Art of Assembly Language, 1st Edition

5.10. 5.10 Forward Procedures

As a general rule HLA requires that you declare all symbols before their first use in a program.^[98] 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 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.

^[98] There are a few minor exceptions to this rule, but it is certainly true for procedure calls.

A forward procedure declaration is a familiar ...