Commodore VIC-20 - Page 37

This is a specialized form of the GOTO statement, with the
important difference that GOSUB remembers where it came from.
When the RETURN statement (different from the RETURN key on
the keyboard) is reached in the program, the program jumps back
to the statement immediately following the original GOSUB
statement from which it came.
The major use of a subroutine {GOSUB really means GO to a
SUBroutine)
is when there is a small section of program that is used
by different sections of the program. By using subroutines rather
than repeating the same lines over and over at different places in
the program, you can save lots of program space. In this way,
GOSUB is similar in use to DEF FN: DEF FN lets you save space
when using a formula, and GOSUB saves space when using a
several-line routine.
Here is an inefficient program that doesn't use GOSUB:
100 PRINT "THIS PROGRAM PRINTS'1
110 FOR L - 1 TO 500 : NEXT
120 PRINT "SLOWLY ON THE SCREEN"
130 FOR L = 1 TO 500 : NEXT
140 PRINT "USING A SIMPLE LOOP"
150 FOR L - 1 TO 500 : NEXT
160 PRINT "AS A TIME DELAY."
170 FOR L = 1 TO 500 : NEXT
Here is the same program using GOSUB:
100 PRINT "THIS PROGRAM PRINTS'1
110 GOSUB 200
120 PRINT "SLOWLY ON THE SCREEN"
130 GOSUB 200
140 PRINT "USING A SIMPLE LOOP"
150 GOSUB 200
160 PRINT "AS A TIME DELAY."
170 GOSUB 200
180 END
200 FOR L = 1 TO 500 : NEXT
210 RETURN
Each time the program executes a GOSUB, the line number and
position in the program line are saved in a special area called the
"stack," which takes up 256 bytes of your memory. This limits the
amount of data that can be stored in the stack. Therefore, the
numberofsubroutine return addresses that can be stored is limited,
and care should be taken to make sure every GOSUB hits the
corresponding RETURN, or else you'll run out of memory even
though you have plenty of bytes free.
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