The DEFINE, DELETE, DISCONN, QUERY, and STARl commands are processed
entirely by the system control task, as they may involve the referencing
and updating of the system status tables (DMTSYS). For the CHANGE, PURGE, and ORDER commands, £MTCMX builds a formatted
table called a command element and passes it, via an ALERT request, to
the DMTAXS task for execution.
The BACKSPAC, CMD, DRAIN, HOLD, MSG, and TRACE commands are passed to the line driver task for the associated active
link via a command element and ALERT request.
Rses Message Handling Messages can occur in response to a command or sFontaneously as a result
of a system malfunction.
The task that originates the message
the variable portion of the message text DMTMGX obtains the fixed portion of
information from the DMTMSG module, and
appropriate operator.
passes the message number and
to the message handler, DMTMGX. the message text and routing
then issues the message to the
Messages can be addressed to the local RSCS operator, remote station
operator, local VM/370 virtual machine, VM/370 system operator, or
combinations of these. Messages directed to the VM/370 system operator or VM/370 user are
issued via the CP MSG command using the virtual console function of the DIAGNOSE interface. Messages for the local RSCS operator are enqueued
for output by DMTREX. Messages for the remote station operator are
presented to the line drivers for the associated links via an RSCS MSG command element and ALERT request. Interruption Handling Three types of interruptions are handled
routines: external interruptions, SVC interruptions. EXTERNAL INTERRUPTIONS by the supervisor service
interruptions, and I/O
External interruptions are handled by the DMTEXT module. Each bit cf
the external interruption code (bytes 16-31 of the external old PSW in
low storage) is inspected. When a bit is set to one, a scan of the
external exit request queue is made to locate the first requested exit
for the bit that was set. If one is found, the exit is taken;
otherwise, processing continues until the entire interruption code has
been inspected.
338 IBM VM/370 System programmer's Guide
SVC INTERRUPTIONS The DMTSVC module receives control directly on an SVC interruption. RSCS uses the SVC interruption to "freeze" the execution of a task while
it is waiting for the results of some service that it has requested of
another task. The left half of the SiC old PSi is moved to the left
half of the resume PSi in the task's save area; the right half is loaded
with the contents of register 14 (resume PSi address). The register
contents at interruption time are also stored in the task's save area. DMTSVC returns control to the caller by setting register 14 to the
address of the task element of the "frozen" task and loading a PSi with
all mask bits set off (except machine check) and execution address as
stored in the SVC old PSi. I/O INTERRUPTIONS I/O interruptions are handled by the DMTIOM module at entry point DMTIOMIN. DMTIOM first searches for an active I/O request element cn
the appropriate queue (MPXIOQ or SELIOQ). If one is found, the I/O request table is updated to reflect the new status. If this is not the
final interruption, control is immediately returned to the dispatcher.
If the I/O has completed without unit check, the synchronous lock in the I/O table is posted; and, if there is no further I/O enqueued for that
subchannel, control is passed to the dispatcher. If I/C is enqueued for
that subchannel, it is started.
If the I/O has completed, but there was a unit check and automatic
sense was requested, the sense channel program is built in a new element
and the new element is chained to the request element. The sense
operation is started and, if not completed immediately, control is passed to the dispatcher.
If an active I/O request element was not found, the asynchronous I/O exit queue (IOEXITQ) is scanned for a matching device address. If it is
found, the asynchronous exit is taken.
If neither an active I/O request element nor an asynchronous exit
request element is found, the interrupt is iqnored and control is passed
to the dispatcher. Part 4. Remote Spooling Communications Subsystem (RSCS) 339
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