INT 15 - TopView - "OSTACK" - SWITCH TO TASK'S INTERNAL STACK
        AX = 101Ah
Return: stack switched
Notes:  this call may not be nested; a second call must be preceded by a call
          to "USTACK" (AX=1025h)
        while TopView requires many API calls to be executed while on the
          task's internal stack, DESQview allows those calls to be executed
          regardless of the current stack