Supported / Installed Codepages
The list of supported and installed codepages on Open32 is not distinguished. This means that EnumSystemCodepages with CP_SUPPORTED and CP_INSTALLED operates on the same set of codepages. This also means that IsValidCodepage returns TRUE for the same set of codepages. Open32 gathers supported/installed codepages by querying to the underlying OS/2. Therefore, whether a certain codepage is supported/installed on Open32 or not depends on its availability on OS/2.
Open32 internally keeps ANSI codepage candidate list and OEM codepage candidate list. These two codepage lists includes all possible ANSI/OEM codepages defined in WIN32 specification. Open32 at first calls WinQueryCpList API, and for each codepage in the result codepage list, it checks whether it is one of ANSI codepages or not, and whether it is one of OEM codepages or not, and then construct the installed/supported ANSI codepage list and the supported/installed OEM codepage list.