HP Business BASIC/XL Reference Manual - HP 3000 MPE/iX Computer Systems - Edition 1 (32715-90001)

8- 3
The resulting branch ( GOTO, GOSUB or CALL ) to be taken is specified in
the ON KEY statement used to define the branch-during-input key. The
definition of the branch-during-input key overwrites the current typing
aid definition for that key. However, the HP Business BASIC/XL
interpreter remembers the last previous typing aid definition for that
key. When an OFF KEY statement for that user-definable key is executed,
the typing aid definition is restored.
SAVE KEY fname and RESAVE KEY fname save only the typing aid definitions
for the keys. If a key is currently defined as a branch-during-input
key, the last previous typing aid definition is written to the file if
either of these statements execute. Remember that the last previous
typing aid definition is set by either a SAVE KEY, SAVE or RESAVE KEY
fname , GET KEY fname , or SCRATCH KEY.
Priority of Handling the Branch after Pressing Branch-During-Input Keys.
The branching that is performed in response to the ON KEY statement can
be considered a restricted interrupt of the normal program flow. As
such, the order it is handled in depends on the number of higher priority
interrupts that must be handled when the branch-during-input key is
pressed. Chapter 4 contains the statements for interrupt handling for
DBERROR, EOF (end of file), run-time errors, and HALT ( CONTROL Y). The
priority for handling these interrupts is:
HALT 16
SHIFT HALT 17
EOF (end of file) 17
run-time errors 17
The priority level for the branch-during-input keys can be set to any
integer between 1 and 15, inclusive. If a priority level is not
specified in the ON KEY statement, the priority is set to 1. The
branches specified by the interrupt handlers and the branch-during-input
keys are added to the interrupt queue. The branch with the highest
associated priority is processed first. If there is more than one
key-generated branch in the interrupt queue with the same priority, the
branch resulting from pressing the highest numbered key is processed
first.
There are now two conditions to consider:
* If the specified branch is a GOSUB or CALL, then the interrupt queue
for the program unit that the key was pressed in is checked
immediately following the execution of the RETURN statement that
returns control to the calling program unit.
* If the branch is a GOTO, then the statement that is the target of the
branch is executed. Following execution of the target statement, the
interrupt queue is checked again.
In either case, if the interrupt queue is not empty, then the next branch
in the queue with equal priority to that just executed or the branch with
the highest remaining priority executes. The process continues until
there are no more branches to execute remaining in the queue. At this
point, program execution continues at the next executable statement in
the program.
If only one GOSUB or CALL branch generated by a branch-during-input key
is in the interrupt queue when the ENABLE statement executes, the GOSUB
or CALL executes and then execution resumes at the statement following
the input statement.
Execution of RUN, STOP, END, SCRATCH PROG, or SCRATCH ALL clears the
interrupt queue of any key generated branches remaining to be executed.
The DISABLE statement lets the program add branches to the interrupt
queue, but delays execution of the branches. The ENABLE statement allows
the handling of queued branch information to continue or begin.