The virtual communications driver (VCOMM) is a non-replaceable system component that manages all access to communications devices. Port drivers use VCOMM services to register themselves and block access to communications hardware. Other virtual devices (VxDs), referred to as client VxDs, use VCOMM services to access communications resources. Finally, the Microsoft® Windows™ communications driver (COMM.DRV) uses VCOMM's protected-mode application programming interface (API) to access communications resources. VCOMM uses port drivers to carry out all hardware operations on communications resources.