Subsystem Structure; Subsystem Startup And Lockout - IBM 5170 Technical Reference

Subsystem Structure
The following figure shows three subsystems which have multiple
tasks. They are arranged in order of hierarchy. Tasks in
subsystem B can only run when Task "Other" A is active in
subsystem A and tasks in subsystem C can only run when Task
"Other" B is active in subsystem B.
Task 1A
I
Task 2A
I
Task 3A
I
Task "Othen" A
Subsystem B Inhibited
I
Task 1 B
I
Task 2B
Task B
"Other"
Subsystem C Inhibited
Task 1C
Task 2C
Multiple Task Subsystems
The order in which subsystems were installed (loaded into main
storage) determines their priority. The first one installed is higher
on the hierarchy. An inhibit mechanism provided at startup time
enforces the hierarchy. As a subsystem starts, it broadcasts to the
rest of the subsystems, previously installed, that it is starting and
at the same time, provides the address of a lock. This lock must
~
be set (incremented) by subsystems higher in the hierarchy
whenever they wish to run one of their own tasks. This flag must
be set for each subsystem lower on the hierarchy, for example,
when subsystem A is about to start Task 2A, the dispatcher must
set subsystem B inhibit and subsystem C inhibit.
Subsystem Startup and Lockout
In order for multiple subsystems to cooperate, there must be
communication between subsystems when a subsystem is loaded
into storage and initialized.
The subsystem being loaded tells the previously loaded
subsystems that it is being loaded and broadcasts the address of
its synchronization lock. Higher priority subsystems use this lock
to exclude the new subsystem from accessing any system
resources (DOS, interrupts, etc.).
9-16 Compatibility