Texas Instruments TI-99/4A - Page 133

Subroutines
Introduction
Subroutines may be thought of as separate self'contained programs
within a main program. They usually perform a certain action. such
as printing some information, performing a calculation, or reading
values into an array, Putting these actions into a subroutine allows
you to type
that set of statements only once and then perform that
set of statements from anyv.'"here in the program with a GOSUB
statement.
The GOSUB statement initially behaves like a GOTO statement. It
causes the computer to jump to the line'number listed. However,
subroutine programming gives the computer the capability to
"remember" where the branch occurred in the main program and
return to that point when it finishes the subroutine. This technique
requires
that the last statement in the subroutine be a RETURN
statement Tht' progr:lm normally has either a STOP statement
or some other unconditional branching statement immediately
before
the subroutines so that the computer doesn't accidentally
"£'al1 into" the subroutines The suhroutines should be entered
only by a GOSUB instruction and may be entered at any line-
number within the subroutine,
The example on the right illustrates how the GOSUB and
RETURN statements might be arranged in your program, The
program begins running at line 100. At line 300 it skips to the first
subroutine,
performs lines 700 through 780, and returns to line 310,
When it reaches line 400, it goes to the second subroutine. performs
lines
900 through 980. returns to line 410, and continues running,
At line 450 it again goes to subroutine 1, this time entering at Ime
750 and continuing to the RETURN, Then it goes back to the
main program at line 460 and continues running. At line 480 it
again jumps to the first subroutine, runs lines 700 through 7HO.
returns to line 490, then stops runninj?; at line 600. The STOP
statement in line 600 keeps the computer from performing the
subroutines unless you specifically direct it there with a GOSUB.
User's Reference Guide
Examples:
>N!::W
>100 REM MAIN PROGRAM
>300
GOSU8
700
>310
>400 GOSUB
900
>410 •
> 45 0
GOSU8 750
>460
>480 GDSUB
700
>490
-
>600 STOP
>700 REM SUBROUTlNE1
>750 •
>780 RETURN
>900 REM SU8RDUTINE2
>980 RETURN
>990 END
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