# Logic in Computer Science: Logic Gates

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1 Logic in Computer Science: Logic Gates Lila Kari The University of Western Ontario Logic in Computer Science: Logic Gates CS2209, Applied Logic for Computer Science 1 / 49

2 Logic and bit operations Computers represent information by bits. A bit has two possible values, namely zero and one. This meaning of the word comes from binary digit, since zeroes and ones are the digits used in binary representations of numbers. The statistician John Tukey introduced this terminology in (There were several other suggested words for a binary digit, including binit and bigit, that never were widely accepted.) John Tukey ( ) Logic in Computer Science: Logic Gates CS2209, Applied Logic for Computer Science 2 / 49

3 Logic and bit operations A bit can be used to represent a truth value, since there are two truth values, true and false. A variable is called a Boolean variable if its values are either true or false. Computer bit operations correspond to the logical connectives. We will also use the notation OR, AND and XOR for, and exclusive. A bit string is a sequence of zero or more bits. The length of the string is the number of bits in the string. Logic in Computer Science: Logic Gates CS2209, Applied Logic for Computer Science 3 / 49

4 Bitwise operations We can extend bit operations to bit strings. We define bitwise OR, bitwise AND and bitwise XOR of two strings of the same length to be the strings that have as their bits the OR, AND and XOR of the corresponding bits in the two strings. Logic in Computer Science: Logic Gates CS2209, Applied Logic for Computer Science 4 / 49

5 Example Example: Find the bitwise OR, bitwise AND and bitwise XOR of the bit strings Logic in Computer Science: Logic Gates CS2209, Applied Logic for Computer Science 5 / 49

6 Example Example: Find the bitwise OR, bitwise AND and bitwise XOR of the bit strings Solution: The bitwise OR is Logic in Computer Science: Logic Gates CS2209, Applied Logic for Computer Science 5 / 49

7 Example Example: Find the bitwise OR, bitwise AND and bitwise XOR of the bit strings Solution: The bitwise OR is The bitwise AND is Logic in Computer Science: Logic Gates CS2209, Applied Logic for Computer Science 5 / 49

8 Example Example: Find the bitwise OR, bitwise AND and bitwise XOR of the bit strings Solution: The bitwise OR is The bitwise AND is and the bitwise XOR is Logic in Computer Science: Logic Gates CS2209, Applied Logic for Computer Science 5 / 49

9 Boolean algebra Circuits in computers and other electronic devices have inputs, each of which is either a 0 or a 1, and produce outputs that are also 0s and 1s. Circuits can be constructed using any basic element that has two different states. Such elements include switches that can be in either the on or the off position and optical devices that can either be lit or unlit. In 1938 Claude Shannon showed how the basic rules of logic, first given by George Boole in 1854 in his The Laws of Thought, could be used to design circuits. Claude Shannon ( ) Logic in Computer Science: Logic Gates CS2209, Applied Logic for Computer Science 6 / 49

10 Boolean algebra George Boole ( ) Boole s rules of logic form the basis for Boolean algebra. In the following we develop the basic properties of Boolean algebra. The operation of a circuit is defined by a Boolean function that specifies the value of an output for each set of inputs. The first step in constructing a circuit is to represent its Boolean function by an expression built up using the basic operations of Boolean algebra. Logic in Computer Science: Logic Gates CS2209, Applied Logic for Computer Science 7 / 49

11 Minimization of circuits The expression that we obtain may contain many more operations than are necessary to represent the function. We will describe later methods for finding an expression with the minimum number of sums and products that represents a Boolean function. The procedures that we will develop are important in the design of efficient circuits. Logic in Computer Science: Logic Gates CS2209, Applied Logic for Computer Science 8 / 49

12 Boolean operations Boolean algebra provides the operations and the rules for working with the set {0, 1}. The three most used operations in Boolean algebra are complementation, the Boolean sum and the Boolean product. They correspond to the logical connectives, and. The complement of an element, denoted by bar, is defined by 0 = 1 and 1 = 0. The Boolean sum, denoted by + or OR has the following values = 1, = 1, = 1, = 0. The Boolean product, denoted by or by AND, has the following values: 1 1 = 1, 1 0 = 0, 0 1 = 0, 0 0 = 0. Logic in Computer Science: Logic Gates CS2209, Applied Logic for Computer Science 9 / 49

13 Boolean operations When there is no danger of confusion, the symbol can be deleted, just as in writing algebraic products. The rules of precedence for connectives still apply: complements have precedence over products and products have precedence over sums. Example: Find the value of (0 + 1). The results about propositional calculus can be translated in results about Boolean algebras. Logic in Computer Science: Logic Gates CS2209, Applied Logic for Computer Science 10 / 49

14 Boolean variables and Boolean functions Let B = {0, 1}. The variable x is called a Boolean variable if it assumes only values from B. A function from B n = {(x 1, x 2,... x n ) x i B, 1 i n} to B is called a Boolean function of degree n. Logic in Computer Science: Logic Gates CS2209, Applied Logic for Computer Science 11 / 49

15 Boolean functions and truth tables The values of a Boolean function are often displayed in tables (truth tables). For instance, the Boolean function F (x, y) with the value 1 when x = 1 and y = 0 and the value 0 for all other choices of x and y can be represented by: x y F (x, y) Exercise: Find the values of the Boolean function represented by F (x, y, z) = xy + z. Logic in Computer Science: Logic Gates CS2209, Applied Logic for Computer Science 12 / 49

16 Boolean functions contd. Two Boolean functions F and G of n variables are equal iff F (b 1,..., b n ) = G(b 1,..., b n ) whenever b 1,... b n belong to B. Boolean functions can be represented using expressions made up from variables and Boolean operations. Each Boolean expression (corresponding to a formula in propositional calculus) represents a Boolean function. Two Boolean expressions that represent the same function are call equivalent. Exercise: How many different Boolean functions of degree n are there? Logic in Computer Science: Logic Gates CS2209, Applied Logic for Computer Science 13 / 49

17 The number of Boolean functions of degree n. Degree Number , ,294,967, ,446,744,073,709,551,616 Logic in Computer Science: Logic Gates CS2209, Applied Logic for Computer Science 14 / 49

18 The abstract definition of a Boolean algebra Until now we have focused on Boolean functions and expressions. However, the results that can be established in this framework can be translated into results about propositions or sets. Because of this, it is useful to define Boolean algebras abstractly. Once it is shown that a particular structure is a Boolean algebra, then all results established about Boolean algebras apply to this particular structure. Boolean algebras can be defined by specifying the properties that operations must satisfy, as is done in the following definition. Logic in Computer Science: Logic Gates CS2209, Applied Logic for Computer Science 15 / 49

19 Definition of a Boolean algebra A Boolean algebra is a set B with two binary operations + and, elements 0 and 1 and a unary operation, such that the following properties hold for all x, y, z in B: Identity laws: x + 0 = x and x 1 = x. Domination laws: x + x = 1, x x = 0. Associative laws: (x + y) + z = x + (y + z), (x y) z = x (y z) Commutative laws: x + y = y + x, x y = y x Distributive laws: x + (y z) = (x + y) (x + z) and x (y + z) = (x y) + (x z). Using the laws given in this definition, it is possible to prove many other laws that hold for every Boolean algebra. Logic in Computer Science: Logic Gates CS2209, Applied Logic for Computer Science 16 / 49

20 Boolean algebra examples B = {0, 1}, with the OR (+) and AND ( ) operations and the complement operator satisfy all these properties. The set of propositions in n variables, with the and operators, 1 and 0, and the negation operator, also satisfy all the properties of a Boolean algebra. The set of subsets of a universal set U, with the union and intersection operations, the empty set and the universal set, and the set complementation operator, is a Boolean algebra. So, to establish results about each of Boolean expressions, propositions, and sets, we need only prove results about abstract Boolean algebras. Logic in Computer Science: Logic Gates CS2209, Applied Logic for Computer Science 17 / 49

21 Representing Boolean functions Given the values of a Boolean function, how can a Boolean expression that represents the function be found? Logic in Computer Science: Logic Gates CS2209, Applied Logic for Computer Science 18 / 49

22 Representing Boolean functions Given the values of a Boolean function, how can a Boolean expression that represents the function be found? This problem is solved by showing that any Boolean function may be represented by a Boolean sum of Boolean products of the variables and their complements. Logic in Computer Science: Logic Gates CS2209, Applied Logic for Computer Science 18 / 49

23 Representing Boolean functions Given the values of a Boolean function, how can a Boolean expression that represents the function be found? This problem is solved by showing that any Boolean function may be represented by a Boolean sum of Boolean products of the variables and their complements. The method is similar to the solution to a problem encountered in propositional calculus: Given a truth table, how can a formula that has that truth table be found? Logic in Computer Science: Logic Gates CS2209, Applied Logic for Computer Science 18 / 49

24 Representing Boolean functions Given the values of a Boolean function, how can a Boolean expression that represents the function be found? This problem is solved by showing that any Boolean function may be represented by a Boolean sum of Boolean products of the variables and their complements. The method is similar to the solution to a problem encountered in propositional calculus: Given a truth table, how can a formula that has that truth table be found? Answer: The solution is a disjunction of minterms (conjunction of literals and their negations). Logic in Computer Science: Logic Gates CS2209, Applied Logic for Computer Science 18 / 49

25 The answer to this problem shows that every Boolean function can be represented using the three Boolean operators +,,. Logic in Computer Science: Logic Gates CS2209, Applied Logic for Computer Science 19 / 49

26 The answer to this problem shows that every Boolean function can be represented using the three Boolean operators +,,. Is there a smaller set of operators that can be used to represent all Boolean functions? Logic in Computer Science: Logic Gates CS2209, Applied Logic for Computer Science 19 / 49

27 The answer to this problem shows that every Boolean function can be represented using the three Boolean operators +,,. Is there a smaller set of operators that can be used to represent all Boolean functions? The answer is similar to the solution to the problem: Does there exist an adequate set of connectives with fewer than three elements? Logic in Computer Science: Logic Gates CS2209, Applied Logic for Computer Science 19 / 49

28 The answer to this problem shows that every Boolean function can be represented using the three Boolean operators +,,. Is there a smaller set of operators that can be used to represent all Boolean functions? The answer is similar to the solution to the problem: Does there exist an adequate set of connectives with fewer than three elements? Answer: Yes (For example, the Sheffer stroke). Logic in Computer Science: Logic Gates CS2209, Applied Logic for Computer Science 19 / 49

29 The answer to this problem shows that every Boolean function can be represented using the three Boolean operators +,,. Is there a smaller set of operators that can be used to represent all Boolean functions? The answer is similar to the solution to the problem: Does there exist an adequate set of connectives with fewer than three elements? Answer: Yes (For example, the Sheffer stroke). Both of these problems have practical importance in circuit design. Logic in Computer Science: Logic Gates CS2209, Applied Logic for Computer Science 19 / 49

30 Example Find a Boolean expression that represents the function F (x, y, z) given in the following table: x y z F Logic in Computer Science: Logic Gates CS2209, Applied Logic for Computer Science 20 / 49

31 Example Find a Boolean expression that represents the function F (x, y, z) given in the following table: Answer: xyz + xyz x y z F Logic in Computer Science: Logic Gates CS2209, Applied Logic for Computer Science 20 / 49

32 Example Find a Boolean expression that represents the function F (x, y, z) given in the following table: x y z F Answer: xyz + xyz The disjunctive normal form obtained is also called sum-of-products expansion. Logic in Computer Science: Logic Gates CS2209, Applied Logic for Computer Science 20 / 49

33 Functional completeness Every Boolean function can be expressed as a Boolean sum of minterms (products of Boolean variables and their complements). Since every Boolean function can be represented using the operators {+,, }, we say that this set of operators is functionally complete. Logic in Computer Science: Logic Gates CS2209, Applied Logic for Computer Science 21 / 49

34 Functional completeness Every Boolean function can be expressed as a Boolean sum of minterms (products of Boolean variables and their complements). Since every Boolean function can be represented using the operators {+,, }, we say that this set of operators is functionally complete. Can we find a smaller functionally complete set of operators? Logic in Computer Science: Logic Gates CS2209, Applied Logic for Computer Science 21 / 49

35 Functional completeness Every Boolean function can be expressed as a Boolean sum of minterms (products of Boolean variables and their complements). Since every Boolean function can be represented using the operators {+,, }, we say that this set of operators is functionally complete. Can we find a smaller functionally complete set of operators? Yes. {+, }, {, }, { } (The NAND operator or Sheffer stroke) and { } (The NOR operator) are all functionally complete. The NAND operator is defined by 1 1 = 0 and 1 0 = 0 1 = 0 0 = 1. The NOR operator is defined by 1 1 = 1 0 = 0 1 = 0 and 0 0 = 1. Logic in Computer Science: Logic Gates CS2209, Applied Logic for Computer Science 21 / 49

36 Electronic circuits and logic gates Boolean algebra is used to model the circuitry of electronic devices, including electronic computers. Each input and output of such a device can be thought of as a member of the set {0, 1}. An electronic computer is made up of a number of circuits. Each circuit can be designed using the rules of Boolean algebra. The basic elements of circuits are called logic gates. A gate is an electronic device that operates on a collection of binary inputs and produces a binary output. Logic in Computer Science: Logic Gates CS2209, Applied Logic for Computer Science 22 / 49

37 Logic gates Each type of gate implements a Boolean operation. We will define several types of gates. Using these gates we will apply the rules of Boolean algebra to design circuits that perform a variety of tasks. The circuits that we study give output that depends only on the inputs, and not the current state of the circuit. In other words, these circuits have no memory abilities. Such circuits are called combinatorial circuits. Logic in Computer Science: Logic Gates CS2209, Applied Logic for Computer Science 23 / 49

38 The inverter The inverter accepts a Boolean value as input and produces the complement of this value as its output. The symbol used for an inverter is shown below The input to the inverter is shown on the left side entering the element, and the output is shown on the right side leaving the element. Logic in Computer Science: Logic Gates CS2209, Applied Logic for Computer Science 24 / 49

39 The OR gate The second type gate is the OR gate. The input to this gate are the values of two or more Boolean variables. The output is the Boolean sum or their values. The symbol used for an OR gate is shown below. The inputs to the OR gate are shown on the left side entering the element and the output is shown on the right side leaving the element. Logic in Computer Science: Logic Gates CS2209, Applied Logic for Computer Science 25 / 49

40 The AND gate The third type of gate is the AND gate. The inputs to this gate are the values of two or more Boolean variables. The output is the Boolean product or their values. The symbol used for an AND gate is shown below. The inputs to an AND gate are shown on the left side entering the element, and the output is shown on the right side leaving the element. Logic in Computer Science: Logic Gates CS2209, Applied Logic for Computer Science 26 / 49

41 Gates with multiple inputs We will permit multiple inputs to AND and OR gates, as below. Logic in Computer Science: Logic Gates CS2209, Applied Logic for Computer Science 27 / 49

42 Circuits: Combinations of gates Combinatorial circuits can be constructed using a combination of inverters, OR gates and AND gates. When combinations of circuits are formed, some gates may share inputs. This is shown in one of two ways in depictions of circuits. One method is to use branchings to indicate all the gates that use a given input. The other method is to indicate this input separately for each gate. Note that output from a gate may be used as input by one or more elements, Logic in Computer Science: Logic Gates CS2209, Applied Logic for Computer Science 28 / 49

43 How to draw circuits Both drawings below depict the circuit with output xy + xy. Logic in Computer Science: Logic Gates CS2209, Applied Logic for Computer Science 29 / 49

44 Example Example: Construct the circuits that produce the following outputs: (a) (x + y)x (b) x(y + z) (c) (x + y + z)(x y z). Logic in Computer Science: Logic Gates CS2209, Applied Logic for Computer Science 30 / 49

45 Example answers (a) (x + y)x Logic in Computer Science: Logic Gates CS2209, Applied Logic for Computer Science 31 / 49

46 (b) x(y + z) Logic in Computer Science: Logic Gates CS2209, Applied Logic for Computer Science 32 / 49

47 (c) (x + y + z)(x y z). Logic in Computer Science: Logic Gates CS2209, Applied Logic for Computer Science 33 / 49

48 Examples of circuits Example 1. A committee of three individuals decides issues for an organization. Each individual votes either yes or no for each proposal that arises. A proposal is passed if it receives at least two yes votes. Design a circuit that determines whether a proposal passes. Logic in Computer Science: Logic Gates CS2209, Applied Logic for Computer Science 34 / 49

49 Examples of circuits Example 1. A committee of three individuals decides issues for an organization. Each individual votes either yes or no for each proposal that arises. A proposal is passed if it receives at least two yes votes. Design a circuit that determines whether a proposal passes. Solution: Let x = 1 be 1 if the first individual says yes and x = 0 if this individual says no. Similarly for y and z. Then a circuit must be designed that produces output 1 from the inputs x, y, z when two or more of x, y, z are 1. Logic in Computer Science: Logic Gates CS2209, Applied Logic for Computer Science 34 / 49

50 Examples of circuits Example 1. A committee of three individuals decides issues for an organization. Each individual votes either yes or no for each proposal that arises. A proposal is passed if it receives at least two yes votes. Design a circuit that determines whether a proposal passes. Solution: Let x = 1 be 1 if the first individual says yes and x = 0 if this individual says no. Similarly for y and z. Then a circuit must be designed that produces output 1 from the inputs x, y, z when two or more of x, y, z are 1. One representation of the Boolean function that has these output values is xy + xz + yz. Logic in Computer Science: Logic Gates CS2209, Applied Logic for Computer Science 34 / 49

51 Circuit for majority voting Logic in Computer Science: Logic Gates CS2209, Applied Logic for Computer Science 35 / 49

52 Example 2. Example 2. Sometimes light fixtures are controlled by more than one switch. Circuits need to be designed so that flipping any one of the switches for the fixture turns the light on when it is off and turns the light off when it is on. Design circuits that accomplish this when there are two switches and when there are three switches. Logic in Computer Science: Logic Gates CS2209, Applied Logic for Computer Science 36 / 49

53 Example 2. Example 2. Sometimes light fixtures are controlled by more than one switch. Circuits need to be designed so that flipping any one of the switches for the fixture turns the light on when it is off and turns the light off when it is on. Design circuits that accomplish this when there are two switches and when there are three switches. Solution: The two-switch case. Let x = 1 when the first switch is closed and 0 when it is open, and let y = 1 when the second switch is closed and 0 when it is open. Logic in Computer Science: Logic Gates CS2209, Applied Logic for Computer Science 36 / 49

54 Example 2. Example 2. Sometimes light fixtures are controlled by more than one switch. Circuits need to be designed so that flipping any one of the switches for the fixture turns the light on when it is off and turns the light off when it is on. Design circuits that accomplish this when there are two switches and when there are three switches. Solution: The two-switch case. Let x = 1 when the first switch is closed and 0 when it is open, and let y = 1 when the second switch is closed and 0 when it is open. Let F (x, y) = 1 when the light is on and F (x, y) = 0 when the light is off. Logic in Computer Science: Logic Gates CS2209, Applied Logic for Computer Science 36 / 49

55 We can arbitrarily decide that the light will be on when both switches are closed, so that F (1, 1) = 1. Logic in Computer Science: Logic Gates CS2209, Applied Logic for Computer Science 37 / 49

56 We can arbitrarily decide that the light will be on when both switches are closed, so that F (1, 1) = 1. This determines all the other values of F. Logic in Computer Science: Logic Gates CS2209, Applied Logic for Computer Science 37 / 49

57 We can arbitrarily decide that the light will be on when both switches are closed, so that F (1, 1) = 1. This determines all the other values of F. When one of the two switches is opened, the light goes off, so F (0, 1) = F (1, 0) = 0. When the other switch is opened the light goes on, so that F (0, 0) = 1. Logic in Computer Science: Logic Gates CS2209, Applied Logic for Computer Science 37 / 49

58 We can arbitrarily decide that the light will be on when both switches are closed, so that F (1, 1) = 1. This determines all the other values of F. When one of the two switches is opened, the light goes off, so F (0, 1) = F (1, 0) = 0. When the other switch is opened the light goes on, so that F (0, 0) = 1. We see that F (x, y) = xy + x y. Logic in Computer Science: Logic Gates CS2209, Applied Logic for Computer Science 37 / 49

59 Circuit for two-switch light fixture Logic in Computer Science: Logic Gates CS2209, Applied Logic for Computer Science 38 / 49

60 Example 2 contd. Example: The three-switch case. In this case we have three variables, x, y, z that take value 1 (switch closed) or 0 (switch open). Logic in Computer Science: Logic Gates CS2209, Applied Logic for Computer Science 39 / 49

61 Example 2 contd. Example: The three-switch case. In this case we have three variables, x, y, z that take value 1 (switch closed) or 0 (switch open). Let F (x, y, z) = 1 when the light is on and 0 when it is off. Logic in Computer Science: Logic Gates CS2209, Applied Logic for Computer Science 39 / 49

62 Example 2 contd. Example: The three-switch case. In this case we have three variables, x, y, z that take value 1 (switch closed) or 0 (switch open). Let F (x, y, z) = 1 when the light is on and 0 when it is off. We can arbitrarily specify that the light be on when all three switches are closed, so that F (1, 1, 1) = 1. Logic in Computer Science: Logic Gates CS2209, Applied Logic for Computer Science 39 / 49

63 Example 2 contd. Example: The three-switch case. In this case we have three variables, x, y, z that take value 1 (switch closed) or 0 (switch open). Let F (x, y, z) = 1 when the light is on and 0 when it is off. We can arbitrarily specify that the light be on when all three switches are closed, so that F (1, 1, 1) = 1. This determines all the other values of F. Logic in Computer Science: Logic Gates CS2209, Applied Logic for Computer Science 39 / 49

64 Example 2 contd. Example: The three-switch case. In this case we have three variables, x, y, z that take value 1 (switch closed) or 0 (switch open). Let F (x, y, z) = 1 when the light is on and 0 when it is off. We can arbitrarily specify that the light be on when all three switches are closed, so that F (1, 1, 1) = 1. This determines all the other values of F. The function F can be represented as xyz + xy z + x yz + x y z. Logic in Computer Science: Logic Gates CS2209, Applied Logic for Computer Science 39 / 49

65 Circuit for three-switch light fixture Logic in Computer Science: Logic Gates CS2209, Applied Logic for Computer Science 40 / 49

66 Adders We will illustrate how logic circuits can be used to carry out addition of two positive integers from their binary expansions. We will build up the circuitry to do this addition from some component circuits. First we build a circuit that can be used to find x + y when x and y are two bits. The input to our circuit will be x and y, since each of these have value 0 or 1. The output will consist of two bits, namely s and c where s is the sum bit and c is the carry bit. Logic in Computer Science: Logic Gates CS2209, Applied Logic for Computer Science 41 / 49

67 The half-adder This circuit is called a multiple output circuit. The circuit we are designing is called the half-adder since it adds two bits, without considering a carry from the previous addition. We show the input and output for the half adder in the following table. Logic in Computer Science: Logic Gates CS2209, Applied Logic for Computer Science 42 / 49

68 The multiple output half-adder truth table x y s c From the preceding table we see that c = xy and xy + xy = (x + y)(xy). Logic in Computer Science: Logic Gates CS2209, Applied Logic for Computer Science 43 / 49

69 Circuit for half-adder Logic in Computer Science: Logic Gates CS2209, Applied Logic for Computer Science 44 / 49

70 The full adder Input: bits x and y and the carry bit c i. Output: The sum bit s and the carry bit c i+1. Input Output x y c i s c i Logic in Computer Science: Logic Gates CS2209, Applied Logic for Computer Science 45 / 49

71 Boolean functions for full adder Note that s = xyc i + xy c i + x yc i + x y c i. c i+1 = xyc i + xyc i + xy c i + x yc i. Logic in Computer Science: Logic Gates CS2209, Applied Logic for Computer Science 46 / 49

72 Circuit for full adder using half adders However, instead of building the full adder from the scratch, we will use half adders to produce the desired outputs. Logic in Computer Science: Logic Gates CS2209, Applied Logic for Computer Science 47 / 49

73 Adding three-bit integers Finally, we show how full and half adders can be used to add the three-bit integers (x 2 x 1 x 0 ) 2 and (y 2 y 1 y 0 ) 2 to produce the sum (s 3 s 2 s 1 s 0 ) 2. Note that s 3, the highest-order bit in the sum is given by the carry c 2. Logic in Computer Science: Logic Gates CS2209, Applied Logic for Computer Science 48 / 49

74 Acknowledgements The figures in this set of notes are from K.Rosen, Discrete Mathematics and Its Applications, 7th edition, McGraw Hill, Logic in Computer Science: Logic Gates CS2209, Applied Logic for Computer Science 49 / 49

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