Reference Guide

ZCOM Message Handling
Priorities
Chapter 2 29
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.