gtpc3m14 | Concepts and Structures |
Within the channel subsystem, the input/output processor (IOP) supervises the flow of data from shared main storage to I/O engines that, in essence, represent devices. The IOP does dynamic pathing (routing) to connect a device to an I-stream engine. The term subchannel is synonymous with device in TPF terminology; that is, there is a unique subchannel address for each device.
The TPF common I/O handler (CIO) manages I/O operations through a clearly defined macro interface that permits the set of CIO macros supporting each I/O function to make use of a centralized service structure. The I-stream engine processing related to I/O instructions, channel programs, and related I/O addressing schemes of the past are essentially untouched. CIO, however, takes advantage of the benefits of the ESA channel subsystem such as dynamic pathing.