AVR IAR Assembler Reference Guide

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Conditional directives
Use the #if...#else...#endif directives to control the assembly process at assembly
time. If the condition following the #if directive is not true, the subsequent instructions
will not generate any code (i.e. it will not be assembled or syntax checked) until a
#endif or #else directive is found.
All assembler directives (except for END) and file inclusion may be disabled by the
conditional directives. Each #if directive must be terminated by a #endif directive.
The #else directive is optional and, if used, it must be inside a #if...#endif block.
#if...#endif and #if...#else...#endif blocks may be nested to any level.
Use #ifdef to assemble instructions up to the next #else or #endif directive only if
a symbol is defined.
Use #ifndef to assemble instructions up to the next #else or #endif directive only if
a symbol is undefined.