1.7: Boolean Logical and Bitwise Operators

1.7.2 Logical and Bitwise Boolean Operators

1.7.1 Boolean Operators

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Boolean operators are operators which are designed to operate on a Boolean or binary data.
They take in one or more input values of 0/1⁴ and combine those bits to create an output value which is either 0/1. This text will only deal with the most common Boolean operators, the unary operator NOT (or inverse), and the binary operators5 AND, OR, NAND, NOR, and XOR. These operators are usually characterized by their truth tables, and two truth tables are given below for these operators.

Table 1-6: Truth table for NOT operator
A	NOT
0	1
1	0

Output

There are two kinds of Boolean operators implemented in many programming languages. They are logical operators and bitwise operators. Logical operators perform Boolean operations to obtain a single value at the end. For example, in Java a programmer might write:

if ((x != 0) && (y / x > 4))

The purpose of this statement is to decide whether or not to enter the statement or code block associated with the if test. What is desired is a single answer, or a single bit, which is either true (1) or false (0). This has two consequences. The first is that in some programming languages (e.g. C/C++) a variable other than a Boolean can be used in the statement, with the consequence that 0 is false, and anything but 0 is true. For example, in the statement if(x=64), the result is true. The equal operator returns a non-zero value, 64, which is true. This has been the cause of many bugs in C/C++, and most modern compilers at least complain about this. This will also be the result of some expressions in assembler, and there will be no compiler to complain about it. So be careful.

The second consequence of the logical Boolean operator is that once a part of it has failed, the entire statement has to fail, and so the rest of the statement will not be executed. This is called short circuiting, and all logical operators are thus short circuiting operators. To see why this is true, consider the if test about. In this if test, if x is 0, then (x != 0) is false. Since false and anything is false, there is no need to evaluate the second part of this equation, and so the statement (y / x > 4) is not executed. Many programmers will recognize this code, as it is a common pattern for protecting against a zero divide.

The important take away from this is that logical operators are short circuiting operators.

On the other hand, bit-wise operators are not short circuiting. Consider the following problem. A programmer wants to write a toLower method which will convert an upper case letter to a lower case letter. In chapter 1.3 it was pointed out that the difference between an upper case letter and a lower case letter is that in a lower case letter the bit 0x20 (001000002) is 1, whereas in the upper case letter it is zero. So to convert from upper case letter to letter case, it is only necessary to OR the upper case letter with 0x20. In pseudo code this could be implemented as follows:

char toLower(char c) {
          return (c | 0x20)
          }

In this case the OR operator, |, needs to operator on every bit in the variables. Therefore, the | operator is not short circuiting, it will process every bit regardless of whether or not a previous bit would cause the operation to fail.

It is possible to use bitwise operators in place of logical operators, but it is usually incorrect to do so. For example, in the previous if statement, if a bitwise operator had been used no short circuiting would have occurred and the zero divide could occur.

if ((x != 0) & (y / x > 4))

Many languages such as C/C++, Java, C#, etc, have both logical (short circuiting) and bitwise operators. In most cases the single character operator is a bit wise operator (e.g. &, |, etc.) and the double character operator is the logical operator (&&, ||, etc.).

To make things more confusing, in MIPS only non-short circuiting operators are used, however they are often called logical operators. There is no good way to reconcile this, so the user is cautioned to read the material and programs carefully.

⁴ Note that the values 0/1 are used here rather than F/T. These operators will be described through the rest of the book using the binary values 0/1, so there is no reason to be inconsistent here.
⁵ The term unary operator means having one input. The term binary operator means having two inputs. Be careful reading this sentence, as binary is used in two different contexts. The binary operator AND operates on binary data.