Codepages
The meaning of the term "codepage" has evolved over time. Only one definition concerns us now: In Windows 95 and NT, a codepage is a list of selected character codes in a certain order. Codepages are usually defined to support specific languages or groups of languages which share common writing systems. For example, codepage 1253 provides character codes required in the Greek writing system.
The order of the character codes in a codepage allows the system to provide the appropriate character code to an application when a user presses a key on the keyboard. When a new codepage is loaded, different character codes are provided to the application.
In Windows 95, codepages can be changed on-the-fly by the user, without changing the default language system in use. An application can determine which codepages a specific font supports and can then present language options to the user.
Preparing for TrueType Open:
Saving Writing System Information within a Text Stream
When a user changes codepages, character codes from the new codepage are stored in the text stream. However, most codepages support multiple writing systems, each of which may have special rules about substituting or placing glyphs. TrueType Open will allow the flexibility for multiple writing systems to be supported by a single character set. Glyph substitution and placement rules can be associated with a writing system and stored in the font. Applications requiring these advanced features will need to save in the document an indication of the writing system in use, as well as the character codes entered.