Arithmetic Logic UnitALU
By: Ahmad YazdankhahCS 147
Fall 2008Prof: Dr. Sin-Min Lee
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Table of Contents• Assumptions• Review: Bus Concept• Review: CPU Building Block• The Simplest Computer Building Blocks • What’s ALU?• Typical Schematic Symbol of an ALU• Let’s Build a 1-Bit ALU• Useful Links• Referencesadmission.edhole.com
Assumptions
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: CPU Building Blocks
Registers (IR, PC, ACC)
Control Unit(CU)
Arithmetic Logic Unit (ALU)
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The Simplest Computer Building Blocks
Instruction Register (IR) Program Counter (PC)
Control Unit (CU)
ALU
Accumulator (ACC)
0
1
2
3
4
5
.
CPU RAM
Status Register (FLAG)
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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 . . .
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Let’s Build a 1-Bit ALUThis 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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Adding a full adder to our ALU
Building a 1-Bit ALU (cont’d)
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A 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 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:
• 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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