Sys Key Interfaces - IBM 5170 Technical Reference

Multiple Press Mode:
This mode allows the user to sequence
through subsystems. Subsystems are displayed in the reverse
order of their installation.
Super Shift Mode:
This mode allows the user direct access to
any subsystem regardless of the priority. The user activates this
mode by holding the SYS key pressed and pressing another key
which designates another subsystem.
Multiple Key Sequence
If a subsystem is to be used on the IBM Personal Computer and
the IBM Personal Computer XT, a multiple key sequence must be
used to access the SYS key functions.
SYS Key Interfaces
There are four interfaces needed by the SYS code to support a
subsystem: startup, activation, cancellation, and completion. The
subsystem activates two of these: startup and completion. The
SYS code in conjunction with user input activates the other two.
~
The following is a description, in tabular form, of the states,
transitions, and actions needed to implement the SYS REQ
functions.
Subsystem Entry Points
subsys A
code A
subsys B
code B
subsys C code C
Entry Points
#
subsystems current subsystem
#
num cur
9-18 Compatibility