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28.6: International Support on MS-DOS

Emacs on MS-DOS supports the same international character sets as it does on Unix and other platforms (see International), including coding systems for converting between the different character sets. However, due to incompatibilities between MS-DOS/MS-Windows and Unix, there are several DOS-specific aspects of this support that users should be aware of. This section describes these aspects.

MS-DOS is designed to support one character set of 256 characters at any given time, but gives you a variety of character sets to choose from. The alternative character sets are known as DOS codepages. Each codepage includes all 128 ASCII characters, but the other 128 characters (codes 128 through 255) vary from one codepage to another. Each DOS codepage is identified by a 3-digit number, such as 850, 862, etc.

In contrast to X Windows, which lets you use several fonts at the same time, MS-DOS doesn't allow use of several codepages in a single session. Instead, MS-DOS loads a single codepage at system startup, and you must reboot MS-DOS to change it[1]. Much the same limitation applies when you run DOS executables on other systems such as MS-Windows.

If you invoke Emacs on MS-DOS with the `--unibyte' option (see Initial Options), Emacs does not perform any conversion of non-ASCII characters. Instead, it reads and writes any non-ASCII characters verbatim, and sends their 8-bit codes to the display verbatim. Thus, unibyte Emacs on MS-DOS supports the current codepage, whatever it may be, but cannot even represent any other characters.

For multibyte operation on MS-DOS, Emacs needs to know which characters the chosen DOS codepage can display. So it queries the system shortly after startup to get the chosen codepage number, and stores the number in the variable dos-codepage. Some systems return the default value 437 for the current codepage, even though the actual codepage is different. (This typically happens when you use the codepage built into the display hardware.) You can specify a different codepage for Emacs to use by setting the variable dos-codepage in your init file.

Multibyte Emacs supports only certain DOS codepages: those which can display Far-Eastern scripts, like the Japanese codepage 932, and those that encode a single ISO 8859 character set.

The Far-Eastern codepages can directly display one of the MULE character sets for these countries, so Emacs simply sets up to use the appropriate terminal coding system that is supported by the codepage. The special features described in the rest of this section mostly pertain to codepages that encode ISO 8859 character sets.

For the codepages which correspond to one of the ISO character sets, Emacs knows the character set name based on the codepage number. Emacs automatically creates a coding system to support reading and writing files that use the current codepage, and uses this coding system by default. The name of this coding system is cpnnn, where nnn is the codepage number.[2]

All the cpnnn coding systems use the letter `D' (for ``DOS'') as their mode-line mnemonic. Since both the terminal coding system and the default coding system for file I/O are set to the proper cpnnn coding system at startup, it is normal for the mode line on MS-DOS to begin with `-DD\-'. See Mode Line. Far-Eastern DOS terminals do not use the cpnnn coding systems, and thus their initial mode line looks like on Unix.

Since the codepage number also indicates which script you are using, Emacs automatically runs set-language-environment to select the language environment for that script (see Language Environments).

If a buffer contains a character belonging to some other ISO 8859 character set, not the one that the chosen DOS codepage supports, Emacs displays it using a sequence of ASCII characters. For example, if the current codepage doesn't have a glyph for the letter `ó' (small `o' with a grave accent), it is displayed as `{`o}', where the braces serve as a visual indication that this is a single character. (This may look awkward for some non-Latin characters, such as those from Greek or Hebrew alphabets, but it is still readable by a person who knows the language.) Even though the character may occupy several columns on the screen, it is really still just a single character, and all Emacs commands treat it as one.

Not all characters in DOS codepages correspond to ISO 8859 characters---some are used for other purposes, such as box-drawing characters and other graphics. Emacs cannot represent these characters internally, so when you read a file that uses these characters, they are converted into a particular character code, specified by the variable dos-unsupported-character-glyph.

Emacs supports many other characters sets aside from ISO 8859, but it cannot display them on MS-DOS. So if one of these multibyte characters appears in a buffer, Emacs on MS-DOS displays them as specified by the dos-unsupported-character-glyph variable; by default, this glyph is an empty triangle. Use the C-u C-x = command to display the actual code and character set of such characters. See Position Info.

By default, Emacs defines a coding system to support the current codepage. To define a coding system for some other codepage (e.g., to visit a file written on a DOS machine in another country), use the M-x codepage-setup command. It prompts for the 3-digit code of the codepage, with completion, then creates the coding system for the specified codepage. You can then use the new coding system to read and write files, but you must specify it explicitly for the file command when you want to use it (see Specify Coding).

These coding systems are also useful for visiting a file encoded using a DOS codepage, using Emacs running on some other operating system.

[1] Normally, one particular codepage is burnt into the display memory, while other codepages can be installed by modifying system configuration files, such as `CONFIG.SYS', and rebooting.

[2] The standard Emacs coding systems for ISO 8859 are not quite right for the purpose, because typically the DOS codepage does not match the standard ISO character codes. For example, the letter `ç' (`c' with cedilla) has code 231 in the standard Latin-1 character set, but the corresponding DOS codepage 850 uses code 135 for this glyph.

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