PATH Mac OS 8 and 9 Developer Documentation > Text Encoding Conversion Manager
Programming With the Text Encoding Conversion Manager

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Why You Need to Convert Text From One Encoding to Another

This section explains in broad terms why you need to convert text from one encoding used to represent the text to another. Like the following sections that introduce the two Text Encoding Conversion Manager converters, this section uses terminology fundamental to the text encoding conversion process. These terms and the concepts they represent are explored in depth later in Character Encoding and Other Concepts Fundamental to Text Encoding Conversion and in Appendix B.

Central to any discussion of text encoding and text encoding conversion is the concept of a character, which is an abstract unit of text context. Characters are often identified with or confused with one or more of the following concepts, but it is important to keep the notion of an abstract character separate from these concepts:

• A graphic representation corresponding to a character (this graphic representation is what most people think of as the character)

• A key or key sequence used to input a character

• A number or number sequence used in a computer system to represent a character

In this book we are concerned primarily with abstract characters and with their numeric representation in a computer system. In order to represent textual characters in a file or in a computer's memory, some sort of mapping must be used to assign numeric values to the textual characters. The mapping can vary depending on the character set, which may depend on the language being used and other factors.

For example, in the ASCII character set, the character A is represented by the value 65, B is represented by 66, and so on. Because ASCII has 128 characters, 7 bits is enough to represent any member of the set (7-bit ASCII characters are usually stored in 8-bit bytes). Each integer value represented by a bit combination is called a code point. (The terms bit combination and code point are further explained in Character Encoding and Other Concepts Fundamental to Text Encoding Conversion .) Larger character sets, such as the Japanese Kanji set, must use more bytes to represent each of their members.

Interpretive problems can occur if a computer attempts to read data that was encoded using a mapping different from what it expects. The other mapping might contain similar characters mapped in a different order, different characters altogether, or the characters may be specially encoded for data transmission. To handle text correctly in these and other similar cases, some method of identifying the various mappings and converting between them is necessary. Text encoding conversion addresses these problems and requirements.

Here are two examples of the many cases for which text conversion is necessary:

• A Mac OS computer receives text in asynchronous packets over the Internet from a remote server. The Mac OS expects text to use the Mac OS Arabic character set, while the server uses the ISO 8859-6 standard.

• A Mac OS application attempts to read a text file created on a Windows 95 computer. The Mac OS application expects text to use the Mac OS Roman character set, while the Windows 95 file uses the Windows Latin-1 character set.