Throughout this manual, references are made to terminal ZLU or just terminal. This does not mean a physical
terminal device type but is a generic term that can refer to any kind
of physical or logical entity. Examples of physical entities are
printers, an HDLC link, terminals, etc. Examples of logical entities
might be an X.25 Permanent or Switched Virtual Circuit.
In the ZCOM Subsystem, messages are sent directly from sender
to receiver (where the receiver is another program or a terminal
ZLU). They are routed using a pointer called a ‘ZLU’ which
stands for ‘ZCOM Logical Unit’. ZCOM uses the
ZLU for its addressing mechanism. A ZLU may represent a terminal
(for instance, an X.25 virtual circuit) or an application program. Messages
are sent to and received by ZLUs.
Figure 1-3 “Definition and Features of ZLUs” shows a
sample set of program and terminal ZLUs, and their interaction with
each other and a Mux card:
To communicate to another program or to a terminal, a program
uses ZCOM library calls to write to a program or terminal ZLU. Each
program ZLU points to a queue that contains all messages, including
timeout and status messages, that were delivered to the program
ZLU. These pointers are maintained automatically by the ZCOM Subsystem.
When a message is sent to a ZLU, the data is moved to a system buffer
first and the address of the message is moved to the attached input
queue.
Program ZLUs |
 |
Each program may open one or more program ZLUs which provides
an input message queue to that program. Think of a program’s
input message queue as a mailbox where messages can be delivered.
All messages delivered are queued at the end of the input queue
for a program in the order in which they were received. Note that
the message is not actually delivered to the program, but simply
placed into the input queue. The program must issue a ZCOM library
call to retrieve the messages one-by-one from the input queue. The
method used to cause a message to be placed into a program’s
input queue will be different depending on whether the message was
created by another program or a physical/logical device.
If a message is sent (generated) by another program, then
the sending program must specify the ZLU of the program’s
input queue to place the message into. The receiving program need
not do anything for the message to be successfully delivered to
their mail box (program input queue).
However, when a message is generated by a terminal ZLU, any
program that wishes to receive messages from that device (terminal
ZLU) must inform the ZCOM Subsystem first. The program should issue
a ZCOM call (zset_rcvr) to tell the ZCOM Subsystem that all data
coming from the specified terminal ZLU should be delivered to its
input queue. Whenever data arrives from the device, it will be placed
into the input queues of all programs that have requested this data.
Terminal
ZLUs |
|
Terminal ZLUs represent a physical or logical device and do
not have input queues of their own. Any data coming from a terminal
ZLU will be delivered to the input queues of all applications that
have requested it. If there is no application interested in receiving
the data, then the data is thrown away and a message is logged to
that effect.
Terminal ZLUs do however have three different output (transmit) queues.
When a program sends data to a terminal ZLU, it can select one of
three queues to place that data on. Each queue represents a different level
of priority. The Express Queue is
used for extremely high-priority protocol dependent requests. The High-Priority Queue is used for high priority
data and the Low-Priority Queue is
used for low priority data. When a message is generated by an application,
the buffers involved are queued to the selected transmit queue and
the ZCOM driver is notified. The driver is then responsible for
handling the message. Messages on the express queue are sent before
those on the high priority queue which are sent before those on
the low priority queue.
Because the ZCOM Subsystem itself takes responsibility for
the delivery of messages, the sending program is able to specify
a number of options when it passes the message to the system. These
options include:
Continue execution
whether the message is sent successfully or not.
Continue execution but report
any error status (status messages are queued to the ZLU of the calling
program).
Continue execution; a definite
response will be queued to the program ZLU when the send has completed.
Suspend execution until the
send has completed (i.e., acknowledgment that the message was transmitted).
On completion, the message buffer may also be passed back
to the sending application program if required.
Each message on the ZLU has a ‘type’ attribute
so that the program reading the ZLU is able to differentiate between
status messages, timeout messages and data messages from terminals
or other programs.
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