if conditional assembly directive is not the same as a Pascal or C "if" statement. If you are familiar with C, the #ifdef directive in C is roughly equivalent to some of MASM's conditional assembly directives.
if directive uses the following syntax:
if expression
<sequence of statements>
else ;This is optional!
<sequence of statements>
endif
MASM evaluates expression. If it is a non-zero value, then MASM will assemble the statements between the if and else directives (or endif, if the else isn't present). If the expression evaluates to zero (false) and an else section is present, MASM will assemble the statements between the else directive and the endif directive. If the else section is not present and expression evaluates to false, then MASM will not assemble any of the code between the if and endif directives.
The important thing to remember is that expression has to be an expression that MASM can evaluate at assembly time. That is, it must evaluate to a constant. Manifest constants (equates) and values that MASM's type operators produce are commonly found in if directive expressions. For example, suppose you want to assemble code for two different processors as described above. You could use statements like the following:
Processor = 80386 ;Set to 8086 for 8086-only code
.
.
.
if Processor eq 80386
shl ax, 4
else ;Must be 8086 processor.
mov cl, 4
shl ax, cl
endif
There are other ways to accomplish this same thing. MASM provides built-in variables that tell you if you are assembling code for some specific processor. More on that later.
ife directive is used exactly like the if directive, except it assembles the code after the ife directive only if the expression evaluates to zero (false), rather than true (non-zero).
Ifdef will assemble the associated code if the symbol is defined, Ifndef will assemble the associated code if the symbol isn't defined. Use else and endif to terminate the conditional assembly sequences.
ifdef DEBUG
<place debugging statements here>
endif
To activate the debugging code, simply define the symbol DEBUG somewhere at the beginning of your program (before the first ifdef referencing DEBUG). To automatically eliminate the debugging code, simply delete the definition of DEBUG. You may define DEBUG using a simple statement like:
DEBUG = 0
Note that the value you assign to DEBUG is unimportant. Only the fact that you have defined (or have not defined) this symbol is important.
ifb) or not blank (ifnb). Consider the following code:
Blank textequ <>
NotBlank textequ <not blank>
ifb Blank
<this code will assemble>
endif
ifb NotBlank
<this code will not>
endif
The ifnb works in an opposite manner to ifb. That is, it would assemble the statements above that ifb does not and vice versa.
ifidn), different (ifdif), identical ignoring case (ifidni), or different ignoring case (ifdifi). The syntax is
ifidn op1, op2
<statements to assemble if <op1> == <op2>>
endif
ifdif op1, op2
<statements to assemble if <op1> != <op2>>
endif
ifidni op1, op2
<statements to assemble if <op1> == <op2>>
endif
ifdifi op1, op2
<statements to assemble if <op1> != <op2>>
endif
The difference between the IFxxx and IFxxxI statements above is that the IFxxxI statements ignore differences in alphabetic case when comparing operands.