By: Ahmad YazdankhahCS 147

Fall 2008Prof: Dr. Sin-Min Lee

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

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, …)

Registers (IR, PC, ACC)

Control Unit(CU)

Arithmetic Logic Unit (ALU)

Instruction Register (IR)

Program Counter (PC)

Control Unit (CU)

ALU

Accumulator (ACC)

0

1

2

3

4

5

.

CPU RAM

Status Register (FLAG)

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 . . .

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.

Adding a full adder to our ALU

By paralleling the one-bit ALUs and some other modification on the logical circuits, we can create bigger ALUs.

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

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