Arithmetic Logic UnitALU

By: Ahmad YazdankhahCS 147

Fall 2008Prof: Dr. Sin-Min Lee

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• References

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, …)

Review: Bus Concept

Review: CPU Building Blocks

Registers (IR, PC, ACC)

Control Unit(CU)

Arithmetic Logic Unit (ALU)

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)

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.

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 . . .

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.

Adding a full adder to our ALU

Building a 1-Bit ALU (cont’d)

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

logical circuits, we can create bigger ALUs.

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

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