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
Figure
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.
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