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ALU

This document discusses the arithmetic logic unit (ALU) and its role in a central processing unit (CPU). It begins with an overview of the ALU and its functions, including that it performs arithmetic and logical operations. The document then shows a typical schematic symbol for an ALU and builds a sample 1-bit ALU circuit. It concludes by mentioning how ALUs can be expanded by connecting more 1-bit circuits in parallel.

Education◦
By: Ahmad Yazdankhah CS 147 Fall 2008 Prof: 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 concept 4. 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 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.
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 Simulator http://courses.cs.vt.edu/~csonline/MachineArchitecture/L  Virginia Tech Microprocessor Simulation: http://courses.cs.vt.edu/~csonline/MachineArchitect ure/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