Practical C Programming, 3rd Edition

Chapter 11. Bit Operations

To be or not to be, that is the question

—— Shakespeare, on boolean algebra [Hamlet, Act 3, Scene 1]

This chapter discusses bit-oriented operations. A bit is the smallest unit of information. Normally, it is represented by the values 1 and 0. (Other representations include on/off, true/false, and yes/no.) Bit manipulations are used to control the machine at the lowest level. They allow the programmer to get under the hood of the machine. Many higher-level programs will never need bit operations. Low-level coding, like writing device drivers or pixel-level graphic programming, requires bit operations.

Eight bits together form a byte, represented by the C data type char.^[13]

A byte might contain the following bits:

01100100

This bit structure can also be written as the hexadecimal number 0x64. (C uses the prefix “0x” to indicate a hexadecimal (base 16) number.) Hexadecimal is convenient for representing binary data because each hexadecimal digit represents 4 binary bits. Table 11-1 gives the hexadecimal-to-binary conversion:

Table 11-1. Hexadecimal and Binary

Hexadecimal	Binary	Hexadecimal	Binary
0	0000	8	1000
1	0001	9	1001
2	0010	A	1010
3	0011	B	1011
4	0100	C	1100
5	0101	D	1101
6	0110	E	1110
7	0111	F	1111

So the hexadecimal number 0xAF represents the binary number 10101111.

The printf format for hexadecimal is %x; for octal the format is %o. So:

int number = 0xAF;
printf("Number is %x %d %o\n", number, number, number);

prints:

af 175 257

Many novice programmers get a number confused with its representation ...