Line 28
If
a function key
is
pressed, execution will be directed according
to
the
subroutine line numbers in the ON KEY GOSUB statement. For example,
if
Function
Key
(Ij)
is
pressed, execution goes
to
the subroutine in line number
100.
If
the
second Function Key,
(£2),
is
pressed, execution goes to the subroutine in line number
200.
If
the third Function Key,
CE3),
is
pressed, execution goes
to
the subroutine in
line number 300.
If
any
other function key
is
pressed, nothing happens, since there are no more line
numbers in the
ON
KEY
GOSUB statement.
The general form of the statement
is:
ON
KEY GOSUB <Ust
of
line
numbers>
where the list of line numbers corresponds to subroutines. The first line number
in
the
list corresponds
to
Function Key
(fj),
the second line number,
if
present, corresponds
to
Function Key
(£2),
and
so
on.
The ON KEY GOSUB statement must be placed
in
the program
so
that it will be
executed
before the Function Key is pressed. Otherwise, the Function Keys will
be
ignored during program execution.
Line
38 A continuous loop prints the integers starting at zero and incrementing
in
steps of one. These statements keep right
on
printing, so long
as
a Function Key
is
not
pressed.
Line
lee
This two statement interrupt subroutine
is
executed if the Function Key (Ij)
is
pressed. The statement which is being executed when the Function Key is pressed
is
allowed
to
finish before execution is transferred
to
the subroutine. This subroutine
simply prints the message
SUBROUTINE
1
and returns execution to the statement after the one which was interrupted. For
example,
if
the interrupt occurs while the statement
PRINT I
is
being executed, then execution returns
to
the next statement
1=1+1.
Line 288 This interrupt subroutine is executed
if
Function Key
~
is
pressed.
Line
388 This interrupt subroutine
is
executed
if
Function Key (l3)
is
pressed.
Experiment
#3
Interrupting
an
Interrupt
It
is
possible to interrupt an interrupt subroutine
by
pressing a Function Key while the
interrupt subroutine is executing. You can modify the current program
to
illustrate
this.
Change line
200
to:
200
FOR
J = 1
TO
20
158
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