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Directives are words of special significance which provide additional functionality
regarding compilation and output.

The following directives are at your disposal:

- Compiler directives for conditional compilation, 
- Linker directives for object distribution in memory. 

Any line in source code with leading 

#

is taken as a compiler directive. The initial #

can be preceded or followed by whitespace (excluding new lines). The compiler
directives are not case sensitive.

You can use conditional compilation to select particular sections of code to compile
while excluding other sections. All compiler directives must be completed in the
source file in which they begun.

Use directive 

#DEFINE

to define a conditional compiler constant (“flag”). You can use

any identifier for a flag, with no limitations. No conflicts with program identifiers are
possible because the flags have a separate name space. Only one flag can be set
per directive.

For example:

#DEFINE extended_format

Use 

#UNDEFINE

to undefine (“clear”) previously defined flag.

Conditional compilation is carried out by the 

#IFDEF

directive. 

#IFDEF

tests whether

a flag is currently defined or not; i.e. whether the previous 

#DEFINE

directive has

been processed for that flag and is still in force.

The directive 

#IFDEF

is terminated by the 

#ENDIF

directive and can have any num-

ber of the 

#ELSEIF

clauses and an optional 

#ELSE

clause:

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- SOFTWARE AND HARDWARE SOLUTIONS FOR EMBEDDED WORLD

CHAPTER 5