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(emacs)MS-DOS and MULE

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International Support on MS-DOS
===============================

   Emacs on MS-DOS supports the same international character sets as it
does on GNU, Unix and other platforms (Note: International), 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.

`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(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 (Note:
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\-'.  Note: 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 (Note: 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 `o`' (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.  Note: 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 (Note: 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.

   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.

   ---------- Footnotes ----------

   (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.  While
third-party software is known to exist that allows to change the
codepage without rebooting, we describe here how a stock MS-DOS system
behaves.

   (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 `c,'
(`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.