If the SVC number is not found in the user-defined SVC table (or if
the table is nonexistent), DMSITS either transfers control to the CMSDCS shared segment (if SETDOS ON has issuedj, UL the standard system table (contained in DMSSVT) of as calls is searched for that SVC number. SVC number is found, control is transferred to the corresponding
address in the usual manner. If the SVC is not in either table, then
the supervisor call is treated as an abend call.
The DMSHDS initialization program sets up the user-defined SVC table.
It is possible for a user to provide his own SVC routines. Input/Output Interruptions
All input/output interruptions are received by the I/O interrupt
handler, DMSITI. DMSITI saves the I/O old PSi and the CSW (channel
status word). It then determines the status and requirements of the
device causing the interruption and passes control to the routine that
processes interruptions from that device. DMSI!I scans the entries in
the device table until it finds the one containing the device address
that is the same as that of the interrupting device. The device table (DEVTAB) contains an entry for each device in the system. Each entry
for a particular device contains, among other things, the address of the
program that processes interruptions from that device. When the appropriate interrupt handling routine completes its
processing, it returns control to DMSITI. At this point, tests
the wait bit in the saved I/O old PSi. If this bit is off, the
interruption was probably caused by a terminal (asynchronous) I/O operation. DMSITI then returns control to the interrupted program by
loading the I/O old PSi. If the wait bit is on, the interruption was probably caused by a
nonterminal (synchronous) I/O operation. The program that initiated the
operation most likely called the DMSIOi function routine to wait for a
particular type of interruption (l1sually a device end) • In this case, DMSITI checks the pseudo-wait bit in the device table entry for the
interrupting device. If this bit is off, the system is waiting for some
event other than the interruption from the interrupting device; returns to the wait state by loading the saved I/O old PSi. (This PSi has the wait bit on.)
If the pseudo-wait bit is on, the system is waiting for an
interruption from that particular device. If this interruption is not
the one being waited for, DMSITI loads the saved I/O old PSi. This will
again place the machine in the wait state. Thus, the program that is
waiting for a particular interruption will be kept waiting until that
interruption occurs.
If the interruption is the one being waited for, DMSITI resets both
the pseudo-wait bit in the device table entry and the wait bit in the I/O old PSi. It then loads that PSi. This causes control to be
returned to the DMSIOW function routine, Which, in turn, returns control
to the program that called it to wait for the interruption. Part 3. Conversational Monitor System (CMS) 237

Terminal Interruptions Terminal input/output interruptions are handled by the DMSCIT module.
All interruptions other than those containing device -end, channel end,
attention, or unit exception status are ignored. If device end status
is present with attention and a write ccw was terminated, its buffer is
unstacked. An attention interrupt causes a read to be issued to the
terminal, unless attention exits have been queued via tne STAX macrc.
The attention exit with the highest priority is given control at each
attention until the queue is exhausted, then a read is issued. Device
end status indicates that the last I/O operation has been completed. If
the last I/O operation -was a write, the line is deleted from the output
buffer and the next write, if any, is started. If the last I/O operation was a normal read, the buffer is put on the finished read list
and the next operation is started. If the read vas caused by an
attention interrupt, the line is first checked for the commands RT, He,
HT, or HX, and the appropriate flags are set if one is found. Unit exception indicates a canceled read. The read is reissued, unless it
had been issued with ATTREST=NO, in which case unit exception is treated
as device end.
Reader I Pu nchl Printer Interruptions Interruptions from these devices are handled by the routines that
actually issue the corresponding I/O operations. When an interruption froa any of these devices occurs, control passes to DMSITI. Then DMSITI passes control to DMSIOW, which returns control to the routine that
issued the I/O operation. This routine can then analyze the cause of
the interruption. User-Controlled Device Interruptions Interrupts from devices under user control are serviced the same as CMS devices except that DMSIOW and DMSITI manipulate a user-created device
table, and DMSITI passes control to any user-written interrupt
processing routine that is specified in the user device table. Otherwise, the processing program regains control directly.
Program Interruptions The program interruption handler, DMSITP, receives control when a
program interruption occurs. When DMSITP gets control, it stores the
Frogram old PSi and the contents of the registers 14, 15, 0, 1, and 2
into the program interruption element (PIE). (the routine that handles
the SPIE macro instruction has already placed the address of the program
interruption control area (PICA) into PIE.) DMSITP then determines wb9ther or not the event that caused the interruption was one of those
selected by a SPIE macro instruction. If it was not, DMSITF passes
control to the DMSABN abend recovery routine.
If the cause of the interruption was one of those selected in a SPIE macro instruction, DMSITP picks up the exit routine address from the PICA and passes control to the exit routine. Upon return from the exit
routine, DMSITP returns to the interrupted program by loading the original program check old PSW. The address field of the PSi vas modified by a SPIE exit routine in the PIE. 238 IBM VM/370 System programmer's Guide