1. Arithmetic Logic Unit ALU
By: Ahmad Yazdankhah
CS 147
Fall 2008
Prof: Dr. Sin-Min Lee

2. 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

3. Assumptions You are familiar with:
Number representations (1s complement, 2s comp.)
Arithmetic operations (Add, Sub, . . .)
Floating Point concept
Registers in CPU (Accumulator, Program Counter, Instruction Register, …)

4. Review: Bus Concept

5. Review: CPU Building Blocks
Registers (IR, PC, ACC)
Control Unit (CU)
Arithmetic Logic Unit (ALU)

6. The Simplest Computer Building Blocks
Instruction Register (IR)
Program Counter (PC) 1 2 3 4 5 .
Control Unit (CU)
ALU
Status Register (FLAG)
Accumulator (ACC)
CPU
RAM

7. What’s ALU? ALU stands for: Arithmetic Logic Unit
ALU is a digital circuit that performs Arithmetic (Add, Sub, . . .) and Logical (AND, OR, NOT) operations.
John Von Neumann proposed the ALU in 1945 when he was working on EDVAC.

8. 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 . . .

9. Let’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.

10. Building a 1-Bit ALU (cont’d)
Adding a full adder to our ALU

11. A 32-Bit ALU
By paralleling the one-bit ALUs and some other modification on the logical circuits, we can create bigger ALUs.

12. Useful Links
Virginia Tech online courses:
Virginia Tech online Circuit Simulator
Virginia Tech Microprocessor Simulation:

13. References:
Text book: Computer System Architecture (By: Rob Williams, 2nd Edition, Prentice Hall, 2006)
Site Wikipedia:
Previous Student Lectures: (By: Anna Kurek, Spring 2008)
Professor’s Lecture Notes.
Virginia Tech online courses: