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Int 2Fh / 4B02h         Detect Switcher                              DOS 5.0+

    Determines whether a task switcher is loaded. Client programs
    (such as a session manager) that need to prevent or control the
    interruptions causes by task switching should call this function
    during initialization.

        Entry   AX = 4B02h

        Return  AX    = 00h
                ES:DI = Address of service-function handler
                or
                ES:DI = 0000:0000h

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