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
structure returned by that call.
Task switchers that intercept this call can enable interrupts can