CS1104: Computer Organisation cs1104 Lecture 3: Boolean Algebra cs1104

CS1104: Computer Organisation

http://www.comp.nus.edu.sg/~cs1104

Lecture 3: Boolean Algebra

CS1104-3 Lecture 3: Boolean Algebra 2


Digital circuits

Boolean Algebra

Two-Valued Boolean Algebra

Boolean Algebra Postulates

Precedence of Operators

Truth Table & Proofs

Duality

CS1104-3 Lecture 3: Boolean Algebra 3


Basic Theorems of Boolean Algebra

Boolean Functions

Complement of Functions

Standard Forms

Minterm & Maxterm

Canonical Forms

Conversion of Canonical Forms

Binary Functions

CS1104-3 Introduction 4

IntroductionBoolean algebra forms the basis of logic circuit design. Consider very simple but common example: if (A is true) and (B is false) then print “the solution is found”. In this case, two Boolean expressions (A is true) and (B is false) are related by a connective ‘and’. How do we define these? This and related things are discussed in this chapter.

In typical circuit design, there are many conditions to be taken care of (for example, when the ‘second counter’ = 60, the ‘minute counter’ is incremented and ‘second counter’ is made 0. Thus it is quite important to understand Boolean algebra. In subsequent chapters, we are going to further study how to minimize the circuit using laws of Boolean algebra (that is very interesting…)

CS1104-3 Digital Circuits 5

Digital Circuits (1/2)

Digital circuit can be represented by a black-box with inputs on one side, and outputs on the other.

The input/output signals are discrete/digital in nature, typically with two distinct voltages (a high voltage and a low voltage).

In contrast, analog circuits use continuous signals.

Digital circuit

inputs outputs: :

High

Low

CS1104-3 Digital Circuits 6

Digital Circuits (2/2)

Advantages of Digital Circuits over Analog Circuits: more reliable (simpler circuits, less noise-prone) specified accuracy (determinable) but slower response time (sampling rate)

Important advantages for two-valued Digital Circuit:

Mathematical Model – Boolean Algebra

Can help design, analyse, simplify Digital Circuits.

CS1104-3 Boolean Algebra 7

Boolean Algebra (1/2)

Boolean Algebra named after George Boole who used it to study human logical reasoning – calculus of proposition.

Events : true or false

Connectives : a OR b; a AND b, NOT a

Example: Either “it has rained” OR “someone splashed water”, “must be tall” AND “good vision”.

What is an Algebra? (e.g. algebra of integers)set of elements (e.g. 0,1,2,..)set of operations (e.g. +, -, *,..)postulates/axioms (e.g. 0 + x = x,..)

CS1104-3 Boolean Algebra 8

Boolean Algebra (2/2)

a b a AND bF F FF T FT F FT T T

a b a OR bF F FF T TT F TT T T

a NOT aF TT F

Later, Shannon introduced switching algebra (two-valued Boolean algebra) to represent bi-stable switching circuit.

CS1104-3 Two-valued Boolean Algebra 9

Two-valued Boolean Algebra Set of elements: {0,1}

Set of operations: { ., + , ¬ }

x y x . y0 0 00 1 01 0 01 1 1

x y x + y0 0 00 1 11 0 11 1 1

x ¬x0 11 0

Signals: High = 5V = 1; Low = 0V = 0

x

yx.y

x

yx+y x x'

Sometimes denoted by ’, for example a’

CS1104-3 Boolean Algebra Postulates 10

Boolean Algebra Postulates (1/3)

The set B contains at least two distinct elements x and y.

Closure: For every x, y in B, x + y is in B x . y is in B

Commutative laws: For every x, y in B, x + y = y + x

x . y = y . x

A Boolean algebra consists of a set of elements B, with two binary operations {+} and {.} and a unary operation {'}, such that the following axioms hold:



Associative laws: For every x, y, z in B, (x + y) + z = x + (y + z) = x + y + z (x . y) . z = x .( y . z) = x . y . z

Identities (0 and 1): 0 + x = x + 0 = x for every x in B 1 . x = x . 1 = x for every x in B

Distributive laws: For every x, y, z in B, x . (y + z) = (x . y) + (x . z) x + (y . z) = (x + y) . (x + z)



Complement: For every x in B, there exists an element x' in B such that x + x' = 1 x . x' = 0

The set B = {0, 1} and the logical operations OR, AND and NOT satisfy all the axioms of a Boolean algebra.

A Boolean function maps some inputs over {0,1} into {0,1}

A Boolean expression is an algebraic statement containing Boolean variables and operators.

CS1104-3 Precedence of Operators 13

Precedence of Operators (1/2)

To lessen the brackets used in writing Boolean expressions, operator precedence can be used.

Precedence (highest to lowest): ' . +

Examples:

a . b + c = (a . b) + c

b' + c = (b') + c

a + b' . c = a + ((b') . c)

CS1104-3 Precedence of Operators 14

Precedence of Operators (2/2)

Use brackets to overwrite precedence.

Examples:

a . (b + c)

(a + b)' . c

CS1104-3 Truth Table 15

Truth Table (1/2)

Provides a listing of every possible combination of inputs and its corresponding outputs.

Example (2 inputs, 2 outputs):

x y x . y x + y0 0 0 00 1 0 11 0 0 11 1 1 1

INPUTS OUTPUTS… …… …

CS1104-3 Truth Table 16

Truth Table (2/2)

Example (3 inputs, 2 outputs):

x y z y + z x.(y + z)0 0 0 0 00 0 1 1 00 1 0 1 00 1 1 1 01 0 0 0 01 0 1 1 11 1 0 1 11 1 1 1 1

CS1104-3 Proof using Truth Table 17

Proof using Truth Table

Can use truth table to prove by perfect induction. Prove that: x . (y + z) = (x . y) + (x . z)(i) Construct truth table for LHS & RHS of above equality.

(ii) Check that LHS = RHSPostulate is SATISFIED because output column 2 & 5 (for

LHS & RHS expressions) are equal for all cases.

x y z y + z x.(y + z) x.y x.z (x.y)+(x.z)0 0 0 0 0 0 0 00 0 1 1 0 0 0 00 1 0 1 0 0 0 00 1 1 1 0 0 0 01 0 0 0 0 0 0 01 0 1 1 1 0 1 11 1 0 1 1 1 0 11 1 1 1 1 1 1 1

CS1104-3 Quick Review Questions (1) 18

Quick Review Questions (1)

Textbook page 54.

Question 3-1.

CS1104-3 Duality 19

Duality (1/2)

Duality Principle – every valid Boolean expression (equality) remains valid if the operators and identity elements are interchanged, as follows:

+ .1 0

Example: Given the expressiona + (b.c) = (a+b).(a+c)

then its dual expression isa . (b+c) = (a.b) + (a.c)

CS1104-3 Duality 20

Duality (2/2)

Duality gives free theorems – “two for the price of one”. You prove one theorem and the other comes for free!

If (x+y+z)' = x'.y.'z' is valid, then its dual is also valid:

(x.y.z)' = x'+y'+z’

If x + 1 = 1 is valid, then its dual is also valid:

x . 0 = 0

CS1104-3 Basic Theorems of Boolean Algebra

21

Basic Theorems of Boolean Algebra (1/5)

Apart from the axioms/postulates, there are other useful theorems.

1. Idempotency.

(a) x + x = x (b) x . x = x

Proof of (a):

x + x = (x + x).1 (identity)

= (x + x).(x + x') (complementarity)

= x + x.x' (distributivity)

= x + 0 (complementarity)

= x (identity)


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2. Null elements for + and . operators.

(a) x + 1 = 1 (b) x . 0 = 0

3. Involution. (x')' = x

4. Absorption.

(a) x + x.y = x (b) x.(x + y) = x

5. Absorption (variant).

(a) x + x'.y = x+y (b) x.(x' + y) = x.y


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6. DeMorgan.

(a) (x + y)' = x'.y'

(b) (x.y)' = x' + y'

7. Consensus.

(a) x.y + x'.z + y.z = x.y + x'.z

(b) (x+y).(x'+z).(y+z) = (x+y).(x'+z)


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Theorems can be proved using the truth table method. (Exercise: Prove De-Morgan’s theorem using the truth table.)

They can also be proved by algebraic manipulation using axioms/postulates or other basic theorems.


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Theorem 4a (absorption) can be proved by:

x + x.y = x.1 + x.y (identity)

= x.(1 + y) (distributivity)

= x.(y + 1) (commutativity)

= x.1 (Theorem 2a)

= x (identity)

By duality, theorem 4b:

x.(x+y) = x

Try prove this by algebraic manipulation.

CS1104-3 Boolean Functions 26

Boolean Functions (1/2)

Boolean function is an expression formed with binary variables, the two binary operators, OR and AND, and the unary operator, NOT, parenthesis and the equal sign.

Its result is also a binary value.

We usually use . for AND, + for OR, and ' or ¬ for NOT. Sometimes, we may omit the . if there is no ambiguity.

CS1104-3 Boolean Functions 27

Boolean Functions (2/2)

Examples: F1= x.y.z' F2= x + y'.z F3=(x'.y'.z)+(x'.y.z)+(x.y') F4=x.y'+x'.z

x y z F1 F2 F3 F40 0 0 0 0 0 00 0 1 0 1 1 10 1 0 0 0 0 00 1 1 0 0 1 11 0 0 0 1 1 11 0 1 0 1 1 11 1 0 1 1 0 01 1 1 0 1 0 0

From the truth table, F3=F4.Can you also prove by algebraic manipulation that F3=F4?

CS1104-3 Complement of Functions 28

Complement of Functions (1/2)

Given a function, F, the complement of this function, F', is obtained by interchanging 1 with 0 in the function’s output values.

x y z F1 F1'0 0 0 0 10 0 1 0 10 1 0 0 10 1 1 0 11 0 0 0 11 0 1 0 11 1 0 1 01 1 1 0 1

Example: F1 = x.y.z'

Complement: F1' = (x.y.z')' = x' + y' + (z')' DeMorgan = x' + y' + z Involution

CS1104-3 Complement of Functions 29

Complement of Functions (2/2)

More general DeMorgan’s theorems useful for obtaining complement functions:

(A + B + C + ... + Z)' = A' . B' . C' . … . Z'

(A . B . C ... . Z)' = A' + B' + C' + … + Z'

CS1104-3 Standard Forms 30

Standard Forms (1/3)

Certain types of Boolean expressions lead to gating networks which are desirable from implementation viewpoint.

Two Standard Forms: Sum-of-Products and Product-of-Sums

Literals: a variable on its own or in its complemented form. Examples: x, x' , y, y'

Product Term: a single literal or a logical product (AND) of several literals.

Examples: x, x.y.z', A'.B, A.B, e.g'.w.v



Sum Term: a single literal or a logical sum (OR) of several literals.

Examples: x, x+y+z', A'+B, A+B, c+d+h'+j Sum-of-Products (SOP) Expression: a product term

or a logical sum (OR) of several product terms.

Examples: x, x+y.z', x.y'+x'.y.z, A.B+A'.B', A + B'.C + A.C' + C.D

Product-of-Sums (POS) Expression: a sum term or a logical product (AND) of several sum terms.

Examples: x, x.(y+z'), (x+y').(x'+y+z), (A+B).(A'+B'), (A+B+C).D'.(B'+D+E')



Every Boolean expression can either be expressed as sum-of-products or product-of-sums expression.

Examples:

SOP: x.y + x.y + x.y.z

POS: (x + y).(x + y).(x + z)both: x + y + z or x.y.zneither: x.(w + y.z) or z + w.x.y + v.(x.z + w)

CS1104-3 Minterm & Maxterm 33

Minterm & Maxterm (1/3)

Consider two binary variables x, y.

Each variable may appear as itself or in complemented form as literals (i.e. x, x' & y, y' )

For two variables, there are four possible combinations with the AND operator, namely:

x'.y', x'.y, x.y', x.y

These product terms are called the minterms.

A minterm of n variables is the product of n literals from the different variables.



In general, n variables can give 2n minterms.

In a similar fashion, a maxterm of n variables is the sum of n literals from the different variables.

Examples: x'+y', x'+y, x+y',x+y

In general, n variables can give 2n maxterms.



The minterms and maxterms of 2 variables are denoted by m0 to m3 and M0 to M3 respectively:

Minterms Maxterms x y term notation term notation 0 0 x'.y' m0 x+y M0 0 1 x'.y m1 x+y' M1 1 0 x.y' m2 x'+y M2 1 1 x.y m3 x'+y' M3

Each minterm is the complement of the corresponding maxterm: Example: m2 = x.y'

m2' = (x.y')' = x' + (y')' = x'+y = M2

CS1104-3 Canonical Form: Sum-of-Minterms

36

Canonical Form: Sum of Minterms (1/3)

What is a canonical/normal form? A unique form for representing something.

Minterms are product terms. Can express Boolean functions using Sum-of-Minterms

form.


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a) Obtain the truth table. Example:

x y z F1 F2 F30 0 0 0 0 00 0 1 0 1 10 1 0 0 0 00 1 1 0 0 11 0 0 0 1 11 0 1 0 1 11 1 0 1 1 01 1 1 0 1 0


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b) Obtain Sum-of-Minterms by gathering/summing the minterms of the function (where result is a 1)F1 = x.y.z' = m(6)

F2 = x'.y'.z + x.y'.z' + x.y'.z + x.y.z' + x.y.z = m(1,4,5,6,7)

F3 = x'.y'.z + x'.y.z + x.y'.z' +x.y'.z = m(1,3,4,5)

x y z F1 F2 F30 0 0 0 0 00 0 1 0 1 10 1 0 0 0 00 1 1 0 0 11 0 0 0 1 11 0 1 0 1 11 1 0 1 1 01 1 1 0 1 0

CS1104-3 Canonical Form: Product-of-Maxterms

39

Canonical Form: Product of Maxterms (1/4)

Maxterms are sum terms.

For Boolean functions, the maxterms of a function are the terms for which the result is 0.

Boolean functions can be expressed as Products-of-Maxterms.


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E.g.: F2 = M(0,2,3) = (x+y+z).(x+y'+z).(x+y'+z')

F3 = M(0,2,6,7)

= (x+y+z).(x+y'+z).(x'+y'+z).(x'+y'+z') x y z F1 F2 F30 0 0 0 0 00 0 1 0 1 10 1 0 0 0 00 1 1 0 0 11 0 0 0 1 11 0 1 0 1 11 1 0 1 1 01 1 1 0 1 0


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Why is this so? Take F2 as an example.

F2 = m(1,4,5,6,7)

The complement function of F2 is:

F2' = m(0,2,3)

= m0 + m2 + m3

(Complement functions’ minterms are the opposite of their original functions, i.e. when original function = 0)

x y z F2 F2'0 0 0 0 10 0 1 1 00 1 0 0 10 1 1 0 11 0 0 1 01 0 1 1 01 1 0 1 01 1 1 1 0


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From previous slide, F2' = m0 + m2 + m3

Therefore:

F2 = (m0 + m2 + m3 )'

= m0' . m2' . m3' DeMorgan

= M0 . M2 . M3 mx' = Mx

= M(0,2,3)

Every Boolean function can be expressed as either Sum-of-Minterms or Product-of-Maxterms.

CS1104-3 Quick Reviwe Questions (2) 43


Textbook pages 54-55.

Questions 3-2 to 3-11.

CS1104-3 Conversion of Canonical Forms 44

Conversion of Canonical Forms (1/3)

Sum-of-Minterms Product-of-Maxterms Rewrite minterm shorthand using maxterm shorthand. Replace minterm indices with indices not already used.

Eg: F1(A,B,C) = m(3,4,5,6,7) = M(0,1,2)

Product-of-Maxterms Sum-of-Minterms Rewrite maxterm shorthand using minterm shorthand. Replace maxterm indices with indices not already used.

Eg: F2(A,B,C) = M(0,3,5,6) = m(1,2,4,7)



Sum-of-Minterms of F Sum-of-Minterms of F' In minterm shorthand form, list the indices not already used

in F.

Eg: F1(A,B,C) = m(3,4,5,6,7)

F1'(A,B,C) = m(0,1,2)

Product-of-Maxterms of F Prod-of-Maxterms of F' In maxterm shorthand form, list the indices not already used

in F.

Eg: F1(A,B,C) = M(0,1,2)

F1'(A,B,C) = M(3,4,5,6,7)



Sum-of-Minterms of F Product-of-Maxterms of F' Rewrite in maxterm shorthand form, using the same indices

as in F.

Eg: F1(A,B,C) = m(3,4,5,6,7)

F1'(A,B,C) = M(3,4,5,6,7)

Product-of-Maxterms of F Sum-of-Minterms of F' Rewrite in minterm shorthand form, using the same indices

as in F.

Eg: F1(A,B,C) = M(0,1,2)

F1'(A,B,C) = m(0,1,2)

CS1104-3 Quick Review Questions (3) 47


Textbook page 55.

Question 3-12.

CS1104-3 Binary Functions 48

Binary Functions (1/2)

Given n variables, there are 2n possible minterms.

As each function can be expressed as sum-of-minterms, there could be 22n

different functions.

In the case of two variables, there are 22 =4 possible minterms; and 24=16 different possible binary functions.

The 16 possible binary functions are shown in the next slide.

CS1104-3 Binary Functions 49

Binary Functions (2/2)x y F0 F1 F2 F3 F4 F5 F6 F7

0 0 0 0 0 0 0 0 0 00 1 0 0 0 0 1 1 1 11 0 0 0 1 1 0 0 1 11 1 0 1 0 1 0 1 0 1Symbol . / / +Name AND XOR OR

x y F8 F9 F10 F11 F12 F13 F14 F15

0 0 1 1 1 1 1 1 1 10 1 0 0 0 0 1 1 1 11 0 0 0 1 1 0 0 1 11 1 0 1 0 1 0 1 0 1Symbol ' ' Name NOR XNOR NAND

CS1103 Chapter 3: Boolean Algebra/Logic Gates

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