V1[]
GOSUB and RETURN are commands which allow use of subroutines in Petit Computer BASIC.
GOSUB must be followed by a label. When a GOSUB is encountered while running the program, that point in the program is pushed onto a 'call stack', then execution of the program continues from the label (like GOTO). When a RETURN is encountered, the last value added to the 'call stack' is removed, and execution continues from that point again (i.e. the location of the corresponding GOSUB).
Essentially, the idea is that you can create a small program (a "subroutine"), put a label at the beginning of it, and put RETURN at the end of it. You can execute this subroutine by using GOSUB with that label. If you have a task that needs to be performed many times, this can save a lot of typing and make your code more robust to change. Even if a section of code is used only once, using subroutines to separate different tasks can help make your program tidy and more easily understood and maintained.
The 'call stack' has a maximum depth of 255. This means that from the start of the program you can run 255 consecutive GOSUBs, but the next one will generate an Out of memory error. If the system encounters a RETURN when the call stack is empty, it generates a RETURN without GOSUB error.
The GOSUB stack memory is shared with the FOR stack memory... so, if the very start of a program is a FOR loop, you can only run 254 consecutive GOSUBs within that loop before getting an Out of memory error. Any additions to the FOR stack made in a subroutine, that are not removed in the normal way by the execution of a NEXT ending the loop, are removed by the RETURN. So, a single RETURN can free up two or more spaces in the stack memory. In contrast, additions to the call stack made during a FOR loop are not removed by a NEXT.
GOSUB is not allowed at the interactive command-line; this generates a Syntax error.
V2[]
GOSUB and RETURN in SmileBasic V2 are the same as in SmileBasic V1, except GOSUB does not have to be follwed by a label: it may instead be followed by a string value representing a label. The first character of the string must be "@", or the program will generate a Syntax error.
V3[]
No information on SmileBasic V3.