Sections of a source file may be assembled conditionally, only if certain conditions are true. The [ and ] (if and endif) directives are used to mark their start and finish; | provides an else construct. The syntax is:

[ logical-expression
...code...
|
...code...
]

Note that [, | and ] may not be the first character of a line. If the logical-expression is true, the section will be assembled; if it is false, the second piece of code, the beginning of which is marked by | and the end of which is marked by ], will be assembled instead. Lines of code skipped during conditional assembly will not be listed unless the assembler is switched from its default TERSE mode by the -NOTERSE command-line switch.

The directives IF, ELSE and ENDIF may be used instead of [, | and ] respectively.