HP UX 11i v1 I/O Cards manual Physical Terminal Table

	ZCOM Message Handling
	Message Queuing
	Queues for terminal ZLUs are located in the physical terminal tables in
	the ZCOM memory area as shown in Figure 2-2. The physical terminal
	tables (one per terminal) maintain four queues for each terminal.
Figure 2-2	Physical Terminal Table
		Physical Terminal Table
		Express Queue
		High Priority Queue
		Low Priority Queue
		Unacknowledged Queue
	The driver reads messages first from the express queue until there are
	no remaining express queue messages, then from the high priority queue
	until there are no high priority messages, at which stage the low priority
	queue is read. Any further additions to the higher priority queues will be
	read before the driver continues with the lower priority queues. When
	the interface card is ready to accept messages, the driver moves the
	message to the unacknowledged queue and transfers the message to the
	interface card (through the unacknowledged queue). Both the interface
	card and the driver use DMA (Direct Memory Access) hardware to
	transfer the messages. If the number of buffers on the unacknowledged
	queue is less than the limit specified to TTGEN, more messages will be
	transferred, otherwise the transfer will be suspended until some
	outstanding messages are acknowledged.
	Once the interface card has transmitted the message and has responded
	with a definite status, the driver passes the status (if requested to do so)
	back to the calling program. If the calling program also requires the
	message buffer to be sent back with the status response, it is picked up
	from the unacknowledged queue. Once the driver has passed the
	message to the program, the message is removed from the
	unacknowledged queue.
	Messages in the unacknowledged queues for each terminal are also used
	in a Mux restart: the messages are moved back to the high priority queue
	to be resent. The size of the unacknowledged queue is configurable in the
	TTGEN configuration file.

28 Chapter 2