gtpn1m0qNon-SNA Data Communications Reference

Synchronous Link Control Package

See Synchronous Link Control Functional Specification for more information about the terminology and protocols unique to SLC that are used in this chapter.

Support for synchronous link control is in three major areas. The CCP includes the logic directly concerned with communications I/O handling on synchronous link lines. This is subdivided into the core-resident CCP segments and supporting file-resident CCP segments.

Secondly, a group of ECB controlled programs forms an interface between system programs and the CCP for processing input messages (input message handler and SLC communications source) and output messages (output message handler).

Thirdly, another group of ECB controlled programs maintains overall control of the link via keypoint records and other tables and handle the special status messages unique to synchronous link control, that is, link control blocks (LCBs). These programs comprise the link controller.

Each of these groups are described in more detail in subsequent sections.

The link trace function is described in Synchronous Link Control Link Trace Package.

In addition, there are the following facilities:

The AMQ and MCR records are discussed under output message handler and the link routing table under input and output message handlers and SLC source communications. The other facilities are discussed in General Facilities.

A system may have more than one link, with differing options as described in Synchronous Link Control Functional Specification. In general, control of a Link is effected through the link and channel keypoint records. The link controller programs establish operational conditions on a link when cycling to CRAS state, or above. They are also driven by file-resident CCP and incoming link control blocks (LCBs), and generate and time-out routine monitors conditions (it is activated through the timer and CPU loop) and adds pseudo LCBs to the input LCB queue for processing by the link controller. file-resident CCP performs transient CCP functions and acts on command entries. Input message handler receives all incoming information blocks, through CCP post interrupt and link OPZO. SLC communications source manipulates them to suit applications requirements and passes them to the appropriate programs. Output message handler accepts applications messages for transformation into link message blocks and queues them for transmission using the macro PLONC-Place on Queue.

Core-resident CCP performs the normal I/O functions, adding incoming LCBs other than ENQs to the input LCB queue. ENQs and all message blocks are added to an input list (which may be the ready or input list as a system option) and subsequently passed to link OPZO that adds them to the appropriate channel input block queue for eventual processing by the input message handler. It also includes logic for queueing output applications messages to be processed by output message handler.