BBC BASIC Programmers' Reference

by Richard Russell and Michael Hutton, July 2009

The right-click context menu in the BBC BASIC for Windows IDE allows you to navigate around your program quickly and easily, by providing a means to 'jump' to the beginning of a specified function or procedure with just a couple of mouse clicks. However there may be circumstances when you would like to jump not to the beginning of a PROC or FN, but to some other significant point in your code.

You can do that by inserting a bookmark in your code at the appropriate point, which will also appear in the context menu. Just as with PROCs and FNs (and other context menu items such as ON CLOSE or ON ERROR) clicking on the name of the bookmark will take you there. If you want to return to where you were before, the Go back menu item does that.

Another use for bookmarks is to act as separators in the context menu. For example you might want to organise your procedures and functions into groups of related routines, in order to make the program structure clearer.

Bookmarks inserted 'inline' within your code will be ignored when the program is executed, although they may slow it down very slightly. Bookmarks used as separators between blocks of FNs or PROCs should have no significant impact, other than making your program a little bigger than it would otherwise be.

There are two ways in which you can insert bookmarks into your program:

Normally you use DEF with the keywords PROC (to define a procedure) or FN (to define a function). However, if you include a line in your program which begins with DEF but is not followed by PROC or FN that line will act as a bookmark. For example:

        DEF _this_is_important_
        REM important code starts here...

The name of the bookmark must follow normal procedure and function naming conventions, that is it must consist only of alphabetic characters (A-Z, a-z), digits (0-9) or the characters _ (underscore), @ (commercial-at) and ` (backwards quote or grave accent).

If you incorporate a space character in the bookmark the context menu will show only what precedes it, for example:

        DEF take me here

will result in take appearing in the menu.

You can incorporate labels in your program, which are primarily intended to be used as the targets for GOTO, GOSUB or RESTORE statements (although their use is deprecated). However you can also use labels as bookmarks, for example:

        (_this_is_important_)
        REM important code starts here...

The same naming restrictions apply as with the previous method, and again if you incorporate a space character the context menu will show only what precedes it (in this case take):

        (take me here)

Using labels as bookmarks does have one minor disadvantage in that if you also use 'genuine' labels in your program (which hopefully you won't!) the bookmarks will use up some space on the heap, and could corrupt the value of a normal variable that happens to have the same name as the bookmark. A similar thing can happen if your program deliberately generates a No such variable error which is trapped, therefore allowing execution to continue.

However these are unlikely circumstances and in most respects there is little to choose between the two methods of incorporating bookmarks.