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

Logging I/O Activity
The RSCS component of VM/370 contains a facility for logging all I/O act1v1ty on a particular teleprocessing link. this logging feature can be utilized if a problem arises where tracing I/O activity on a line
becomes a necessity.
The RSCS operator can turn the feature on and off by issuing the RSCS CMD command with the LOG or NOLOG operand. The format of the CMD command, when used to control logging, is as follows: CMD L
linkid LOG NOLOG linkid {LOG } NOLOG , I I is the location identifier for the link on which logging is to
be performed.
is the keyword that starts the logging of I/C activity.
is the keyword that stops the logging of I/O activity.
The loggIng output Is a printer spool file containing a one-line
record for each I/O transaction on the teleprocessing line. A
transaction is defined as any read or write of a teleprocessing buffer. When logging is turned off, the output is spooled to a printer. The distribution code on the printer output is the linkid that
was specified in the CMD command.
The output log record is printed in hexadecimal notation unless
otherwise noted. 340 IBM VMj370 System Programmer's Guide