Priorities

Priorities only apply to messages that are destined for terminals. Messages to program ZLUs are simply added to the end of the input queue for that ZLU.

Messages destined for terminal ZLUs may have either express, high or low priority. The priority for a message is determined by bits ZCOM_ZSEND_LPR and ZCOM_ZSEND_XPS of the mode parameter, in the zsend and zcntl routine calls. If the ZCOM_ZSEND_XPS bit is set, then the message is queued on the express queue in the physical terminal table for that terminal ZLU. If the ZCOM_ZSEND_LPR bit is set, then the message is queued on the low priority queue in the physical terminal table for that terminal ZLU. If neither bit is set, then the message is queued on the high priority queue. This enables background tasks to be sent at lower priorities, without impacting any on-line throughput.

The driver reads the information from the terminal tables, looking at the high priority queue for each terminal and sending the message at the head of that queue. The driver only checks the low priority queue if there are no messages on the high priority queue. This means that any message on the high priority queue will be sent first.

	NOTE: The Express Queue should be reserved for Protocol Control events, such as, interrupts or resets. Normally, data should not be sent using the Express Queue.