At load time, a port driver must register itself with VCOMM by calling the _VCOMM_Register_Port_Driver service. Static port drivers register when they receive the Sys_Device_Init control message. Dynamic (plug-and-play) port drivers register when they receive the Sys_Dynamic_Device_Init control message.
Before the _VCOMM_Register_Port_Driver service returns, VCOMM calls the port driver's DriverControl function, specifying the DC_InitializePort action. For plug-and-play port drivers, VCOMM calls the function once for each port supported by the driver. For other port drivers, VCOMM calls the function exactly once. The port driver must check for the existence of the port (or ports), and then calls the _VCOMM_Add_Port service.
DriverControl, _VCOMM_Add_Port, _VCOMM_Register_Port_Driver