IBM 5170 Technical Reference page 399

Cancellation
This signal from the SYS REQ processing module tells the
subsystem monitor to ignore the previous activation signal and
take the necessary action to return to its previous state.
This entry into the subsystem dispatcher signals that the monitor
task should be deactivated. The subsystem may not have control
of the system. It is necessary for the subsystem to note that a
cancellation has occurred and to wait until it has a valid
opportunity to run through its dispatcher code in a normal
fashion. The subsystem entry point is CALLED with the AH
register set to 1.
Completion
The following call signals completion. Completion constitutes
any action taken by the user when the subsystem's menu is
displayed.
The completion call causes the activation pointer to be reset to
~
the lowest priority subsystem. All lower priority subsystems also
receive a cancellation notification.
MOV AX,SEG
entry
Joint
;address for SYS to call
MOVES,AX
MOV BX,OFFSET
entry
Joint ;ES:BX must contain the same
;values as the startup call
MOV AX,2011H ;AH=20H, AL=IIH
INT 15H
Compatibility 9-21