Int 2Fh / 4B03h         Allocate Switcher ID                         DOS 5.0+

    Returns a unique switcher identifier (in the range 01h through
    0Fh). A task switcher (or controlling session manager) calls this
    function on initialization and then uses the switcher identifier
    to create session identifiers for programs that it manages. The
    first-loaded task switcher is responsible for processing this
    function.

    Client programs must not call this function.

        Entry   AX    = 4B03h
                BX    = 00h   (Reserved, must be zero)
                ES:DI = Address of service-function handler

        Return  AX = 00h
                BX = New task switcher's identifier
                or
                BX = 00h

    [...]