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My BASIC Notes

GOSUB
Abbreviation: GO <SHIFT+S>

TYPE: Statement
FORMAT: GOSUB <line number>

Action: This is a specialized form of the GOTO statement, with one important difference: 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.

The major use of a subroutine (GOSUB really means GO to a SUBroutine) is when a small section of program 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 to DEF FN. DEF FN lets you save space when using a formula, while GOSUB saves space when using a severalline routine. Here is an inefficient program that doesn't use GOSUB:

100 PRINT "THIS PROGRAM PRINTS"
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"
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 number of subroutine 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.