Availability Guide for Change Management

Making Communications Subsystem Changes
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Availability Guide for Change Management–125506
5-2
I/O Processes
To use any device or communications line, other processes send requests to the I/O
process that owns the device or line. The I/O process, in turn, calls its own procedures
to handle the protocols of the devices or lines it controls and to achieve the physical
transfer of data.
Figure 5-1 illustrates how calls are passed from a user process to an I/O process, and
then to physical devices or communications lines:
1. The user process obtains access to subsystem resources by issuing operating system
procedure calls to the file system, identifying the resource and the action to be
performed.
2. The file system determines the location of the resource and passes the request to the
message system, which in turn passes it to the appropriate I/O process.
3. The I/O process manipulates the data communications resource on behalf of the user
process, using a ServerNet addressible controller.
Figure 5-1. User Process Communicates With I/O Process
CDT 009
User
Process
File System
Message System
I/O
Process
ServerNet
Addressable
Controller
Device or
Network
1
2
3