Subroutines

When the same few lines need to happen in three different places, put them in one place and call them.

GOSUB and RETURN

10 GOSUB 100
20 PRINT "BACK"
30 END
100 PRINT "IN THE SUBROUTINE"
110 RETURN

RUN
IN THE SUBROUTINE
BACK

OK

GOSUB jumps like GOTO, but it remembers where it came from. RETURN goes back to the line after the GOSUB.

Where to put them

Subroutines go after the main program, and the main program ends with END so it doesn't wander into them:

10 REM MAIN PROGRAM
20 GOSUB 1000
30 END
1000 REM DRAW THE BOARD
1010 RETURN

Without that END on line 30, the program falls straight into line 1000, prints the board a second time, hits RETURN with nowhere to return to, and stops with ?RETURN WITHOUT GOSUB. It's the classic mistake, and now you know what the message means.

Passing values

There are no parameters. A subroutine works on ordinary variables, so you set them before you call:

10 R = 5
20 GOSUB 500
30 PRINT "AREA IS"; A
40 END
500 A = 3.14159 * R * R
510 RETURN

Everything is shared with everything. Pick a couple of variables and use them consistently as "the ones subroutines talk through", and write down which they are in a REM.

A menu with ON

ON picks a destination by number: 1 takes the first line in the list, 2 the second:

10 PRINT "1 - SAY HELLO"
20 PRINT "2 - TELL A JOKE"
30 PRINT "3 - ADD UP"
40 PRINT
50 PRINT "CHOOSE";
60 INPUT C
70 IF C < 1 OR C > 3 THEN PRINT "NO SUCH CHOICE" : END
80 ON C GOSUB 100, 200, 300
90 END
100 PRINT "HELLO THERE"
110 RETURN
200 PRINT "WHY DID THE 6502 CROSS THE BUS"
210 RETURN
300 PRINT "2 + 2 ="; 2 + 2
310 RETURN

RUN
1 - SAY HELLO
2 - TELL A JOKE
3 - ADD UP

CHOOSE? 2
WHY DID THE 6502 CROSS THE BUS

OK

ON C GOSUB 100, 200, 300 replaces three IFs and stays readable when the menu grows. ON … GOTO works exactly the same way without the coming back.

A number outside the list — 0, or 4 when there are only three — quietly falls through to the next line, which is why line 70 checks the range first. A negative number is an error rather than a fall-through.

How deep you can go

A subroutine can call a subroutine. Each GOSUB costs a little of the machine's working space, and around two dozen levels deep you'll get ?OUT OF MEMORY.

That's plenty for ordinary nesting and not enough for recursion. If you write a subroutine that calls itself, expect to hit the ceiling almost immediately:

10 N = N + 1
20 GOSUB 10

RUN

?OUT OF MEMORY ERROR IN 20

OK

Next: holding a hundred numbers without inventing a hundred names.