The Art of Assembly Language, 2nd Edition

5.9 Forward Procedures

As a general rule, HLA requires that you declare all symbols before their first use in a program.^[80] Therefore, you must define all procedures before their first call. There are two reasons this isn't always practical: mutual recursion (two procedures call each other) and source code organization (you prefer to place a procedure in your code after the point where you've first called it). Fortunately, HLA lets you use a forward procedure definition to declare a procedure prototype. Forward declarations let you define a procedure before you actually supply the code for that procedure.

A forward procedure declaration is a familiar procedure declaration that uses the reserved word forward in place of the procedure's declaration ...