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(nasm.info)Section 4.4

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4.4. Conditional Assembly
=========================

   Similarly to the C preprocessor, NASM allows sections of a source
file to be assembled only if certain conditions are met. The general
syntax of this feature looks like this:

     %if<condition>
         ; some code which only appears if <condition> is met
     %elif<condition2>
         ; only appears if <condition> is not met but <condition2> is
     %else
         ; this appears if neither <condition> nor <condition2> was met
     %endif

   The `%else' clause is optional, as is the `%elif' clause. You can
have more than one `%elif' clause as well.

Section 4.4.1

`%ifdef': Testing Single-Line Macro Existence

Section 4.4.2

`ifmacro': Testing Multi-Line Macro Existence

Section 4.4.3

`%ifctx': Testing the Context Stack

Section 4.4.4

`%if': Testing Arbitrary Numeric Expressions

Section 4.4.5

`%ifidn' and `%ifidni': Testing Exact Text Identity

Section 4.4.6

`%ifid', `%ifnum', `%ifstr': Testing Token Types

Section 4.4.7

`%error': Reporting User-Defined Errors