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G.10.6 International Support on MS-DOS
Emacs on MS-DOS supports the same international character sets as it does on GNU, Unix and other platforms (see section International Character Set Support), including coding systems for converting between the different character sets. However, due to incompatibilities between MS-DOS/MS-Windows and other systems, there are several DOS-specific aspects of this support that you should be aware of. This section describes these aspects.
The description below is largely specific to the MS-DOS port of Emacs, especially where it talks about practical implications for Emacs users. For other operating systems, see the ‘code-pages.el’ package, which implements support for MS-DOS- and MS-Windows-specific encodings for all platforms other than MS-DOS.
- M-x dos-codepage-setup
Set up Emacs display and coding systems as appropriate for the current DOS codepage.
- M-x codepage-setup
Create a coding system for a certain DOS codepage.
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, which lets you use several fonts at the same time, MS-DOS normally doesn't allow use of several codepages in a single session. MS-DOS was designed to load a single codepage at system startup, and require you to reboot in order to change it(26). 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 section 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.(27)
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 section The Mode Line.
Far-Eastern DOS terminals do not use the cpnnn
coding
systems, and thus their initial mode line looks like the Emacs
default.
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 section 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 ‘ò’ (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 maps these characters to two
special character sets called eight-bit-control
and
eight-bit-graphic
, and displays them as their IBM glyphs.
However, you should be aware that other systems might display these
characters differently, so you should avoid them in text that might be
copied to a different operating system, or even to another DOS machine
that uses a different codepage.
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 section Cursor Position Information.
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 section Specifying a Coding System for File Text).
These coding systems are also useful for visiting a file encoded using a DOS codepage, using Emacs running on some other operating system.
MS-Windows provides its own codepages, which are different from the DOS codepages for the same locale. For example, DOS codepage 850 supports the same character set as Windows codepage 1252; DOS codepage 855 supports the same character set as Windows codepage 1251, etc. The MS-Windows version of Emacs uses the current codepage for display when invoked with the ‘-nw’ option. Support for codepages in the Windows port of Emacs is part of the ‘code-pages.el’ package.
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