INT 2fH 4B02H: Is Switcher Installed? / Get Service Addr

                                                         Compatibility: 5.0+ 
 Expects: AX    4B02H  (INT 2fH)
          BX    0000H (must be 0)
          ES    0000H (must be 0)
 Returns: AX    0000H if task switcher is installed
          ES:BX address of service-function handler (if switcher installed)
                0000:0000 if switched is not installed
    Info: This determines whether a task switcher has been installed and
          obtains the service function address (used for accessing switcher

          It is possible that more than one task switcher is active (for
          instance, if DOSSHELL is executed while in a Windows command
          prompt session).  To see if that's the case, use SwSrvcFn 0000H
          and examine the pfPrevSwitcher field of the SwVersionRec
          structure returned by that call.

          Task switchers that intercept this call can enable interrupts can
          use any DOS fns.

See Also: INT 2fH 4BxxH: Task Switcher Functions
          Switcher Services and Notifications
          INT 2fH: Multiplex Interrupt
          DOS Functions