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Chapter 10 Writing Assembler Control Statements

Conditional Assembly 191

10.4.1 #ifdef, #ifndef

Syntax

Syntax for #ifdef

Syntax for #ifndef

#ifdef

identifier

#ifndef

identifier

block1

[#else

block2]

#endif

Functional description

#ifdef

If the identifier has been defined by a #define directive before the #ifdef statement, block1 will be

assembled. If it has not been defined and an #else directive has been coded, block2 will be assembled.

#ifndef

If the identifier has not been defined by a #define directive before the #ifndef statement, block1 will be

assembled. If it has been defined and an #else directive has been coded, block2 will be assembled.

Coding rules

These directives can be used within macro definitions and wherever machine language instructions can

be coded.

If an identifier is defined after #ifdef or #ifndef, it will be considered undefined.

Identifiers can be specified by the D option when the assembler is started, even if they are not defined

with #define directives.