Communications I/O Subsystem

The Communication I/O Subsystem consists of one or more physical device handlers (PDHs) that control various communication adapters. The interface to the physical device handlers can support any number of processes, the limit being device-dependent.

The Communication I/O Subsystem design introduces a more efficient, streamlined approach to attaching data link control (DLC) processes to communication and LAN adapters.

A communications PDH is a special type of multiplexed character device driver. Information common to all communications device handlers is discussed here. Additionally, individual communications PDHs have their own adapter-specific sets of information. Refer to the following to learn more about the adapter types:

• MPQP Device Handler Interface Overview
• Serial Optical Link Device Handler Overview

Each adapter type requires a device driver. Each PDH can support one or more adapters of the same type.

There are two interfaces a user can use to access a PDH. One is from a user-mode process (application space), and the other is from a kernel-mode process (within the kernel).