CDA3101-L12-alu-MIPSalu

CDA3101-L12-alu-MIPSalu - CDA 3101 Fall 2011 Introduction...

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CDA 3101 Fall 2011 Introduction to Computer Organization The Arithmetic Logic Unit (ALU) and MIPS ALU Support 28 September 2011
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Overview Hardware building blocks ALU design ALU implementation 1-bit ALU 32-bit ALU
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Hardware Building Blocks ALUs are implemented using lower level components (logic gates) Gate (review) Hardware element that receives a certain number of inputs and produces one output Can be represented as a truth table or logic equation Gates in turn are implemented with transistors ALU Building Blocks (review) And gate Or gate Inverter (not gate) Multiplexor (mux)
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Basic Gates
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Modular ALU Design Facts Building blocks work with individual (I/O) bits ALU works with 32-bit registers ALU performs a variety of tasks (+, -, *, /, shift, etc) Principles Build 32 separate 1-bit ALUs Build separate hardware blocks for each task Perform all operations in parallel Use a mux to choose the actual operation (make decision) Advantages Easy to add new operations (instructions) Add new data lines into the muxes; inform “control” of the change.
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ALU Implementation 1. 32-bit ALU uses 32 muxes ( one for each output bit ) 2. Go through instruction set and add data (and control) lines to implement the corresponding operations. Control lines (n) Output : one per mux Data lines (2 n )
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One-Bit Logical Instructions Map directly onto hardware components AND instruction One of data lines should be a simple AND gate OR instruction Another data line should be a simple OR gate A B C 0 1 Op (control) Definition Op C 0 A and B 1 A or B
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One-Bit Full Adder Each bit of addition has – Three input bits: A i , B i , CarryIn i – Two output bits: Sum i , CarryOut i ( CarryIn i+1 = CarryOut i ) CarryIn . . . (0) (1) (0) (0) (0) . . . 0 0 1 0 1 . . . 0 0 1 1 0 . . . 0 (0) 1 (1) 0 (0) 1 (0) 1 + Inputs Outputs A: B: Sum : CarryOut
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Full Adder’s Truth Table Sum
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This note was uploaded on 12/10/2011 for the course CDA 3101 taught by Professor Small during the Fall '08 term at University of Florida.

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CDA3101-L12-alu-MIPSalu - CDA 3101 Fall 2011 Introduction...

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