A code page is simply a character set.  Code page support was added to
DOS 3.3 to make it possible for DOS to be translated into international
languages.  DOS also makes it possible for a program to be written in a
"language-independent" manner; for instance, there are DOS functions which
help you to sort a list of text strings that contain international
characters such as Р and А.

Code page support provides a "behind-the-scenes" way to process non-
English keystrokes and a way to reprogram EGA and VGA adapters to display
certain characters that are not defined by standard IBM-compatible
hardware.  It also provides some limited printer font-definition support.

Code pages (character sets) defined and supported in DOS include:

CodePage Language
437      US et.al.
850      Multilingual (Latin I)
852      Slavic (Latin II)
860      Portuguese
863      Canadian French
866      Norwegian and Danish

Here is some information relevant to code pages:

  The CONFIG.SYS file contains a COUNTRY= directive which selects a
default code page and country information file.  See also fn 65H.

  One or more DEVICE= directives in CONFIG.SYS install 'code page
aware' device drivers DISPLAY.SYS or PRINTER.SYS.

  The '.CPI' files that come with DOS contain 'Code Page Information'
for a selected device.  See Code Page Font Files.

  The Nlsfunc command installs code page switching (optionally
overriding the country information file set in CONFIG.SYS) and must
be used before any code page activities take place.

  The Mode and Chcp commands (3.3+) can prepare and select a code page.
You can perform the same functions as Mode via fn 440cH (but it is a
pain). And you can use fn 6602H to activate a prepared code page.

  When you 'prepare' a code page via the Mode command, the information
in the .CPI file is used to redefine a font.  For instance, the '$'
character can be changed into the 'Ь' character.  This takes place at
the device driver level via Fn 44H (IOCTL) commands.  For the EGA,
the appropriate INT 10H BIOS calls are made to redefine the character
set.  For a printer, a character set can be downloaded into it and
selected as the default.

Code Page Font Files describes the layout of the font-definition files
used by code page aware device drivers.

fn 44H (IOCTL functions)
fn 66H (code page switching)
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Code Page