Two kinds of breaks can be made available to users running
in session mode. System break, the so called "hard"
break, lets the terminal user suspend the currently running process
and access the Command Interpreter to enter MPE/iX commands. The
user can then type :RESUME to return to the interrupted
process or :ABORT to terminate the suspended
process. System break is enabled by default; you must explicitly
disable system break through the FCONTROL (or FDEVICECONTROL) intrinsic.
The second kind of break that can be made available, subsystem
break, must be explicitly enabled by your program. Subsystem break
allows you to specify the exact way your program will respond to
a user who invokes subsystem break while the program is executing.
Subsystem break is often referred to as [CTRL]Y
break, because the common default for subsystem break is [CTRL]Y,
activated by pressing the [CTRL] key and the Y
key simultaneously. This sends an end-of-medium (EM) signal when
entered from any MPE/iX supported terminal. If subsystem break is
enabled for the program running on the terminal, this signal causes
the program to pass control to a break handler subroutine.
It is a good idea to include subsystem break processing capabilities
in any program that may be used to perform time consuming tasks,
such as listing a large file or searching a database for specific
information. By including a subsystem break routine you make it
possible for the user to change his mind and interrupt a task without
aborting the application.
A subsystem break procedure will usually allow the user to
interrupt the program at any point to return to the main program
prompt or menu. The procedure may also write a line of information
to the terminal to indicate that a subsystem break has occurred.
You, as the programmer, control exactly what will take place when
subsystem break is entered by the way you program the break handler
subroutine.
Subsystem break may be invoked in either standard or transparent
editing mode, but the program must be running in a session in order
for it to have any effect. You should not attempt to enable subsystem
break if your program is performing block mode reads. Subsystem
break is automatically disabled at FCLOSE.
