The document discusses the arithmetic logic unit (ALU), a basic component of central processing units. It defines the ALU as a digital circuit that performs arithmetic and logical operations. The document then reviews computer building blocks like registers and control units. It proceeds to illustrate how to build 1-bit and 3-bit ALUs using logic gates. Useful online resources and references on ALUs and computer architecture are also provided.

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By: Ahmad Yazdankhah
CS 147
Fall 2008
Prof: Dr. Sin-Min Lee
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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
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4. You are familiar with:
1.Number representations (1s complement,
2s comp.)
2.Arithmetic operations (Add, Sub, . . .)
3.Floating Point concept
4.Registers in CPU (Accumulator, Program
Counter, Instruction Register, …)
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 Registers
(IR, PC, ACC)
 Control Unit
(CU)
 Arithmetic
Logic Unit
(ALU)
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Instruction Register
(IR)
Program Counter
(PC)
Control Unit
(CU)
ALU
Status Register
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Accumulator (ACC)
0
1
2
3
4
5
.
CPU RAM

8. 1. ALU stands for: Arithmetic Logic Unit
2. 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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9. 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
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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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Adding a full adder to our ALU
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By paralleling the one-bit ALUs and some other modification on the
logical circuits, we can create bigger ALUs.
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13. Virginia Tech online courses:
http://courses.cs.vt.edu/~csonline/index.html
Virginia Tech online Circuit Simulator
http://courses.cs.vt.edu/~csonline/MachineArchitecture/Virginia Tech Microprocessor Simulation:
http://courses.cs.vt.edu/~csonline/MachineArchitecture/School.edhole.com
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14. Text book: Computer System Architecture
(By: Rob Williams, 2nd Edition, Prentice Hall,
2006)
Site Wikipedia:
http://en.wikipedia.org/wiki/Arithmetic_logic_uni
t
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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