# 4.3: Unary Operations

• • Charles W. Kann III
• Adjunct Professor (Computer Science) at Gettysburg College

Boolean logic consists of a set of values (t and f) and the operations which can be performed on the binary values. In this book, the three most important Boolean operations are AND, OR, and NOT.

The operation of a Boolean operator is usually shown (or characterized) using truth-tables. A truth-table is a table which gives the input value for the operation, and the output values for that operation. For example, there are only two unary operations. Unary operations are operations which take only one input, the NULL operator and the NOT operator. An easy way to characterize these functions is in a table where the input values of 0 and 1 are input, and the output values from the function are given as 0 and 1.

A truth table is normally shown with the input values for a function on the left, and the name of the function at the top of the column. The following truth table characterizes the NULL and NOT operations. It shows what the outputs of the NULL and NOT operator are for an input value of A.

 Input Output A NULL NOT 0 0 1 1 1 0

As this table shows, if the input value A is 0, the NULL gate will produce an output value of 0, and the NOT operator will produce a value of 1 (the inverse). If the input value of A is 1, the NULL operator will produce a value of 1, and the NOT operator will produce an output value of 0. The NOT operator in this text will be written as a quote ('), so NOT-A will be A'.

Gates are the physical implementation of Boolean operators in a circuit. Since the NOT operator always inverts the input value, the NOT gate is often referred to as an inverter, and is represented by the triangle with a circle symbol in Figure $$\PageIndex{1}$$ below. The NULL gate is usually either absent, or implemented as a buffer. The symbol for the buffer is represented by the triangle symbol in Figure $$\PageIndex{1}$$ below. Figure $$\PageIndex{1}$$: Buffer and inverter gates

Figure $$\PageIndex{2}$$ shows a circuit with the buffer and inverter in Logisim. Figure $$\PageIndex{2}$$: Buffer and inverter circuit in Logisim