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COMBINATIONAL LOGIC CIRCUITS
expressed in a variety of ways. The particular expression used to represent the
function dictates the interconnection of gates in the logic circuit diagram. By
manipulating a Boolean expression according to Boolean algebraic rules, it is often
possible to obtain a simpler expression for the same function. This simpler expres-
sion reduces both the number of gates in the circuit and the numbers of inputs to
the gates. To see how this is done, we must first study the basic rules of Boolean
algebra.
Basic Identities of Boolean Algebra
Table 3 lists the most basic identities of Boolean algebra. The notation is simplified
by omitting the symbol for AND whenever doing so does not lead to confusion.
The first nine identities show the relationship between a single variable X, its com-
plement X, and the binary constants 0 and 1. The next five identities, 10 through 14,
have counterparts in ordinary algebra. The last three, 15 through 17, do not apply
in ordinary algebra, but are useful in manipulating Boolean expressions.
The basic rules listed in the table have been arranged into two columns that
demonstrate the property of duality of Boolean algebra. The dual of an algebraic
expression is obtained by interchanging OR and AND operations and replacing 1s
by 0s and 0s by 1s. An equation in one column of the table can be obtained from
the corresponding equation in the other column by taking the dual of the expres-
sions on both sides of the equals sign. For example, relation 2 is the dual of relation
1 because the OR has been replaced by an AND and the 0 by 1. It is important to
note that most of the time the dual of an expression is not equal to the original
expression, so that an expression usually cannot be replaced by its dual.
The nine identities involving a single variable can be easily verified by sub-
stituting each of the two possible values for X. For example, to show that X ! 0 "
X, let X " 0 to obtain 0 ! 0 " 0, and then let X " 1 to obtain 1 ! 0 " 1. Both
TABLE 3
Basic Identities of Boolean Algebra
1. 2.
3. 4.
5. 6.
7. 8.
9.
10. 11. Commutative
12. 13. Associative
14. 15. Distributive
16. 17. DeMorgan’s
X 0
! X
" X 1
# X
"
X 1
! 1
" X 0
# 0
"
X X
! X
" X X
# X
"
X X
! 1
" X X
# 0
"
X X
"
X Y
! Y X
!
" XY YX
"
X Y Z
!
( )
! X Y
!
( ) Z
!
" X YZ
( ) XY
( )Z
"
X Y Z
!
( ) XY XZ
!
" X YZ
! X Y
!
( ) X Z
!
( )
"
X Y
! X Y
#
" X Y
# X Y
!
"

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Boolean Identities

  • 1. COMBINATIONAL LOGIC CIRCUITS expressed in a variety of ways. The particular expression used to represent the function dictates the interconnection of gates in the logic circuit diagram. By manipulating a Boolean expression according to Boolean algebraic rules, it is often possible to obtain a simpler expression for the same function. This simpler expres- sion reduces both the number of gates in the circuit and the numbers of inputs to the gates. To see how this is done, we must first study the basic rules of Boolean algebra. Basic Identities of Boolean Algebra Table 3 lists the most basic identities of Boolean algebra. The notation is simplified by omitting the symbol for AND whenever doing so does not lead to confusion. The first nine identities show the relationship between a single variable X, its com- plement X, and the binary constants 0 and 1. The next five identities, 10 through 14, have counterparts in ordinary algebra. The last three, 15 through 17, do not apply in ordinary algebra, but are useful in manipulating Boolean expressions. The basic rules listed in the table have been arranged into two columns that demonstrate the property of duality of Boolean algebra. The dual of an algebraic expression is obtained by interchanging OR and AND operations and replacing 1s by 0s and 0s by 1s. An equation in one column of the table can be obtained from the corresponding equation in the other column by taking the dual of the expres- sions on both sides of the equals sign. For example, relation 2 is the dual of relation 1 because the OR has been replaced by an AND and the 0 by 1. It is important to note that most of the time the dual of an expression is not equal to the original expression, so that an expression usually cannot be replaced by its dual. The nine identities involving a single variable can be easily verified by sub- stituting each of the two possible values for X. For example, to show that X ! 0 " X, let X " 0 to obtain 0 ! 0 " 0, and then let X " 1 to obtain 1 ! 0 " 1. Both TABLE 3 Basic Identities of Boolean Algebra 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. Commutative 12. 13. Associative 14. 15. Distributive 16. 17. DeMorgan’s X 0 ! X " X 1 # X " X 1 ! 1 " X 0 # 0 " X X ! X " X X # X " X X ! 1 " X X # 0 " X X " X Y ! Y X ! " XY YX " X Y Z ! ( ) ! X Y ! ( ) Z ! " X YZ ( ) XY ( )Z " X Y Z ! ( ) XY XZ ! " X YZ ! X Y ! ( ) X Z ! ( ) " X Y ! X Y # " X Y # X Y ! "