Source code that controls what XL Fortran does rather than what the user program does.

A specially-formatted comment within a source program that affects how the program is compiled. Compiler directives are not part of the Fortran 90 Standard. In HP Fortran 90, compiler directives provide control over source listing, optimization, and other features.

A special comment added to a program which directs the compiler to perform certain actions, such as data parallelisation or vectorisation