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Arithmetic Logic UnitArithmetic Logic UnitALUALU

By: Ahmad YazdankhahCS 147

Fall 2008Prof: Dr. Sin-Min Lee

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Table of ContentsTable of Contents

AssumptionsReview: Bus ConceptReview: CPU Building BlockThe Simplest Computer Building Blocks What’s ALU?Typical Schematic Symbol of an ALULet’s Build a 1-Bit ALUUseful LinksReferences

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AssumptionsAssumptions

You are familiar with:1.Number representations (1s complement,

2s comp.)2.Arithmetic operations (Add, Sub, . . .)3.Floating Point concept4.Registers in CPU (Accumulator, Program

Counter, Instruction Register, …)

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Review: Bus ConceptReview: Bus ConceptSchool.edhole.com

Review: CPU Building BlocksReview: CPU Building Blocks

Registers (IR, PC, ACC)

Control Unit(CU)

Arithmetic Logic Unit (ALU)

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The Simplest Computer Building The Simplest Computer Building BlocksBlocks

Instruction Register (IR)

Program Counter (PC)

Control Unit (CU)

ALU

Accumulator (ACC)

0

1

2

3

4

5

.

CPU RAM

Status Register (FLAG)School.edhole.com

What’s ALU?What’s ALU?

1. ALU stands for: Arithmetic Logic Unit2. ALU is a digital circuit that performs

Arithmetic (Add, Sub, . . .) and Logical (AND, OR, NOT) operations.

3. John Von Neumann proposed the ALU in 1945 when he was working on EDVAC.

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Typical Schematic Symbol of an ALU

A and B: the inputs to the ALU(aka operands)R: Output or Result F: Code or Instruction from the Control Unit (aka as op-code)D: Output status; it indicates cases such as:•carry-in•carry-out, •overflow, •division-by-zero•And . . .School.edhole.com

Let’s Build a 1-Bit ALULet’s Build a 1-Bit ALU

This is an one-bit ALU which can do Logical AND and Logical OR operation.

Result = a AND b when operation = 0

Result = a OR b when operation = 1

The operation line is the input of a MUX.

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Building a 1-Bit ALU Building a 1-Bit ALU (cont’d)(cont’d)

Adding a full adder to our ALU

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A 32-Bit ALUA 32-Bit ALUBy paralleling the one-bit ALUs and some other modification on the

logical circuits, we can create bigger ALUs.

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Useful LinksUseful Links

Virginia Tech online courses:http://courses.cs.vt.edu/~csonline/index.html

Virginia Tech online Circuit Simulatorhttp://courses.cs.vt.edu/~csonline/MachineArchitecture/Lessons/Circuits/simcir_frame.html

Virginia Tech Microprocessor Simulation:http://courses.cs.vt.edu/~csonline/MachineArchitecture/Lessons/CPU/sumprogram.html

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References:References:

Text book: Computer System Architecture (By: Rob Williams, 2nd Edition, Prentice Hall, 2006)

Site Wikipedia: http://en.wikipedia.org/wiki/Arithmetic_logic_unit

Previous Student Lectures:(By: Anna Kurek, Spring 2008)

Professor’s Lecture Notes.Virginia Tech online courses:

http://courses.cs.vt.edu/~csonline/index.html

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