Sys Req Key Functions - IBM 5170 Technical Reference

After a subsystem is loaded, it must "listen" for any subsystems
that may be loaded later so that it can lock them out when it is
running. The following describes the code sequence for startup.
Startup Interface
MOV AX,SEG SYSLOCK ;segment of lock
MOVES,AX
MOV BX,OFFSET SYSLOCK ;offset of lock
MOV AX,2000H ;AH=20H, AL=O
INT 15H
Lockout Interface
The register ES:BX points to a byte which initially contains a
value of O. Whenever a higher priority subsystem wishes to run, it
~ increments the lock. When it completes running, it decrements the
lock. This allows proper synchronization of resources and
subsystems.
SYS REQ Key
Functions
During initialization, the subsystem also needs to connect to the
SYS REQ key function.
It
is necessary for the SYS key code to
be included in each subsystem. This startup section determines if
the SYS support is already loaded and loads the support if
necessary.
The SYS functions provide a means for the subsystem's main
screen or menu to be displayed.
If
the subsystem requires no user
action, then these functions need not be provided.
SYS Key Modes
There are two SYS key modes: multiple press and super shift.
Compatibility 9-17