts number left or right and fills with lost bits 1011 ROR1 = 1101 1011 ROL1 = 0111 18: Datapath Functional Units 9 CMOS VLSI Design CMOS VLSI Design 4th Ed....
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lect18-datapath - Lecture 18: Datapath Functional Units 18:...

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