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 0’s detector: A = 00…000 1’s detector: A = 11…111 Equality comparator: A = B Magnitude comparator: A < B 18: Datapath Functional Units 4 CMOS VLSI Design CMOS VLSI Design 4th Ed. 1’s & 0’s Detectors 1’s detector: N-input AND gate 0’s detector: NOTs + 1’s 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) 1’s 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 0’s • 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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This note was uploaded on 01/15/2012 for the course ECE260A 660090 taught by Professor Bendak,michaelbeshara during the Fall '09 term at UCSD.

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

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