Q44306: Method for Clearing GOSUB…RETURN Subroutine Stack in BASIC

Article: Q44306
Product(s): See article
Version(s): 1.00 1.01 1.02 2.00 2.01 3.00 4.00 4.00b 4.50
Operating System(s): MS-DOS
Keyword(s): ENDUSER | B_GWBasicI B_BasicCom B_BasicInt B_BBasic B_MQuickB | mspl13_basic
Last Modified: 15-DEC-1989

When a program jumps out of a GOSUB subroutine without executing a
RETURN statement, the return address is left on the stack. If the
program jumps out multiple times, an "Out of Stack Space" or "Out of
Memory" error eventually occurs. You must make your program execute an
equal number of GOSUB and RETURN statements to avoid this error.

The program below demonstrates a method for clearing the subroutine
stack when a program executes more GOSUB statements than RETURN
statements. Generally, a program should get out of a GOSUB subroutine
with a RETURN statement, but when that is not feasible, the method
below may be used.

This information applies to Microsoft QuickBASIC Versions 1.00, 1.01,
2.00, 2.01, 3.00, 4.00, 4.00b, and 4.50 for MS-DOS; Microsoft GW-BASIC
Interpreter Versions 3.20, 3.22, and 3.23 for MS-DOS; Microsoft BASIC
Interpreter Version 5.28 for MS-DOS; Microsoft BASIC Compiler Versions
5.35 and 5.36 for MS-DOS; Microsoft BASIC Compiler Versions 6.00 and
6.00b for MS-DOS and MS OS/2; and Microsoft BASIC PDS Version 7.00 for
MS-DOS and MS OS/2.

The following program, STACK.BAS, invokes RETURN in a continuous loop
to clear the stack until a "RETURN Without GOSUB" error occurs. This
error is trapped, and the program resumes execution. The program
STACK.BAS is as follows:

10 A = A + 1
   IF A < 10 THEN GOSUB 10      ' Put 10 GOSUB addresses on the stack.
   ON ERROR GOTO 30
20 RETURN 20                    ' Pop addresses off stack until
30 ON ERROR GOTO 0              ' "RETURN without GOSUB" error.
   END

Note: This method will not work with the SUB...END SUB subprogram
procedure. A SUB...END SUB procedure should always be exited with
either an EXIT SUB or an END SUB statement.

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