GOSUB

GOSUB is a control flow instruction found in many BASIC dialects. It transfers program execution to a subroutine located at a specified line number or label and, when the subroutine completes, returns to the statement immediately following the GOSUB via the RETURN command. The language generally implements this with a return stack that records the address to resume after each GOSUB call, allowing nested subroutines up to the interpreter’s limit.

In typical usage, a GOSUB 1000 would jump to line 1000. The subroutine runs and ends with RETURN, which returns control to the line after the original GOSUB. If RETURN is executed without a preceding GOSUB, or if the program falls off the end of the subroutine without returning, a runtime error or a mismatch error is often produced. Some dialects distinguish between successful returns and errors with messages such as “RETURN without GOSUB.”

GOSUB does not provide formal parameter passing; any data required by the subroutine is usually accessed through global variables or by setting values before issuing GOSUB. This can lead to tighter coupling between code blocks. Because of its reliance on a return stack and potential for tangled control flow, GOSUB is considered less structured than modern function or procedure calls and is less common in new software. Nevertheless, it remains part of the heritage of classic BASIC interpreters and is still supported in many dialects for backward compatibility. Some newer BASICs offer line-numberless forms or labels for subroutines, but traditional GOSUB primarily uses line numbers.