Chapter 4 Assembly Language and Cross Assembler
Comment
Comments are the descriptions of codes. They are used for documentation only and are ignored
by the Cross Assembler. Any text following a semicolon is considered a comment.
Assembly Directives
Directives give direction to the Cross Assembler, specifying the manner in which the Cross Assem-
bler generates object code at assembly time. Directives can be further classified according to their
behavior as described below.
Conditional Assembly Directives
The conditional block has the following form:
IF
statements
[ELSE
statements]
ENDIF
Syntax
IF
expression
IFE
expression
Description
The directives
IF
and
IFE
test the
expression
following them.
The
IF
directive grants assembly if the value of the
expression
is true, i.e. non-zero.
The
IFE
directive grants assembly if the value of the
expression
is false, i.e. zero.
Example
IF debugcase
ACC1
equ 5
extern username: byte
ENDIF
In this example, the value of the variable
ACC1
is set to 5 and the
username
is declared as an
external variable if the symbol
debugcase
is evaluated as true, i.e. nonzero.
Syntax
IFDEF
name
IFNDEF
name
Description
The directives
IFDEF
and
IFNDEF
test whether or not the given
name
has been defined. The
IFDEF
directive grants assembly only if the
name
is a label, a variable or a symbol. The
IFNDEF
di-
rective grants assembly only if the
name
has not yet been defined. The conditional assembly direc-
tives support a nesting structure, with a maximum nesting level of 7.
Example
IFDEF
ENDIF
In this example, the
buffer
is allocated only if the
buf_flag
has been previously defined.
139
buf_flag
buffer DB
20 dup(?)
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