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I/O Handling
155
Environmental Considerations
Changes in program environment, i.e., calling a subprogram or returning from one, can affect
whether an ON INT is in effect or not.
Once executed, the CALL version of an ON INT is always in effect, whether in the main
program or in any subprogram, until it is redefined by another ON INT or is specifically
disabled (see below).
In the GOSUB or GOTO versions, the statement is in effect only in the same program environ-
ment. This is to say that if you have executed an ON INT statement in your main program, then
it is effective only while your program is executing part of the main program. The instant the
program goes into a subprogram (through a CALL statement), the statement is no longer
effective until the execution returns to the main program. Similarly, if you define an ON INT in
a subprogram, it is effective only while the program is executing that subprogram.
A side-effect occurs here when you use the CALL version of an ON INT. By calling the
subprogram with an ON INT, you have the effect of locking out the other interrupts, except
those which are executed in the subprogram itself and other CALL versions. This is regardless
of priority. In the priority
exampl~
in the previous section, if the ON INT#5 had been a CALL
instead of a GOSUB, then the second interrupt from select code,. 7 would not have been
acknowledged until the subprogram had finished.
Since recursive calls of subprograms are possible, it is also possible that many calls to the same
subprogram may be stacked up because an interrupt from a different select code with a CALL
version of an ON INT in effect may be received while processing the CALL caused by a
previous interrupt.
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