The multiplexing feature of ZCOM allows more than one logical
terminal to be mapped to a single physical terminal. This mechanism
is implemented in ZCOM by allowing messages to and from a terminal ZLU
to be intercepted by a program (which is able to alter the data content,
e.g., adding or removing headers) and then forwarded on to the destination
physical terminal or its receiver program.
Some fields in the logical terminal table and in the physical
terminal table are mentioned in the following descriptions; more
information on these fields can be found in the logical terminal
table and the physical terminal table information in the section
on Tables and Data Structures.
The outbound and the inbound multiplexing are separate, so
although in most situations the multiplexing program will perform
both functions, it is possible to perform only inbound multiplexing
or only outbound multiplexing in any program.
The ltflag field of the
logical terminal table (zltt_type) contains two flag bits ZCOM_LTFLAG_OMX
and ZCOM_LTFLAG_IMX, which indicate whether messages to or from
the logical terminal respectively, are to be handled by a multiplexing
program. These bits are set by TTGEN according to the device type.
When more than one logical terminal is mapped to a single
physical terminal, the ltgzlk field
(<zltt_type>.ltgzlk) linkage forms a circular
list of terminals in the same multiplexed group (i.e., mapped to
the same physical terminal). The ltqzlk field
contains the ZLU number (16-bit) of the next terminal in the group.
For a non-multiplexing terminal, this field contains the ZLU number
of the current terminal (i.e., pointing back to itself, a special
case for circular list).
Outbound Multiplexing |
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The multiplexing program uses the zread call
to determine the origin of the message, after which, the program
would normally attach a protocol header and forward the message
to the destination physical terminal. MZAUXL in the message header
(see the chapter on Tables and Data Structures) can be used to save
the originating source node and ZLU on the outbound message. The
MZSRCE field in the message header is then overridden with the multiplexing
program's primary ZLU, hence status and response messages
may be routed to the multiplexing program instead of the sender.
Routine zsend writes to the
physical terminal (with bit ZCOM_ZSEND_NOMX set in the mode parameter). Routine zsend with user header (mode bit ZCOM_ZSEND_MHD) must also
be used.
The responses to the outbound messages are received by the multiplexing
program as type 6 (ZCOM_MSTYPE_RSLT) messages (request code ZCOM_MRQCODE_WRITE
[2]). The multiplexing program has to strip the protocol header
which it had previously added and forward the response to the ZLU
number saved in MSPARE (using zsend with
bits ZCOM_ZSEND_MHD and ZCOM_ZSEND_NOMX both set).
If the ZCOM_LTFLAG_OMX bit is set in the LTFLAG
word of the logical terminal table, then a logical terminal is enabled
for outbound multiplexing. This means that any message sent (by zsend) to this terminal ZLU will
be formatted as normal (type 4, ZCOM_MSTYPE_LPLT, message with request
descriptor block), but instead of being queued to the physical terminal
transmit queue, it will be queued to the program ZLU defined by
LTZMXP. These fields must have been defined to the system for each
terminal by a prior zset_rcvr call
(mode 2, ZcOUTB_MLTPLX) by
the outbound multiplexing program during its initialization.
The zcntl call works
in a similar way to zsend.
For example, an enable of a multiplexed logical terminal ZLU will
queue an enable request (type 4, ZCOM_MSTYPE_LPLT, request code
7, ZCOM_MRQCODE_ENB) to the outbound multiplexing program's
input queue. These messages would normally be used to maintain the
flags for the logical terminal in LTSTAT. They may also generate
a control command to the physical terminal from the multiplexing
program.
Inbound Multiplexing |
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If the ZCOM_LTFLAG_IMX bit is set in the LTFLAG word of the
logical terminal table, then the logical terminal is enabled for
inbound multiplexing. This means that when an application program
selects the terminal for input (using zset_rcvr default
mode 0), zset_rcvr will insert
the application program's ZLU into LTZRVR (rather than PTZRVR
for a terminal without inbound multiplexing).
It is up to the inbound multiplexing program to define itself
to the ZCOM subsystem as the receiver for messages from the physical
terminal using the zset_rcvr (mode
1, ZcINB_MLTPLX) call. This sets PTZRVR in the physical terminal
table.
The inbound multiplexing program has the task of receiving
all messages from the physical terminal. It determines the origin
of the logical terminal from which the messages were sent. Then
it processes any of the control messages and forwards the data portion
of any data messages to the application receiver.
Thus, to process an inbound message with multiplexing:
zread (the
message type will be type 1, ZCOM_MSTYPE_MSLT - a message from a
terminal ZLU).
The source ZLU is obtained from PTZLU. PTZLU gives
the ZLU of one of the ZLUs in a multiplexed group.
Decode the protocol header to extract the logical
terminal address. Use the ltfind routine
to scan the multiplexed group to find a match of the logical terminal
address with LTADDR.
Build a ZCOM message header where the source ZLU
is the ZLU found by ltfind,
and the destination ZLU is taken from LTZRVR of the logical terminal
able.
Use zsend (with
bit ZCOM_ZSEND_MHD set) to send the message and the ZCOM header
to LTZRVR.
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