The Communication I/O Subsystem design introduces a more efficient, streamlined approach to attaching data link control (DLC) processes to communication and LAN adapters.
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.
Note: A PDH, as used for the Communications I/O, provides both the device head role for interfacing to users, and the device handler role for performing I/O to the device.
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:
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).
The user-mode process uses system calls (open, close, select, poll, ioctl, read, write) to interface to the PDH to send or receive data. The poll or select subroutine notifies a user-mode process of available receive data, available transmit, and status and exception conditions.
The kernel-mode interface to a communications PDH differs from the interface supported for a user-mode process in the following ways:
Status Blocks for Communications Device Handlers Overview.
Communications Physical Device Handler Model Overview.
MPQP Device Handler Interface Overview.
Serial Optical Link Device Handler Overview.
Logical File System Kernel Services.
Some device drivers have a different design and use the services known as Common Data Link Interface (CDLI). The following are device drivers that use CDLI: