INFO: Windows, Code Pages, and Character Sets

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The ASCII (American Standard Code for Information Interchange) character set defines a mapping of the letters, numerals, and specified punctuation and control characters to the numbers from zero to 127. The term "code page" is used to refer to extensions of the ASCII character set that also map specified symbols to the numbers from 128 through 255.

This article discusses how Windows deals with code pages and warns against some of the pitfalls that applications can encounter.

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The ANSI (American National Standards Institute) character set maps the letters and numerals in the same manner as ASCII. However, ANSI does not support control characters and it maps many symbols, including accented letters, that are not mapped in standard ASCII. All Windows fonts are defined in the ANSI character set.

An Original Equipment Manufacturer (OEM) code page is built into the computer hardware. There are a number of OEM code pages, each defined for a particular language. These code pages are referred to by a number; for example, code page 437 is installed in the original IBM PC computer.

MS-DOS uses code pages to change the available character set, depending on user preference. A code page change is implemented by programming a new character set into the video display hardware. By changing to the code page for a particular language, the accented characters appropriate to that language are made available. Each code page is limited to 256 symbols.

For each code page, MS-DOS maintains a mapping table to map lowercase characters to and from uppercase. Because all string parameters to MS-DOS (filenames) are implicitly coded in the current code page, when the table is changed, filenames that were accessible under one case mapping may not be available under another. However, the common code pages were designed to combat this problem.

Windows runs as an extension to MS-DOS. There is a mapping layer that translates between the ANSI character set and an OEM character set. When Windows is installed, the Setup program determines the installed character set and installs the corresponding ANSI-OEM translation tables and Windows OEM fonts.

If the user changes the current MS-DOS code page, Windows does not change its ANSI-OEM mapping tables automatically. It is necessary to run the Windows Setup program to modify these tables and to load the corresponding fonts.

Windows-based applications must use the Windows functions AnsiToOem() and OemToAnsi() when transferring information to and from MS-DOS. In addition, applications must use the correct character set when creating filenames.

There is no one-to-one mapping between the ANSI and OEM character sets. Applying the AnsiToOem() function followed by the OemToAnsi() function to a given string will not always result in the original string. A file that has been named with one of these strings cannot be accessed by any Windows-based application. The filename must be changed by the user from outside of Windows.

The following two scenarios may have differing results.


A lowercase ANSI string is passed to the AnsiToOem() function. The result is passed to MS-DOS, which maps the string to uppercase.


An uppercase ANSI string is passed to the AnsiToOem() function. The string is passed to MS-DOS.

This is caused by the fact that the MS-DOS lowercase to uppercase conversion mapping and the Windows ANSI case conversion do not match. To avoid this problem, use the AnsiUpper() function to convert the ANSI string to uppercase before passing it to the AnsiToOem()function. Also note, this is only a problem with extended characters. These problems are often overlooked until your customers call to complain.

Keep in mind that both ANSI and OEM are 8-bit character sets. In applications, always use the "unsigned char" type instead of "signed char" for character variables. Problems that result from using "signed char" are very hard to track.

The SYSTEM.ini file contains entries that relate to code pages. In the [boot] section, the OEMFONTS.fon line specifies the file that contains the OEM stock font. In the [keyboard] section, the OEMANSI.bin line specifies the ANSI-OEM translation table. If this line is blank, Windows uses the default table built into the keyboard driver. In the [enh] section, the *WOA.fon lines specify the fonts used in an MS-DOS window at various resolutions.

Windows does not provide any mechanism for an application to work with data that is not in the current MS-DOS code page, nor does Windows recognize changing the OEM code page in an MS-DOS window. However, an application is free to provide its own translation tables and provide a data format that includes the code page.

Article ID: 75435 - Last Review: Feb 11, 2005 - Revision: 1