If the symbol specified in the operand is not defined the code following the
conditional statement will is assembled until either an 'else', 'endif', or
other conditional assemby mneumonic is encountered. This statement must be
terminated by a matching 'endif' directive.
Example:
ifndef lodval
lda #$11 * Assembled only if 'lodval' is not defined
else
lda #$22 * Assembled
only if
'lodval'
is defined
endif