CPSC 171 Introduction to Computer Science Boolean Logic, Gates, & Circuits.

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Transcript of CPSC 171 Introduction to Computer Science Boolean Logic, Gates, & Circuits.

  • CPSC 171 Introduction to Computer ScienceBoolean Logic, Gates, & Circuits

  • AnnouncementsRead Chapter 4

    Exam, Oct 2nd in class

  • Boolean LogicA Boolean variable, A, is either true or falseA Boolean expression, (A AND B), evaluates to either true or falseBoolean operators include:AND (& )OR ( + )NOT (a bar ' ~)

  • Boolean Operatorsa AND btrue only when A and B are both truea OR btrue when A is true, B is true, or both are trueNOT atrue when A is false

  • Truth TablesTruth tables can be used to capture when an expression is true, given its inputsYou make truth tables for AND and NOT

  • Example Boolean Expressions(a AND b) OR (NOT a AND c)ab + ~acab+c

    Truth tables can be made for complex expressions as well

  • Boolean Logic (continued)Example: (a AND b) OR ((NOT b) and (NOT a))


  • GatesGatesHardware devices built from transistors to mimic Boolean logicAn electronic device that operates on a collection of binary inputs to produce a single binary output

    AND gate (page 161 in text)Two input lines, one output lineOutputs a 1 when both inputs are 1

  • Gates (continued)OR gate (page 163 in text)Two input lines, one output lineOutputs a 1 when either input is 1

    NOT gate (page 161 in textOne input line, one output lineOutputs a 1 when input is 0 and vice versa

  • Figure 4.15The Three Basic Gates and Their Symbols

  • CircuitsA collection of logic gates that transforms a set of binary inputs into a set of binary outputsWire gates together keeping constraints for the number of inputs to any gate

  • Example CircuitIf a, b, c, and d are all true the output can be determined by tracing through the circuitoutput11111100

  • Designing CircuitsA circuit construction algorithmTruth Table ConstructionDetermine outputs for every possible inputSub-expression Construction (using AND and NOT gates)For each output find the rows that are 1 and build a sub-expression that is true for the exact inputSub-expression combination (using OR gates)Take each subexpression and combine them, 2 at a time, using OR gatesCircuit Diagram ProductionConstruct final circuit by converting Boolean operators into gates

  • Example Circuit DesignDesign a 3-input circuit that is true if exactly two inputs are true, and false otherwise

    You Try it: Design a 2-input circuit that is true if the inputs are the same, and false otherwise

  • Examples of Circuit Design and ConstructionCompare-for-equality circuitAddition circuitBoth circuits can be built using the circuit design algorithm

  • A Compare-for-Equality CircuitCE compares two unsigned binary integers for equalityBuilt by combining together 1-bit comparison circuits (1-CE)Integers are equal if corresponding bits are equal (AND together 1-CD circuits for each pair of bits)

  • A Compare-for-Equality Circuit (continued)

    1-CE circuit truth table

  • A Compare-for-Equality Circuit (continued)1-CE Boolean expressionFirst case: (NOT a) AND (NOT b)Second case: a AND bCombined:((NOT a) AND (NOT b)) OR (a AND b)

  • Figure 4.22One-Bit Compare-for-Equality Circuit

  • N-Bit Compare for Equality Circuit

    AND together the 1-CE circuits, two at a time

  • An Addition CircuitAdds two unsigned binary integers, setting output bits and an overflowBuilt from 1-bit adders (1-ADD)Starting with rightmost bits, each pair producesA value for that orderA carry bit for next place to the left

  • An Addition Circuit (continued)1-ADD truth tableInputOne bit from each input integerOne carry bit (always zero for rightmost bit)OutputOne bit for output place valueOne carry bit

  • Figure 4.24The 1-ADD Circuit and Truth Table

  • An Addition Circuit (continued)Building the full adder

    Put rightmost bits into 1-ADD, with zero for the input carrySend 1-ADDs output value to output, and put its carry value as input to 1-ADD for next bits to leftRepeat process for all bitsSee pg 174, 175, 176