6.1.9.23 If, Elsif, Else And Endif Directives

These directives implement conditional assembly.

The argument to IF and ELSIF must be a constant expression. If this argument is non-zero, then the code following it up to the next matching ELSE, ELSIF or ENDIF will be assembled. If the argument is zero, then the code up to the next matching ELSE or ENDIF will not be output. At an ELSE, the sense of the conditional compilation will be inverted, while an ENDIF will terminate the conditional assembly block. Conditional assembly blocks can be nested.

These directives do not implement a runtime conditional statement in the same way that the C statement if does; they are only evaluated when the code is built. In addition, assembly code in both true and false cases is always scanned and interpreted, but the machine code corresponding to instructions is output only if the condition matches. This implies that assembler directives (e.g., EQU) will be processed regardless of the state of the condition expression and should not be used inside an IF construct.

For example:

IF ABC
    goto aardvark
ELSIF DEF
    goto denver
ELSE
    goto grapes
ENDIF
ENDIF

In this example, if ABC is non-zero, the first goto instruction will be assembled but not the second or third. If ABC is zero and DEF is non-zero, the second goto instruction will be assembled but the first and third will not. If both ABC and DEF are zero, the third goto instruction will be assembled.