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<h1>NASM - The Netwide Assembler</h1>
<span class="subtitle">version 2.14.02</span>
</div>
<div class="contents"
>
<h2 id="chapter-4">Chapter 4: The NASM Preprocessor</h2>
<p>NASM contains a powerful macro processor, which supports conditional
assembly, multi-level file inclusion, two forms of macro (single-line and
multi-line), and a `context stack' mechanism for extra macro power.
Preprocessor directives all begin with a <code>%</code> sign.</p>
<p>The preprocessor collapses all lines which end with a backslash (\)
character into a single line. Thus:</p>
<pre>
%define THIS_VERY_LONG_MACRO_NAME_IS_DEFINED_TO \
THIS_VALUE
</pre>
<p>will work like a single-line macro without the backslash-newline
sequence.</p>
<h3 id="section-4.1">4.1 Single-Line Macros</h3>
<h4 id="section-4.1.1">4.1.1 The Normal Way: <code>%define</code></h4>
<p>Single-line macros are defined using the <code>%define</code>
preprocessor directive. The definitions work in a similar way to C; so you
can do things like</p>
<pre>
%define ctrl 0x1F &
%define param(a,b) ((a)+(a)*(b))
mov byte [param(2,ebx)], ctrl 'D'
</pre>
<p>which will expand to</p>
<pre>
mov byte [(2)+(2)*(ebx)], 0x1F & 'D'
</pre>
<p>When the expansion of a single-line macro contains tokens which invoke
another macro, the expansion is performed at invocation time, not at
definition time. Thus the code</p>
<pre>
%define a(x) 1+b(x)
%define b(x) 2*x
mov ax,a(8)
</pre>
<p>will evaluate in the expected way to <code>mov ax,1+2*8</code>, even
though the macro <code>b</code> wasn't defined at the time of definition of
<code>a</code>.</p>
<p>Macros defined with <code>%define</code> are case sensitive: after
<code>%define foo bar</code>, only <code>foo</code> will expand to
<code>bar</code>: <code>Foo</code> or <code>FOO</code> will not. By using
<code>%idefine</code> instead of <code>%define</code> (the `i' stands for
`insensitive') you can define all the case variants of a macro at once, so
that <code>%idefine foo bar</code> would cause <code>foo</code>,
<code>Foo</code>, <code>FOO</code>, <code>fOO</code> and so on all to
expand to <code>bar</code>.</p>
<p>There is a mechanism which detects when a macro call has occurred as a
result of a previous expansion of the same macro, to guard against circular
references and infinite loops. If this happens, the preprocessor will only
expand the first occurrence of the macro. Hence, if you code</p>
<pre>
%define a(x) 1+a(x)
mov ax,a(3)
</pre>
<p>the macro <code>a(3)</code> will expand once, becoming
<code>1+a(3)</code>, and will then expand no further. This behaviour can be
useful: see <a href="nasmdoc9.html#section-9.1">section 9.1</a> for an
example of its use.</p>
<p>You can overload single-line macros: if you write</p>
<pre>
%define foo(x) 1+x
%define foo(x,y) 1+x*y
</pre>
<p>the preprocessor will be able to handle both types of macro call, by
counting the parameters you pass; so <code>foo(3)</code> will become
<code>1+3</code> whereas <code>foo(ebx,2)</code> will become
<code>1+ebx*2</code>. However, if you define</p>
<pre>
%define foo bar
</pre>
<p>then no other definition of <code>foo</code> will be accepted: a macro
with no parameters prohibits the definition of the same name as a macro
<em>with</em> parameters, and vice versa.</p>
<p>This doesn't prevent single-line macros being <em>redefined</em>: you
can perfectly well define a macro with</p>
<pre>
%define foo bar
</pre>
<p>and then re-define it later in the same source file with</p>
<pre>
%define foo baz
</pre>
<p>Then everywhere the macro <code>foo</code> is invoked, it will be
expanded according to the most recent definition. This is particularly
useful when defining single-line macros with <code>%assign</code> (see
<a href="#section-4.1.7">section 4.1.7</a>).</p>
<p>You can pre-define single-line macros using the `-d' option on the NASM
command line: see <a href="nasmdoc2.html#section-2.1.19">section
2.1.19</a>.</p>
<h4 id="section-4.1.2">4.1.2 Resolving <code>%define</code>: <code>%xdefine</code></h4>
<p>To have a reference to an embedded single-line macro resolved at the
time that the embedding macro is <em>defined</em>, as opposed to when the
embedding macro is <em>expanded</em>, you need a different mechanism to the
one offered by <code>%define</code>. The solution is to use
<code>%xdefine</code>, or it's case-insensitive counterpart
<code>%ixdefine</code>.</p>
<p>Suppose you have the following code:</p>
<pre>
%define isTrue 1
%define isFalse isTrue
%define isTrue 0
val1: db isFalse
%define isTrue 1
val2: db isFalse
</pre>
<p>In this case, <code>val1</code> is equal to 0, and <code>val2</code> is
equal to 1. This is because, when a single-line macro is defined using
<code>%define</code>, it is expanded only when it is called. As
<code>isFalse</code> expands to <code>isTrue</code>, the expansion will be
the current value of <code>isTrue</code>. The first time it is called that
is 0, and the second time it is 1.</p>
<p>If you wanted <code>isFalse</code> to expand to the value assigned to
the embedded macro <code>isTrue</code> at the time that
<code>isFalse</code> was defined, you need to change the above code to use
<code>%xdefine</code>.</p>
<pre>
%xdefine isTrue 1
%xdefine isFalse isTrue
%xdefine isTrue 0
val1: db isFalse
%xdefine isTrue 1
val2: db isFalse
</pre>
<p>Now, each time that <code>isFalse</code> is called, it expands to 1, as
that is what the embedded macro <code>isTrue</code> expanded to at the time
that <code>isFalse</code> was defined.</p>
<h4 id="section-4.1.3">4.1.3 Macro Indirection: <code>%[...]</code></h4>
<p>The <code>%[...]</code> construct can be used to expand macros in
contexts where macro expansion would otherwise not occur, including in the
names other macros. For example, if you have a set of macros named
<code>Foo16</code>, <code>Foo32</code> and <code>Foo64</code>, you could
write:</p>
<pre>
mov ax,Foo%[__BITS__] ; The Foo value
</pre>
<p>to use the builtin macro <code>__BITS__</code> (see
<a href="#section-4.11.5">section 4.11.5</a>) to automatically select
between them. Similarly, the two statements:</p>
<pre>
%xdefine Bar Quux ; Expands due to %xdefine
%define Bar %[Quux] ; Expands due to %[...]
</pre>
<p>have, in fact, exactly the same effect.</p>
<p><code>%[...]</code> concatenates to adjacent tokens in the same way that
multi-line macro parameters do, see <a href="#section-4.3.9">section
4.3.9</a> for details.</p>
<h4 id="section-4.1.4">4.1.4 Concatenating Single Line Macro Tokens: <code>%+</code></h4>
<p>Individual tokens in single line macros can be concatenated, to produce
longer tokens for later processing. This can be useful if there are several
similar macros that perform similar functions.</p>
<p>Please note that a space is required after <code>%+</code>, in order to
disambiguate it from the syntax <code>%+1</code> used in multiline macros.</p>
<p>As an example, consider the following:</p>
<pre>
%define BDASTART 400h ; Start of BIOS data area
struc tBIOSDA ; its structure
.COM1addr RESW 1
.COM2addr RESW 1
; ..and so on
endstruc
</pre>
<p>Now, if we need to access the elements of tBIOSDA in different places,
we can end up with:</p>
<pre>
mov ax,BDASTART + tBIOSDA.COM1addr
mov bx,BDASTART + tBIOSDA.COM2addr
</pre>
<p>This will become pretty ugly (and tedious) if used in many places, and
can be reduced in size significantly by using the following macro:</p>
<pre>
; Macro to access BIOS variables by their names (from tBDA):
%define BDA(x) BDASTART + tBIOSDA. %+ x
</pre>
<p>Now the above code can be written as:</p>
<pre>
mov ax,BDA(COM1addr)
mov bx,BDA(COM2addr)
</pre>
<p>Using this feature, we can simplify references to a lot of macros (and,
in turn, reduce typing errors).</p>
<h4 id="section-4.1.5">4.1.5 The Macro Name Itself: <code>%?</code> and <code>%??</code></h4>
<p>The special symbols <code>%?</code> and <code>%??</code> can be used to
reference the macro name itself inside a macro expansion, this is supported
for both single-and multi-line macros. <code>%?</code> refers to the macro
name as <em>invoked</em>, whereas <code>%??</code> refers to the macro name
as <em>declared</em>. The two are always the same for case-sensitive
macros, but for case-insensitive macros, they can differ.</p>
<p>For example:</p>
<pre>
%idefine Foo mov %?,%??
foo
FOO
</pre>
<p>will expand to:</p>
<pre>
mov foo,Foo
mov FOO,Foo
</pre>
<p>The sequence:</p>
<pre>
%idefine keyword $%?
</pre>
<p>can be used to make a keyword "disappear", for example in case a new
instruction has been used as a label in older code. For example:</p>
<pre>
%idefine pause $%? ; Hide the PAUSE instruction
</pre>
<h4 id="section-4.1.6">4.1.6 Undefining Single-Line Macros: <code>%undef</code></h4>
<p>Single-line macros can be removed with the <code>%undef</code>
directive. For example, the following sequence:</p>
<pre>
%define foo bar
%undef foo
mov eax, foo
</pre>
<p>will expand to the instruction <code>mov eax, foo</code>, since after
<code>%undef</code> the macro <code>foo</code> is no longer defined.</p>
<p>Macros that would otherwise be pre-defined can be undefined on the
command-line using the `-u' option on the NASM command line: see
<a href="nasmdoc2.html#section-2.1.20">section 2.1.20</a>.</p>
<h4 id="section-4.1.7">4.1.7 Preprocessor Variables: <code>%assign</code></h4>
<p>An alternative way to define single-line macros is by means of the
<code>%assign</code> command (and its case-insensitive counterpart
<code>%iassign</code>, which differs from <code>%assign</code> in exactly
the same way that <code>%idefine</code> differs from <code>%define</code>).</p>
<p><code>%assign</code> is used to define single-line macros which take no
parameters and have a numeric value. This value can be specified in the
form of an expression, and it will be evaluated once, when the
<code>%assign</code> directive is processed.</p>
<p>Like <code>%define</code>, macros defined using <code>%assign</code> can
be re-defined later, so you can do things like</p>
<pre>
%assign i i+1
</pre>
<p>to increment the numeric value of a macro.</p>
<p><code>%assign</code> is useful for controlling the termination of
<code>%rep</code> preprocessor loops: see <a href="#section-4.5">section
4.5</a> for an example of this. Another use for <code>%assign</code> is
given in <a href="nasmdoc8.html#section-8.4">section 8.4</a> and
<a href="nasmdoc9.html#section-9.1">section 9.1</a>.</p>
<p>The expression passed to <code>%assign</code> is a critical expression
(see <a href="nasmdoc3.html#section-3.8">section 3.8</a>), and must also
evaluate to a pure number (rather than a relocatable reference such as a
code or data address, or anything involving a register).</p>
<h4 id="section-4.1.8">4.1.8 Defining Strings: <code>%defstr</code></h4>
<p><code>%defstr</code>, and its case-insensitive counterpart
<code>%idefstr</code>, define or redefine a single-line macro without
parameters but converts the entire right-hand side, after macro expansion,
to a quoted string before definition.</p>
<p>For example:</p>
<pre>
%defstr test TEST
</pre>
<p>is equivalent to</p>
<pre>
%define test 'TEST'
</pre>
<p>This can be used, for example, with the <code>%!</code> construct (see
<a href="#section-4.10.2">section 4.10.2</a>):</p>
<pre>
%defstr PATH %!PATH ; The operating system PATH variable
</pre>
<h4 id="section-4.1.9">4.1.9 Defining Tokens: <code>%deftok</code></h4>
<p><code>%deftok</code>, and its case-insensitive counterpart
<code>%ideftok</code>, define or redefine a single-line macro without
parameters but converts the second parameter, after string conversion, to a
sequence of tokens.</p>
<p>For example:</p>
<pre>
%deftok test 'TEST'
</pre>
<p>is equivalent to</p>
<pre>
%define test TEST
</pre>
<h3 id="section-4.2">4.2 String Manipulation in Macros</h3>
<p>It's often useful to be able to handle strings in macros. NASM supports
a few simple string handling macro operators from which more complex
operations can be constructed.</p>
<p>All the string operators define or redefine a value (either a string or
a numeric value) to a single-line macro. When producing a string value, it
may change the style of quoting of the input string or strings, and
possibly use <code>\</code>–escapes inside
<code>`</code>–quoted strings.</p>
<h4 id="section-4.2.1">4.2.1 Concatenating Strings: <code>%strcat</code></h4>
<p>The <code>%strcat</code> operator concatenates quoted strings and assign
them to a single-line macro.</p>
<p>For example:</p>
<pre>
%strcat alpha "Alpha: ", '12" screen'
</pre>
<p>... would assign the value <code>'Alpha: 12" screen'</code> to
<code>alpha</code>. Similarly:</p>
<pre>
%strcat beta '"foo"\', "'bar'"
</pre>
<p>... would assign the value <code>`"foo"\\'bar'`</code> to
<code>beta</code>.</p>
<p>The use of commas to separate strings is permitted but optional.</p>
<h4 id="section-4.2.2">4.2.2 String Length: <code>%strlen</code></h4>
<p>The <code>%strlen</code> operator assigns the length of a string to a
macro. For example:</p>
<pre>
%strlen charcnt 'my string'
</pre>
<p>In this example, <code>charcnt</code> would receive the value 9, just as
if an <code>%assign</code> had been used. In this example,
<code>'my string'</code> was a literal string but it could also have been a
single-line macro that expands to a string, as in the following example:</p>
<pre>
%define sometext 'my string'
%strlen charcnt sometext
</pre>
<p>As in the first case, this would result in <code>charcnt</code> being
assigned the value of 9.</p>
<h4 id="section-4.2.3">4.2.3 Extracting Substrings: <code>%substr</code></h4>
<p>Individual letters or substrings in strings can be extracted using the
<code>%substr</code> operator. An example of its use is probably more
useful than the description:</p>
<pre>
%substr mychar 'xyzw' 1 ; equivalent to %define mychar 'x'
%substr mychar 'xyzw' 2 ; equivalent to %define mychar 'y'
%substr mychar 'xyzw' 3 ; equivalent to %define mychar 'z'
%substr mychar 'xyzw' 2,2 ; equivalent to %define mychar 'yz'
%substr mychar 'xyzw' 2,-1 ; equivalent to %define mychar 'yzw'
%substr mychar 'xyzw' 2,-2 ; equivalent to %define mychar 'yz'
</pre>
<p>As with <code>%strlen</code> (see <a href="#section-4.2.2">section
4.2.2</a>), the first parameter is the single-line macro to be created and
the second is the string. The third parameter specifies the first character
to be selected, and the optional fourth parameter preceeded by comma) is
the length. Note that the first index is 1, not 0 and the last index is
equal to the value that <code>%strlen</code> would assign given the same
string. Index values out of range result in an empty string. A negative
length means "until N-1 characters before the end of string", i.e.
<code>-1</code> means until end of string, <code>-2</code> until one
character before, etc.</p>
<h3 id="section-4.3">4.3 Multi-Line Macros: <code>%macro</code></h3>
<p>Multi-line macros are much more like the type of macro seen in MASM and
TASM: a multi-line macro definition in NASM looks something like this.</p>
<pre>
%macro prologue 1
push ebp
mov ebp,esp
sub esp,%1
%endmacro
</pre>
<p>This defines a C-like function prologue as a macro: so you would invoke
the macro with a call such as</p>
<pre>
myfunc: prologue 12
</pre>
<p>which would expand to the three lines of code</p>
<pre>
myfunc: push ebp
mov ebp,esp
sub esp,12
</pre>
<p>The number <code>1</code> after the macro name in the
<code>%macro</code> line defines the number of parameters the macro
<code>prologue</code> expects to receive. The use of <code>%1</code> inside
the macro definition refers to the first parameter to the macro call. With
a macro taking more than one parameter, subsequent parameters would be
referred to as <code>%2</code>, <code>%3</code> and so on.</p>
<p>Multi-line macros, like single-line macros, are case-sensitive, unless
you define them using the alternative directive <code>%imacro</code>.</p>
<p>If you need to pass a comma as <em>part</em> of a parameter to a
multi-line macro, you can do that by enclosing the entire parameter in
braces. So you could code things like</p>
<pre>
%macro silly 2
%2: db %1
%endmacro
silly 'a', letter_a ; letter_a: db 'a'
silly 'ab', string_ab ; string_ab: db 'ab'
silly {13,10}, crlf ; crlf: db 13,10
</pre>
<h4 id="section-4.3.1">4.3.1 Overloading Multi-Line Macros</h4>
<p>As with single-line macros, multi-line macros can be overloaded by
defining the same macro name several times with different numbers of
parameters. This time, no exception is made for macros with no parameters
at all. So you could define</p>
<pre>
%macro prologue 0
push ebp
mov ebp,esp
%endmacro
</pre>
<p>to define an alternative form of the function prologue which allocates
no local stack space.</p>
<p>Sometimes, however, you might want to `overload' a machine instruction;
for example, you might want to define</p>
<pre>
%macro push 2
push %1
push %2
%endmacro
</pre>
<p>so that you could code</p>
<pre>
push ebx ; this line is not a macro call
push eax,ecx ; but this one is
</pre>
<p>Ordinarily, NASM will give a warning for the first of the above two
lines, since <code>push</code> is now defined to be a macro, and is being
invoked with a number of parameters for which no definition has been given.
The correct code will still be generated, but the assembler will give a
warning. This warning can be disabled by the use of the
<code>-w-macro-params</code> command-line option (see
<a href="nasmdoc2.html#section-2.1.25">section 2.1.25</a>).</p>
<h4 id="section-4.3.2">4.3.2 Macro-Local Labels</h4>
<p>NASM allows you to define labels within a multi-line macro definition in
such a way as to make them local to the macro call: so calling the same
macro multiple times will use a different label each time. You do this by
prefixing <code>%%</code> to the label name. So you can invent an
instruction which executes a <code>RET</code> if the <code>Z</code> flag is
set by doing this:</p>
<pre>
%macro retz 0
jnz %%skip
ret
%%skip:
%endmacro
</pre>
<p>You can call this macro as many times as you want, and every time you
call it NASM will make up a different `real' name to substitute for the
label <code>%%skip</code>. The names NASM invents are of the form
<code>..@2345.skip</code>, where the number 2345 changes with every macro
call. The <code>..@</code> prefix prevents macro-local labels from
interfering with the local label mechanism, as described in
<a href="nasmdoc3.html#section-3.9">section 3.9</a>. You should avoid
defining your own labels in this form (the <code>..@</code> prefix, then a
number, then another period) in case they interfere with macro-local
labels.</p>
<h4 id="section-4.3.3">4.3.3 Greedy Macro Parameters</h4>
<p>Occasionally it is useful to define a macro which lumps its entire
command line into one parameter definition, possibly after extracting one
or two smaller parameters from the front. An example might be a macro to
write a text string to a file in MS-DOS, where you might want to be able to
write</p>
<pre>
writefile [filehandle],"hello, world",13,10
</pre>
<p>NASM allows you to define the last parameter of a macro to be
<em>greedy</em>, meaning that if you invoke the macro with more parameters
than it expects, all the spare parameters get lumped into the last defined
one along with the separating commas. So if you code:</p>
<pre>
%macro writefile 2+
jmp %%endstr
%%str: db %2
%%endstr:
mov dx,%%str
mov cx,%%endstr-%%str
mov bx,%1
mov ah,0x40
int 0x21
%endmacro
</pre>
<p>then the example call to <code>writefile</code> above will work as
expected: the text before the first comma, <code>[filehandle]</code>, is
used as the first macro parameter and expanded when <code>%1</code> is
referred to, and all the subsequent text is lumped into <code>%2</code> and
placed after the <code>db</code>.</p>
<p>The greedy nature of the macro is indicated to NASM by the use of the
<code>+</code> sign after the parameter count on the <code>%macro</code>
line.</p>
<p>If you define a greedy macro, you are effectively telling NASM how it
should expand the macro given <em>any</em> number of parameters from the
actual number specified up to infinity; in this case, for example, NASM now
knows what to do when it sees a call to <code>writefile</code> with 2, 3, 4
or more parameters. NASM will take this into account when overloading
macros, and will not allow you to define another form of
<code>writefile</code> taking 4 parameters (for example).</p>
<p>Of course, the above macro could have been implemented as a non-greedy
macro, in which case the call to it would have had to look like</p>
<pre>
writefile [filehandle], {"hello, world",13,10}
</pre>
<p>NASM provides both mechanisms for putting commas in macro parameters,
and you choose which one you prefer for each macro definition.</p>
<p>See <a href="nasmdoc6.html#section-6.3.1">section 6.3.1</a> for a better
way to write the above macro.</p>
<h4 id="section-4.3.4">4.3.4 Macro Parameters Range</h4>
<p>NASM allows you to expand parameters via special construction
<code>%{x:y}</code> where <code>x</code> is the first parameter index and
<code>y</code> is the last. Any index can be either negative or positive
but must never be zero.</p>
<p>For example</p>
<pre>
%macro mpar 1-*
db %{3:5}
%endmacro
mpar 1,2,3,4,5,6
</pre>
<p>expands to <code>3,4,5</code> range.</p>
<p>Even more, the parameters can be reversed so that</p>
<pre>
%macro mpar 1-*
db %{5:3}
%endmacro
mpar 1,2,3,4,5,6
</pre>
<p>expands to <code>5,4,3</code> range.</p>
<p>But even this is not the last. The parameters can be addressed via
negative indices so NASM will count them reversed. The ones who know Python
may see the analogue here.</p>
<pre>
%macro mpar 1-*
db %{-1:-3}
%endmacro
mpar 1,2,3,4,5,6
</pre>
<p>expands to <code>6,5,4</code> range.</p>
<p>Note that NASM uses comma to separate parameters being expanded.</p>
<p>By the way, here is a trick – you might use the index
<code>%{-1:-1</code>} which gives you the last argument passed to a macro.</p>
<h4 id="section-4.3.5">4.3.5 Default Macro Parameters</h4>
<p>NASM also allows you to define a multi-line macro with a <em>range</em>
of allowable parameter counts. If you do this, you can specify defaults for
omitted parameters. So, for example:</p>
<pre>
%macro die 0-1 "Painful program death has occurred."
writefile 2,%1
mov ax,0x4c01
int 0x21
%endmacro
</pre>
<p>This macro (which makes use of the <code>writefile</code> macro defined
in <a href="#section-4.3.3">section 4.3.3</a>) can be called with an
explicit error message, which it will display on the error output stream
before exiting, or it can be called with no parameters, in which case it
will use the default error message supplied in the macro definition.</p>
<p>In general, you supply a minimum and maximum number of parameters for a
macro of this type; the minimum number of parameters are then required in
the macro call, and then you provide defaults for the optional ones. So if
a macro definition began with the line</p>
<pre>
%macro foobar 1-3 eax,[ebx+2]
</pre>
<p>then it could be called with between one and three parameters, and
<code>%1</code> would always be taken from the macro call. <code>%2</code>,
if not specified by the macro call, would default to <code>eax</code>, and
<code>%3</code> if not specified would default to <code>[ebx+2]</code>.</p>
<p>You can provide extra information to a macro by providing too many
default parameters:</p>
<pre>
%macro quux 1 something
</pre>
<p>This will trigger a warning by default; see
<a href="nasmdoc2.html#section-2.1.25">section 2.1.25</a> for more
information. When <code>quux</code> is invoked, it receives not one but two
parameters. <code>something</code> can be referred to as <code>%2</code>.
The difference between passing <code>something</code> this way and writing
<code>something</code> in the macro body is that with this way
<code>something</code> is evaluated when the macro is defined, not when it
is expanded.</p>
<p>You may omit parameter defaults from the macro definition, in which case
the parameter default is taken to be blank. This can be useful for macros
which can take a variable number of parameters, since the <code>%0</code>
token (see <a href="#section-4.3.6">section 4.3.6</a>) allows you to
determine how many parameters were really passed to the macro call.</p>
<p>This defaulting mechanism can be combined with the greedy-parameter
mechanism; so the <code>die</code> macro above could be made more powerful,
and more useful, by changing the first line of the definition to</p>
<pre>
%macro die 0-1+ "Painful program death has occurred.",13,10
</pre>
<p>The maximum parameter count can be infinite, denoted by <code>*</code>.
In this case, of course, it is impossible to provide a <em>full</em> set of
default parameters. Examples of this usage are shown in
<a href="#section-4.3.8">section 4.3.8</a>.</p>
<h4 id="section-4.3.6">4.3.6 <code>%0</code>: Macro Parameter Counter</h4>
<p>The parameter reference <code>%0</code> will return a numeric constant
giving the number of parameters received, that is, if <code>%0</code> is n
then <code>%</code>n is the last parameter. <code>%0</code> is mostly
useful for macros that can take a variable number of parameters. It can be
used as an argument to <code>%rep</code> (see
<a href="#section-4.5">section 4.5</a>) in order to iterate through all the
parameters of a macro. Examples are given in
<a href="#section-4.3.8">section 4.3.8</a>.</p>
<h4 id="section-4.3.7">4.3.7 <code>%00</code>: Label Preceeding Macro</h4>
<p><code>%00</code> will return the label preceeding the macro invocation,
if any. The label must be on the same line as the macro invocation, may be
a local label (see <a href="nasmdoc3.html#section-3.9">section 3.9</a>),
and need not end in a colon.</p>
<h4 id="section-4.3.8">4.3.8 <code>%rotate</code>: Rotating Macro Parameters</h4>
<p>Unix shell programmers will be familiar with the <code>shift</code>
shell command, which allows the arguments passed to a shell script
(referenced as <code>$1</code>, <code>$2</code> and so on) to be moved left
by one place, so that the argument previously referenced as <code>$2</code>
becomes available as <code>$1</code>, and the argument previously
referenced as <code>$1</code> is no longer available at all.</p>
<p>NASM provides a similar mechanism, in the form of <code>%rotate</code>.
As its name suggests, it differs from the Unix <code>shift</code> in that
no parameters are lost: parameters rotated off the left end of the argument
list reappear on the right, and vice versa.</p>
<p><code>%rotate</code> is invoked with a single numeric argument (which
may be an expression). The macro parameters are rotated to the left by that
many places. If the argument to <code>%rotate</code> is negative, the macro
parameters are rotated to the right.</p>
<p>So a pair of macros to save and restore a set of registers might work as
follows:</p>
<pre>
%macro multipush 1-*
%rep %0
push %1
%rotate 1
%endrep
%endmacro
</pre>
<p>This macro invokes the <code>PUSH</code> instruction on each of its
arguments in turn, from left to right. It begins by pushing its first
argument, <code>%1</code>, then invokes <code>%rotate</code> to move all
the arguments one place to the left, so that the original second argument
is now available as <code>%1</code>. Repeating this procedure as many times
as there were arguments (achieved by supplying <code>%0</code> as the
argument to <code>%rep</code>) causes each argument in turn to be pushed.</p>
<p>Note also the use of <code>*</code> as the maximum parameter count,
indicating that there is no upper limit on the number of parameters you may
supply to the <code>multipush</code> macro.</p>
<p>It would be convenient, when using this macro, to have a
<code>POP</code> equivalent, which <em>didn't</em> require the arguments to
be given in reverse order. Ideally, you would write the
<code>multipush</code> macro call, then cut-and-paste the line to where the
pop needed to be done, and change the name of the called macro to
<code>multipop</code>, and the macro would take care of popping the
registers in the opposite order from the one in which they were pushed.</p>
<p>This can be done by the following definition:</p>
<pre>
%macro multipop 1-*
%rep %0
%rotate -1
pop %1
%endrep
%endmacro
</pre>
<p>This macro begins by rotating its arguments one place to the
<em>right</em>, so that the original <em>last</em> argument appears as
<code>%1</code>. This is then popped, and the arguments are rotated right
again, so the second-to-last argument becomes <code>%1</code>. Thus the
arguments are iterated through in reverse order.</p>
<h4 id="section-4.3.9">4.3.9 Concatenating Macro Parameters</h4>
<p>NASM can concatenate macro parameters and macro indirection constructs
on to other text surrounding them. This allows you to declare a family of
symbols, for example, in a macro definition. If, for example, you wanted to
generate a table of key codes along with offsets into the table, you could
code something like</p>
<pre>
%macro keytab_entry 2
keypos%1 equ $-keytab
db %2
%endmacro
keytab:
keytab_entry F1,128+1
keytab_entry F2,128+2
keytab_entry Return,13
</pre>
<p>which would expand to</p>
<pre>
keytab:
keyposF1 equ $-keytab
db 128+1
keyposF2 equ $-keytab
db 128+2
keyposReturn equ $-keytab
db 13
</pre>
<p>You can just as easily concatenate text on to the other end of a macro
parameter, by writing <code>%1foo</code>.</p>
<p>If you need to append a <em>digit</em> to a macro parameter, for example
defining labels <code>foo1</code> and <code>foo2</code> when passed the
parameter <code>foo</code>, you can't code <code>%11</code> because that
would be taken as the eleventh macro parameter. Instead, you must code
<code>%{1}1</code>, which will separate the first <code>1</code> (giving
the number of the macro parameter) from the second (literal text to be
concatenated to the parameter).</p>
<p>This concatenation can also be applied to other preprocessor in-line
objects, such as macro-local labels (<a href="#section-4.3.2">section
4.3.2</a>) and context-local labels (<a href="#section-4.7.2">section
4.7.2</a>). In all cases, ambiguities in syntax can be resolved by
enclosing everything after the <code>%</code> sign and before the literal
text in braces: so <code>%{%foo}bar</code> concatenates the text
<code>bar</code> to the end of the real name of the macro-local label
<code>%%foo</code>. (This is unnecessary, since the form NASM uses for the
real names of macro-local labels means that the two usages
<code>%{%foo}bar</code> and <code>%%foobar</code> would both expand to the
same thing anyway; nevertheless, the capability is there.)</p>
<p>The single-line macro indirection construct, <code>%[...]</code>
(<a href="#section-4.1.3">section 4.1.3</a>), behaves the same way as macro
parameters for the purpose of concatenation.</p>
<p>See also the <code>%+</code> operator, <a href="#section-4.1.4">section
4.1.4</a>.</p>
<h4 id="section-4.3.10">4.3.10 Condition Codes as Macro Parameters</h4>
<p>NASM can give special treatment to a macro parameter which contains a
condition code. For a start, you can refer to the macro parameter
<code>%1</code> by means of the alternative syntax <code>%+1</code>, which
informs NASM that this macro parameter is supposed to contain a condition
code, and will cause the preprocessor to report an error message if the
macro is called with a parameter which is <em>not</em> a valid condition
code.</p>
<p>Far more usefully, though, you can refer to the macro parameter by means
of <code>%-1</code>, which NASM will expand as the <em>inverse</em>
condition code. So the <code>retz</code> macro defined in
<a href="#section-4.3.2">section 4.3.2</a> can be replaced by a general
conditional-return macro like this:</p>
<pre>
%macro retc 1
j%-1 %%skip
ret
%%skip:
%endmacro
</pre>
<p>This macro can now be invoked using calls like <code>retc ne</code>,
which will cause the conditional-jump instruction in the macro expansion to
come out as <code>JE</code>, or <code>retc po</code> which will make the
jump a <code>JPE</code>.</p>
<p>The <code>%+1</code> macro-parameter reference is quite happy to
interpret the arguments <code>CXZ</code> and <code>ECXZ</code> as valid
condition codes; however, <code>%-1</code> will report an error if passed
either of these, because no inverse condition code exists.</p>
<h4 id="section-4.3.11">4.3.11 Disabling Listing Expansion</h4>
<p>When NASM is generating a listing file from your program, it will
generally expand multi-line macros by means of writing the macro call and
then listing each line of the expansion. This allows you to see which
instructions in the macro expansion are generating what code; however, for
some macros this clutters the listing up unnecessarily.</p>
<p>NASM therefore provides the <code>.nolist</code> qualifier, which you
can include in a macro definition to inhibit the expansion of the macro in
the listing file. The <code>.nolist</code> qualifier comes directly after
the number of parameters, like this:</p>
<pre>
%macro foo 1.nolist
</pre>
<p>Or like this:</p>
<pre>
%macro bar 1-5+.nolist a,b,c,d,e,f,g,h
</pre>
<h4 id="section-4.3.12">4.3.12 Undefining Multi-Line Macros: <code>%unmacro</code></h4>
<p>Multi-line macros can be removed with the <code>%unmacro</code>
directive. Unlike the <code>%undef</code> directive, however,
<code>%unmacro</code> takes an argument specification, and will only remove
exact matches with that argument specification.</p>
<p>For example:</p>
<pre>
%macro foo 1-3
; Do something
%endmacro
%unmacro foo 1-3
</pre>
<p>removes the previously defined macro <code>foo</code>, but</p>
<pre>
%macro bar 1-3
; Do something
%endmacro
%unmacro bar 1
</pre>
<p>does <em>not</em> remove the macro <code>bar</code>, since the argument
specification does not match exactly.</p>
<h3 id="section-4.4">4.4 Conditional Assembly</h3>
<p>Similarly to the C preprocessor, NASM allows sections of a source file
to be assembled only if certain conditions are met. The general syntax of
this feature looks like this:</p>
<pre>
%if<condition>
; some code which only appears if <condition> is met
%elif<condition2>
; only appears if <condition> is not met but <condition2> is
%else
; this appears if neither <condition> nor <condition2> was met
%endif
</pre>
<p>The inverse forms <code>%ifn</code> and <code>%elifn</code> are also
supported.</p>
<p>The <code>%else</code> clause is optional, as is the <code>%elif</code>
clause. You can have more than one <code>%elif</code> clause as well.</p>
<p>There are a number of variants of the <code>%if</code> directive. Each
has its corresponding <code>%elif</code>, <code>%ifn</code>, and
<code>%elifn</code> directives; for example, the equivalents to the
<code>%ifdef</code> directive are <code>%elifdef</code>,
<code>%ifndef</code>, and <code>%elifndef</code>.</p>
<h4 id="section-4.4.1">4.4.1 <code>%ifdef</code>: Testing Single-Line Macro Existence</h4>
<p>Beginning a conditional-assembly block with the line
<code>%ifdef MACRO</code> will assemble the subsequent code if, and only
if, a single-line macro called <code>MACRO</code> is defined. If not, then
the <code>%elif</code> and <code>%else</code> blocks (if any) will be
processed instead.</p>
<p>For example, when debugging a program, you might want to write code such
as</p>
<pre>
; perform some function
%ifdef DEBUG
writefile 2,"Function performed successfully",13,10
%endif
; go and do something else
</pre>
<p>Then you could use the command-line option <code>-dDEBUG</code> to
create a version of the program which produced debugging messages, and
remove the option to generate the final release version of the program.</p>
<p>You can test for a macro <em>not</em> being defined by using
<code>%ifndef</code> instead of <code>%ifdef</code>. You can also test for
macro definitions in <code>%elif</code> blocks by using
<code>%elifdef</code> and <code>%elifndef</code>.</p>
<h4 id="section-4.4.2">4.4.2 <code>%ifmacro</code>: Testing Multi-Line Macro Existence</h4>
<p>The <code>%ifmacro</code> directive operates in the same way as the
<code>%ifdef</code> directive, except that it checks for the existence of a
multi-line macro.</p>
<p>For example, you may be working with a large project and not have
control over the macros in a library. You may want to create a macro with
one name if it doesn't already exist, and another name if one with that
name does exist.</p>
<p>The <code>%ifmacro</code> is considered true if defining a macro with
the given name and number of arguments would cause a definitions conflict.
For example:</p>
<pre>
%ifmacro MyMacro 1-3
%error "MyMacro 1-3" causes a conflict with an existing macro.
%else
%macro MyMacro 1-3
; insert code to define the macro
%endmacro
%endif
</pre>
<p>This will create the macro "MyMacro 1-3" if no macro already exists
which would conflict with it, and emits a warning if there would be a
definition conflict.</p>
<p>You can test for the macro not existing by using the
<code>%ifnmacro</code> instead of <code>%ifmacro</code>. Additional tests
can be performed in <code>%elif</code> blocks by using
<code>%elifmacro</code> and <code>%elifnmacro</code>.</p>
<h4 id="section-4.4.3">4.4.3 <code>%ifctx</code>: Testing the Context Stack</h4>
<p>The conditional-assembly construct <code>%ifctx</code> will cause the
subsequent code to be assembled if and only if the top context on the
preprocessor's context stack has the same name as one of the arguments. As
with <code>%ifdef</code>, the inverse and <code>%elif</code> forms
<code>%ifnctx</code>, <code>%elifctx</code> and <code>%elifnctx</code> are
also supported.</p>
<p>For more details of the context stack, see
<a href="#section-4.7">section 4.7</a>. For a sample use of
<code>%ifctx</code>, see <a href="#section-4.7.6">section 4.7.6</a>.</p>
<h4 id="section-4.4.4">4.4.4 <code>%if</code>: Testing Arbitrary Numeric Expressions</h4>
<p>The conditional-assembly construct <code>%if expr</code> will cause the
subsequent code to be assembled if and only if the value of the numeric
expression <code>expr</code> is non-zero. An example of the use of this
feature is in deciding when to break out of a <code>%rep</code>
preprocessor loop: see <a href="#section-4.5">section 4.5</a> for a
detailed example.</p>
<p>The expression given to <code>%if</code>, and its counterpart
<code>%elif</code>, is a critical expression (see
<a href="nasmdoc3.html#section-3.8">section 3.8</a>).</p>
<p><code>%if</code> extends the normal NASM expression syntax, by providing
a set of relational operators which are not normally available in
expressions. The operators <code>=</code>, <code><</code>,
<code>></code>, <code><=</code>, <code>>=</code> and
<code><></code> test equality, less-than, greater-than,
less-or-equal, greater-or-equal and not-equal respectively. The C-like
forms <code>==</code> and <code>!=</code> are supported as alternative
forms of <code>=</code> and <code><></code>. In addition,
low-priority logical operators <code>&&</code>, <code>^^</code> and
<code>||</code> are provided, supplying logical AND, logical XOR and
logical OR. These work like the C logical operators (although C has no
logical XOR), in that they always return either 0 or 1, and treat any
non-zero input as 1 (so that <code>^^</code>, for example, returns 1 if
exactly one of its inputs is zero, and 0 otherwise). The relational
operators also return 1 for true and 0 for false.</p>
<p>Like other <code>%if</code> constructs, <code>%if</code> has a
counterpart <code>%elif</code>, and negative forms <code>%ifn</code> and
<code>%elifn</code>.</p>
<h4 id="section-4.4.5">4.4.5 <code>%ifidn</code> and <code>%ifidni</code>: Testing Exact Text Identity</h4>
<p>The construct <code>%ifidn text1,text2</code> will cause the subsequent
code to be assembled if and only if <code>text1</code> and
<code>text2</code>, after expanding single-line macros, are identical
pieces of text. Differences in white space are not counted.</p>
<p><code>%ifidni</code> is similar to <code>%ifidn</code>, but is
case-insensitive.</p>
<p>For example, the following macro pushes a register or number on the
stack, and allows you to treat <code>IP</code> as a real register:</p>
<pre>
%macro pushparam 1
%ifidni %1,ip
call %%label
%%label:
%else
push %1
%endif
%endmacro
</pre>
<p>Like other <code>%if</code> constructs, <code>%ifidn</code> has a
counterpart <code>%elifidn</code>, and negative forms <code>%ifnidn</code>
and <code>%elifnidn</code>. Similarly, <code>%ifidni</code> has
counterparts <code>%elifidni</code>, <code>%ifnidni</code> and
<code>%elifnidni</code>.</p>
<h4 id="section-4.4.6">4.4.6 <code>%ifid</code>, <code>%ifnum</code>, <code>%ifstr</code>: Testing Token Types</h4>
<p>Some macros will want to perform different tasks depending on whether
they are passed a number, a string, or an identifier. For example, a string
output macro might want to be able to cope with being passed either a
string constant or a pointer to an existing string.</p>
<p>The conditional assembly construct <code>%ifid</code>, taking one
parameter (which may be blank), assembles the subsequent code if and only
if the first token in the parameter exists and is an identifier.
<code>%ifnum</code> works similarly, but tests for the token being a
numeric constant; <code>%ifstr</code> tests for it being a string.</p>
<p>For example, the <code>writefile</code> macro defined in
<a href="#section-4.3.3">section 4.3.3</a> can be extended to take
advantage of <code>%ifstr</code> in the following fashion:</p>
<pre>
%macro writefile 2-3+
%ifstr %2
jmp %%endstr
%if %0 = 3
%%str: db %2,%3
%else
%%str: db %2
%endif
%%endstr: mov dx,%%str
mov cx,%%endstr-%%str
%else
mov dx,%2
mov cx,%3
%endif
mov bx,%1
mov ah,0x40
int 0x21
%endmacro
</pre>
<p>Then the <code>writefile</code> macro can cope with being called in
either of the following two ways:</p>
<pre>
writefile [file], strpointer, length
writefile [file], "hello", 13, 10
</pre>
<p>In the first, <code>strpointer</code> is used as the address of an
already-declared string, and <code>length</code> is used as its length; in
the second, a string is given to the macro, which therefore declares it
itself and works out the address and length for itself.</p>
<p>Note the use of <code>%if</code> inside the <code>%ifstr</code>: this is
to detect whether the macro was passed two arguments (so the string would
be a single string constant, and <code>db %2</code> would be adequate) or
more (in which case, all but the first two would be lumped together into
<code>%3</code>, and <code>db %2,%3</code> would be required).</p>
<p>The usual <code>%elif</code>..., <code>%ifn</code>..., and
<code>%elifn</code>... versions exist for each of <code>%ifid</code>,
<code>%ifnum</code> and <code>%ifstr</code>.</p>
<h4 id="section-4.4.7">4.4.7 <code>%iftoken</code>: Test for a Single Token</h4>
<p>Some macros will want to do different things depending on if it is
passed a single token (e.g. paste it to something else using
<code>%+</code>) versus a multi-token sequence.</p>
<p>The conditional assembly construct <code>%iftoken</code> assembles the
subsequent code if and only if the expanded parameters consist of exactly
one token, possibly surrounded by whitespace.</p>
<p>For example:</p>
<pre>
%iftoken 1
</pre>
<p>will assemble the subsequent code, but</p>
<pre>
%iftoken -1
</pre>
<p>will not, since <code>-1</code> contains two tokens: the unary minus
operator <code>-</code>, and the number <code>1</code>.</p>
<p>The usual <code>%eliftoken</code>, <code>%ifntoken</code>, and
<code>%elifntoken</code> variants are also provided.</p>
<h4 id="section-4.4.8">4.4.8 <code>%ifempty</code>: Test for Empty Expansion</h4>
<p>The conditional assembly construct <code>%ifempty</code> assembles the
subsequent code if and only if the expanded parameters do not contain any
tokens at all, whitespace excepted.</p>
<p>The usual <code>%elifempty</code>, <code>%ifnempty</code>, and
<code>%elifnempty</code> variants are also provided.</p>
<h4 id="section-4.4.9">4.4.9 <code>%ifenv</code>: Test If Environment Variable Exists</h4>
<p>The conditional assembly construct <code>%ifenv</code> assembles the
subsequent code if and only if the environment variable referenced by the
<code>%!</code><em>variable</em> directive exists.</p>
<p>The usual <code>%elifenv</code>, <code>%ifnenv</code>, and
<code>%elifnenv</code> variants are also provided.</p>
<p>Just as for <code>%!</code><em>variable</em> the argument should be
written as a string if it contains characters that would not be legal in an
identifier. See <a href="#section-4.10.2">section 4.10.2</a>.</p>
<h3 id="section-4.5">4.5 Preprocessor Loops: <code>%rep</code></h3>
<p>NASM's <code>TIMES</code> prefix, though useful, cannot be used to
invoke a multi-line macro multiple times, because it is processed by NASM
after macros have already been expanded. Therefore NASM provides another
form of loop, this time at the preprocessor level: <code>%rep</code>.</p>
<p>The directives <code>%rep</code> and <code>%endrep</code>
(<code>%rep</code> takes a numeric argument, which can be an expression;
<code>%endrep</code> takes no arguments) can be used to enclose a chunk of
code, which is then replicated as many times as specified by the
preprocessor:</p>
<pre>
%assign i 0
%rep 64
inc word [table+2*i]
%assign i i+1
%endrep
</pre>
<p>This will generate a sequence of 64 <code>INC</code> instructions,
incrementing every word of memory from <code>[table]</code> to
<code>[table+126]</code>.</p>
<p>For more complex termination conditions, or to break out of a repeat
loop part way along, you can use the <code>%exitrep</code> directive to
terminate the loop, like this:</p>
<pre>
fibonacci:
%assign i 0
%assign j 1
%rep 100
%if j > 65535
%exitrep
%endif
dw j
%assign k j+i
%assign i j
%assign j k
%endrep
fib_number equ ($-fibonacci)/2
</pre>
<p>This produces a list of all the Fibonacci numbers that will fit in 16
bits. Note that a maximum repeat count must still be given to
<code>%rep</code>. This is to prevent the possibility of NASM getting into
an infinite loop in the preprocessor, which (on multitasking or multi-user
systems) would typically cause all the system memory to be gradually used
up and other applications to start crashing.</p>
<p>Note a maximum repeat count is limited by 62 bit number, though it is
hardly possible that you ever need anything bigger.</p>
<h3 id="section-4.6">4.6 Source Files and Dependencies</h3>
<p>These commands allow you to split your sources into multiple files.</p>
<h4 id="section-4.6.1">4.6.1 <code>%include</code>: Including Other Files</h4>
<p>Using, once again, a very similar syntax to the C preprocessor, NASM's
preprocessor lets you include other source files into your code. This is
done by the use of the <code>%include</code> directive:</p>
<pre>
%include "macros.mac"
</pre>
<p>will include the contents of the file <code>macros.mac</code> into the
source file containing the <code>%include</code> directive.</p>
<p>Include files are searched for in the current directory (the directory
you're in when you run NASM, as opposed to the location of the NASM
executable or the location of the source file), plus any directories
specified on the NASM command line using the <code>-i</code> option.</p>
<p>The standard C idiom for preventing a file being included more than once
is just as applicable in NASM: if the file <code>macros.mac</code> has the
form</p>
<pre>
%ifndef MACROS_MAC
%define MACROS_MAC
; now define some macros
%endif
</pre>
<p>then including the file more than once will not cause errors, because
the second time the file is included nothing will happen because the macro
<code>MACROS_MAC</code> will already be defined.</p>
<p>You can force a file to be included even if there is no
<code>%include</code> directive that explicitly includes it, by using the
<code>-p</code> option on the NASM command line (see
<a href="nasmdoc2.html#section-2.1.18">section 2.1.18</a>).</p>
<h4 id="section-4.6.2">4.6.2 <code>%pathsearch</code>: Search the Include Path</h4>
<p>The <code>%pathsearch</code> directive takes a single-line macro name
and a filename, and declare or redefines the specified single-line macro to
be the include-path-resolved version of the filename, if the file exists
(otherwise, it is passed unchanged.)</p>
<p>For example,</p>
<pre>
%pathsearch MyFoo "foo.bin"
</pre>
<p>... with <code>-Ibins/</code> in the include path may end up defining
the macro <code>MyFoo</code> to be <code>"bins/foo.bin"</code>.</p>
<h4 id="section-4.6.3">4.6.3 <code>%depend</code>: Add Dependent Files</h4>
<p>The <code>%depend</code> directive takes a filename and adds it to the
list of files to be emitted as dependency generation when the
<code>-M</code> options and its relatives (see
<a href="nasmdoc2.html#section-2.1.4">section 2.1.4</a>) are used. It
produces no output.</p>
<p>This is generally used in conjunction with <code>%pathsearch</code>. For
example, a simplified version of the standard macro wrapper for the
<code>INCBIN</code> directive looks like:</p>
<pre>
%imacro incbin 1-2+ 0
%pathsearch dep %1
%depend dep
incbin dep,%2
%endmacro
</pre>
<p>This first resolves the location of the file into the macro
<code>dep</code>, then adds it to the dependency lists, and finally issues
the assembler-level <code>INCBIN</code> directive.</p>
<h4 id="section-4.6.4">4.6.4 <code>%use</code>: Include Standard Macro Package</h4>
<p>The <code>%use</code> directive is similar to <code>%include</code>, but
rather than including the contents of a file, it includes a named standard
macro package. The standard macro packages are part of NASM, and are
described in <a href="nasmdoc5.html">chapter 5</a>.</p>
<p>Unlike the <code>%include</code> directive, package names for the
<code>%use</code> directive do not require quotes, but quotes are
permitted. In NASM 2.04 and 2.05 the unquoted form would be macro-expanded;
this is no longer true. Thus, the following lines are equivalent:</p>
<pre>
%use altreg
%use 'altreg'
</pre>
<p>Standard macro packages are protected from multiple inclusion. When a
standard macro package is used, a testable single-line macro of the form
<code>__USE_</code><em>package</em><code>__</code> is also defined, see
<a href="#section-4.11.9">section 4.11.9</a>.</p>
<h3 id="section-4.7">4.7 The Context Stack</h3>
<p>Having labels that are local to a macro definition is sometimes not
quite powerful enough: sometimes you want to be able to share labels
between several macro calls. An example might be a <code>REPEAT</code> ...
<code>UNTIL</code> loop, in which the expansion of the <code>REPEAT</code>
macro would need to be able to refer to a label which the
<code>UNTIL</code> macro had defined. However, for such a macro you would
also want to be able to nest these loops.</p>
<p>NASM provides this level of power by means of a <em>context stack</em>.
The preprocessor maintains a stack of <em>contexts</em>, each of which is
characterized by a name. You add a new context to the stack using the
<code>%push</code> directive, and remove one using <code>%pop</code>. You
can define labels that are local to a particular context on the stack.</p>
<h4 id="section-4.7.1">4.7.1 <code>%push</code> and <code>%pop</code>: Creating and Removing Contexts</h4>
<p>The <code>%push</code> directive is used to create a new context and
place it on the top of the context stack. <code>%push</code> takes an
optional argument, which is the name of the context. For example:</p>
<pre>
%push foobar
</pre>
<p>This pushes a new context called <code>foobar</code> on the stack. You
can have several contexts on the stack with the same name: they can still
be distinguished. If no name is given, the context is unnamed (this is
normally used when both the <code>%push</code> and the <code>%pop</code>
are inside a single macro definition.)</p>
<p>The directive <code>%pop</code>, taking one optional argument, removes
the top context from the context stack and destroys it, along with any
labels associated with it. If an argument is given, it must match the name
of the current context, otherwise it will issue an error.</p>
<h4 id="section-4.7.2">4.7.2 Context-Local Labels</h4>
<p>Just as the usage <code>%%foo</code> defines a label which is local to
the particular macro call in which it is used, the usage <code>%$foo</code>
is used to define a label which is local to the context on the top of the
context stack. So the <code>REPEAT</code> and <code>UNTIL</code> example
given above could be implemented by means of:</p>
<pre>
%macro repeat 0
%push repeat
%$begin:
%endmacro
%macro until 1
j%-1 %$begin
%pop
%endmacro
</pre>
<p>and invoked by means of, for example,</p>
<pre>
mov cx,string
repeat
add cx,3
scasb
until e
</pre>
<p>which would scan every fourth byte of a string in search of the byte in
<code>AL</code>.</p>
<p>If you need to define, or access, labels local to the context
<em>below</em> the top one on the stack, you can use <code>%$$foo</code>,
or <code>%$$$foo</code> for the context below that, and so on.</p>
<h4 id="section-4.7.3">4.7.3 Context-Local Single-Line Macros</h4>
<p>NASM also allows you to define single-line macros which are local to a
particular context, in just the same way:</p>
<pre>
%define %$localmac 3
</pre>
<p>will define the single-line macro <code>%$localmac</code> to be local to
the top context on the stack. Of course, after a subsequent
<code>%push</code>, it can then still be accessed by the name
<code>%$$localmac</code>.</p>
<h4 id="section-4.7.4">4.7.4 Context Fall-Through Lookup <em>(deprecated)</em></h4>
<p>Context fall-through lookup (automatic searching of outer contexts) is a
feature that was added in NASM version 0.98.03. Unfortunately, this feature
is unintuitive and can result in buggy code that would have otherwise been
prevented by NASM's error reporting. As a result, this feature has been
<em>deprecated</em>. NASM version 2.09 will issue a warning when usage of
this <em>deprecated</em> feature is detected. Starting with NASM version
2.10, usage of this <em>deprecated</em> feature will simply result in an
<em>expression syntax error</em>.</p>
<p>An example usage of this <em>deprecated</em> feature follows:</p>
<pre>
%macro ctxthru 0
%push ctx1
%assign %$external 1
%push ctx2
%assign %$internal 1
mov eax, %$external
mov eax, %$internal
%pop
%pop
%endmacro
</pre>
<p>As demonstrated, <code>%$external</code> is being defined in the
<code>ctx1</code> context and referenced within the <code>ctx2</code>
context. With context fall-through lookup, referencing an undefined
context-local macro like this implicitly searches through all outer
contexts until a match is made or isn't found in any context. As a result,
<code>%$external</code> referenced within the <code>ctx2</code> context
would implicitly use <code>%$external</code> as defined in
<code>ctx1</code>. Most people would expect NASM to issue an error in this
situation because <code>%$external</code> was never defined within
<code>ctx2</code> and also isn't qualified with the proper context depth,
<code>%$$external</code>.</p>
<p>Here is a revision of the above example with proper context depth:</p>
<pre>
%macro ctxthru 0
%push ctx1
%assign %$external 1
%push ctx2
%assign %$internal 1
mov eax, %$$external
mov eax, %$internal
%pop
%pop
%endmacro
</pre>
<p>As demonstrated, <code>%$external</code> is still being defined in the
<code>ctx1</code> context and referenced within the <code>ctx2</code>
context. However, the reference to <code>%$external</code> within
<code>ctx2</code> has been fully qualified with the proper context depth,
<code>%$$external</code>, and thus is no longer ambiguous, unintuitive or
erroneous.</p>
<h4 id="section-4.7.5">4.7.5 <code>%repl</code>: Renaming a Context</h4>
<p>If you need to change the name of the top context on the stack (in
order, for example, to have it respond differently to <code>%ifctx</code>),
you can execute a <code>%pop</code> followed by a <code>%push</code>; but
this will have the side effect of destroying all context-local labels and
macros associated with the context that was just popped.</p>
<p>NASM provides the directive <code>%repl</code>, which <em>replaces</em>
a context with a different name, without touching the associated macros and
labels. So you could replace the destructive code</p>
<pre>
%pop
%push newname
</pre>
<p>with the non-destructive version <code>%repl newname</code>.</p>
<h4 id="section-4.7.6">4.7.6 Example Use of the Context Stack: Block IFs</h4>
<p>This example makes use of almost all the context-stack features,
including the conditional-assembly construct <code>%ifctx</code>, to
implement a block IF statement as a set of macros.</p>
<pre>
%macro if 1
%push if
j%-1 %$ifnot
%endmacro
%macro else 0
%ifctx if
%repl else
jmp %$ifend
%$ifnot:
%else
%error "expected `if' before `else'"
%endif
%endmacro
%macro endif 0
%ifctx if
%$ifnot:
%pop
%elifctx else
%$ifend:
%pop
%else
%error "expected `if' or `else' before `endif'"
%endif
%endmacro
</pre>
<p>This code is more robust than the <code>REPEAT</code> and
<code>UNTIL</code> macros given in <a href="#section-4.7.2">section
4.7.2</a>, because it uses conditional assembly to check that the macros
are issued in the right order (for example, not calling <code>endif</code>
before <code>if</code>) and issues a <code>%error</code> if they're not.</p>
<p>In addition, the <code>endif</code> macro has to be able to cope with
the two distinct cases of either directly following an <code>if</code>, or
following an <code>else</code>. It achieves this, again, by using
conditional assembly to do different things depending on whether the
context on top of the stack is <code>if</code> or <code>else</code>.</p>
<p>The <code>else</code> macro has to preserve the context on the stack, in
order to have the <code>%$ifnot</code> referred to by the <code>if</code>
macro be the same as the one defined by the <code>endif</code> macro, but
has to change the context's name so that <code>endif</code> will know there
was an intervening <code>else</code>. It does this by the use of
<code>%repl</code>.</p>
<p>A sample usage of these macros might look like:</p>
<pre>
cmp ax,bx
if ae
cmp bx,cx
if ae
mov ax,cx
else
mov ax,bx
endif
else
cmp ax,cx
if ae
mov ax,cx
endif
endif
</pre>
<p>The block-<code>IF</code> macros handle nesting quite happily, by means
of pushing another context, describing the inner <code>if</code>, on top of
the one describing the outer <code>if</code>; thus <code>else</code> and
<code>endif</code> always refer to the last unmatched <code>if</code> or
<code>else</code>.</p>
<h3 id="section-4.8">4.8 Stack Relative Preprocessor Directives</h3>
<p>The following preprocessor directives provide a way to use labels to
refer to local variables allocated on the stack.</p>
<ul>
<li>
<p><code>%arg</code> (see <a href="#section-4.8.1">section 4.8.1</a>)</p>
</li>
<li>
<p><code>%stacksize</code> (see <a href="#section-4.8.2">section 4.8.2</a>)</p>
</li>
<li>
<p><code>%local</code> (see <a href="#section-4.8.3">section 4.8.3</a>)</p>
</li>
</ul>
<h4 id="section-4.8.1">4.8.1 <code>%arg</code> Directive</h4>
<p>The <code>%arg</code> directive is used to simplify the handling of
parameters passed on the stack. Stack based parameter passing is used by
many high level languages, including C, C++ and Pascal.</p>
<p>While NASM has macros which attempt to duplicate this functionality (see
<a href="nasmdoc8.html#section-8.4.5">section 8.4.5</a>), the syntax is not
particularly convenient to use and is not TASM compatible. Here is an
example which shows the use of <code>%arg</code> without any external
macros:</p>
<pre>
some_function:
%push mycontext ; save the current context
%stacksize large ; tell NASM to use bp
%arg i:word, j_ptr:word
mov ax,[i]
mov bx,[j_ptr]
add ax,[bx]
ret
%pop ; restore original context
</pre>
<p>This is similar to the procedure defined in
<a href="nasmdoc8.html#section-8.4.5">section 8.4.5</a> and adds the value
in i to the value pointed to by j_ptr and returns the sum in the ax
register. See <a href="#section-4.7.1">section 4.7.1</a> for an explanation
of <code>push</code> and <code>pop</code> and the use of context stacks.</p>
<h4 id="section-4.8.2">4.8.2 <code>%stacksize</code> Directive</h4>
<p>The <code>%stacksize</code> directive is used in conjunction with the
<code>%arg</code> (see <a href="#section-4.8.1">section 4.8.1</a>) and the
<code>%local</code> (see <a href="#section-4.8.3">section 4.8.3</a>)
directives. It tells NASM the default size to use for subsequent
<code>%arg</code> and <code>%local</code> directives. The
<code>%stacksize</code> directive takes one required argument which is one
of <code>flat</code>, <code>flat64</code>, <code>large</code> or
<code>small</code>.</p>
<pre>
%stacksize flat
</pre>
<p>This form causes NASM to use stack-based parameter addressing relative
to <code>ebp</code> and it assumes that a near form of call was used to get
to this label (i.e. that <code>eip</code> is on the stack).</p>
<pre>
%stacksize flat64
</pre>
<p>This form causes NASM to use stack-based parameter addressing relative
to <code>rbp</code> and it assumes that a near form of call was used to get
to this label (i.e. that <code>rip</code> is on the stack).</p>
<pre>
%stacksize large
</pre>
<p>This form uses <code>bp</code> to do stack-based parameter addressing
and assumes that a far form of call was used to get to this address (i.e.
that <code>ip</code> and <code>cs</code> are on the stack).</p>
<pre>
%stacksize small
</pre>
<p>This form also uses <code>bp</code> to address stack parameters, but it
is different from <code>large</code> because it also assumes that the old
value of bp is pushed onto the stack (i.e. it expects an <code>ENTER</code>
instruction). In other words, it expects that <code>bp</code>,
<code>ip</code> and <code>cs</code> are on the top of the stack, underneath
any local space which may have been allocated by <code>ENTER</code>. This
form is probably most useful when used in combination with the
<code>%local</code> directive (see <a href="#section-4.8.3">section
4.8.3</a>).</p>
<h4 id="section-4.8.3">4.8.3 <code>%local</code> Directive</h4>
<p>The <code>%local</code> directive is used to simplify the use of local
temporary stack variables allocated in a stack frame. Automatic local
variables in C are an example of this kind of variable. The
<code>%local</code> directive is most useful when used with the
<code>%stacksize</code> (see <a href="#section-4.8.2">section 4.8.2</a> and
is also compatible with the <code>%arg</code> directive (see
<a href="#section-4.8.1">section 4.8.1</a>). It allows simplified reference
to variables on the stack which have been allocated typically by using the
<code>ENTER</code> instruction. An example of its use is the following:</p>
<pre>
silly_swap:
%push mycontext ; save the current context
%stacksize small ; tell NASM to use bp
%assign %$localsize 0 ; see text for explanation
%local old_ax:word, old_dx:word
enter %$localsize,0 ; see text for explanation
mov [old_ax],ax ; swap ax & bx
mov [old_dx],dx ; and swap dx & cx
mov ax,bx
mov dx,cx
mov bx,[old_ax]
mov cx,[old_dx]
leave ; restore old bp
ret ;
%pop ; restore original context
</pre>
<p>The <code>%$localsize</code> variable is used internally by the
<code>%local</code> directive and <em>must</em> be defined within the
current context before the <code>%local</code> directive may be used.
Failure to do so will result in one expression syntax error for each
<code>%local</code> variable declared. It then may be used in the
construction of an appropriately sized ENTER instruction as shown in the
example.</p>
<h3 id="section-4.9">4.9 Reporting User-Defined Errors: <code>%error</code>, <code>%warning</code>, <code>%fatal</code></h3>
<p>The preprocessor directive <code>%error</code> will cause NASM to report
an error if it occurs in assembled code. So if other users are going to try
to assemble your source files, you can ensure that they define the right
macros by means of code like this:</p>
<pre>
%ifdef F1
; do some setup
%elifdef F2
; do some different setup
%else
%error "Neither F1 nor F2 was defined."
%endif
</pre>
<p>Then any user who fails to understand the way your code is supposed to
be assembled will be quickly warned of their mistake, rather than having to
wait until the program crashes on being run and then not knowing what went
wrong.</p>
<p>Similarly, <code>%warning</code> issues a warning, but allows assembly
to continue:</p>
<pre>
%ifdef F1
; do some setup
%elifdef F2
; do some different setup
%else
%warning "Neither F1 nor F2 was defined, assuming F1."
%define F1
%endif
</pre>
<p><code>%error</code> and <code>%warning</code> are issued only on the
final assembly pass. This makes them safe to use in conjunction with tests
that depend on symbol values.</p>
<p><code>%fatal</code> terminates assembly immediately, regardless of pass.
This is useful when there is no point in continuing the assembly further,
and doing so is likely just going to cause a spew of confusing error
messages.</p>
<p>It is optional for the message string after <code>%error</code>,
<code>%warning</code> or <code>%fatal</code> to be quoted. If it is
<em>not</em>, then single-line macros are expanded in it, which can be used
to display more information to the user. For example:</p>
<pre>
%if foo > 64
%assign foo_over foo-64
%error foo is foo_over bytes too large
%endif
</pre>
<h3 id="section-4.10">4.10 Other Preprocessor Directives</h3>
<h4 id="section-4.10.1">4.10.1 <code>%line</code> Directive</h4>
<p>The <code>%line</code> directive is used to notify NASM that the input
line corresponds to a specific line number in another file. Typically this
other file would be an original source file, with the current NASM input
being the output of a pre-processor. The <code>%line</code> directive
allows NASM to output messages which indicate the line number of the
original source file, instead of the file that is being read by NASM.</p>
<p>This preprocessor directive is not generally used directly by
programmers, but may be of interest to preprocessor authors. The usage of
the <code>%line</code> preprocessor directive is as follows:</p>
<pre>
%line nnn[+mmm] [filename]
</pre>
<p>In this directive, <code>nnn</code> identifies the line of the original
source file which this line corresponds to. <code>mmm</code> is an optional
parameter which specifies a line increment value; each line of the input
file read in is considered to correspond to <code>mmm</code> lines of the
original source file. Finally, <code>filename</code> is an optional
parameter which specifies the file name of the original source file.</p>
<p>After reading a <code>%line</code> preprocessor directive, NASM will
report all file name and line numbers relative to the values specified
therein.</p>
<p>If the command line option <code>--no-line</code> is given, all
<code>%line</code> directives are ignored. This may be useful for debugging
preprocessed code. See <a href="nasmdoc2.html#section-2.1.33">section
2.1.33</a>.</p>
<h4 id="section-4.10.2">4.10.2 <code>%!</code><em>variable</em>: Read an Environment Variable.</h4>
<p>The <code>%!</code><em>variable</em> directive makes it possible to read
the value of an environment variable at assembly time. This could, for
example, be used to store the contents of an environment variable into a
string, which could be used at some other point in your code.</p>
<p>For example, suppose that you have an environment variable
<code>FOO</code>, and you want the contents of <code>FOO</code> to be
embedded in your program as a quoted string. You could do that as follows:</p>
<pre>
%defstr FOO %!FOO
</pre>
<p>See <a href="#section-4.1.8">section 4.1.8</a> for notes on the
<code>%defstr</code> directive.</p>
<p>If the name of the environment variable contains non-identifier
characters, you can use string quotes to surround the name of the variable,
for example:</p>
<pre>
%defstr C_colon %!'C:'
</pre>
<h3 id="section-4.11">4.11 Standard Macros</h3>
<p>NASM defines a set of standard macros, which are already defined when it
starts to process any source file. If you really need a program to be
assembled with no pre-defined macros, you can use the <code>%clear</code>
directive to empty the preprocessor of everything but context-local
preprocessor variables and single-line macros.</p>
<p>Most user-level assembler directives (see
<a href="nasmdoc6.html">chapter 6</a>) are implemented as macros which
invoke primitive directives; these are described in
<a href="nasmdoc6.html">chapter 6</a>. The rest of the standard macro set
is described here.</p>
<h4 id="section-4.11.1">4.11.1 NASM Version Macros</h4>
<p>The single-line macros <code>__NASM_MAJOR__</code>,
<code>__NASM_MINOR__</code>, <code>__NASM_SUBMINOR__</code> and
<code>___NASM_PATCHLEVEL__</code> expand to the major, minor, subminor and
patch level parts of the version number of NASM being used. So, under NASM
0.98.32p1 for example, <code>__NASM_MAJOR__</code> would be defined to be
0, <code>__NASM_MINOR__</code> would be defined as 98,
<code>__NASM_SUBMINOR__</code> would be defined to 32, and
<code>___NASM_PATCHLEVEL__</code> would be defined as 1.</p>
<p>Additionally, the macro <code>__NASM_SNAPSHOT__</code> is defined for
automatically generated snapshot releases <em>only</em>.</p>
<h4 id="section-4.11.2">4.11.2 <code>__NASM_VERSION_ID__</code>: NASM Version ID</h4>
<p>The single-line macro <code>__NASM_VERSION_ID__</code> expands to a
dword integer representing the full version number of the version of nasm
being used. The value is the equivalent to <code>__NASM_MAJOR__</code>,
<code>__NASM_MINOR__</code>, <code>__NASM_SUBMINOR__</code> and
<code>___NASM_PATCHLEVEL__</code> concatenated to produce a single
doubleword. Hence, for 0.98.32p1, the returned number would be equivalent
to:</p>
<pre>
dd 0x00622001
</pre>
<p>or</p>
<pre>
db 1,32,98,0
</pre>
<p>Note that the above lines are generate exactly the same code, the second
line is used just to give an indication of the order that the separate
values will be present in memory.</p>
<h4 id="section-4.11.3">4.11.3 <code>__NASM_VER__</code>: NASM Version string</h4>
<p>The single-line macro <code>__NASM_VER__</code> expands to a string
which defines the version number of nasm being used. So, under NASM 0.98.32
for example,</p>
<pre>
db __NASM_VER__
</pre>
<p>would expand to</p>
<pre>
db "0.98.32"
</pre>
<h4 id="section-4.11.4">4.11.4 <code>__FILE__</code> and <code>__LINE__</code>: File Name and Line Number</h4>
<p>Like the C preprocessor, NASM allows the user to find out the file name
and line number containing the current instruction. The macro
<code>__FILE__</code> expands to a string constant giving the name of the
current input file (which may change through the course of assembly if
<code>%include</code> directives are used), and <code>__LINE__</code>
expands to a numeric constant giving the current line number in the input
file.</p>
<p>These macros could be used, for example, to communicate debugging
information to a macro, since invoking <code>__LINE__</code> inside a macro
definition (either single-line or multi-line) will return the line number
of the macro <em>call</em>, rather than <em>definition</em>. So to
determine where in a piece of code a crash is occurring, for example, one
could write a routine <code>stillhere</code>, which is passed a line number
in <code>EAX</code> and outputs something like `line 155: still here'. You
could then write a macro</p>
<pre>
%macro notdeadyet 0
push eax
mov eax,__LINE__
call stillhere
pop eax
%endmacro
</pre>
<p>and then pepper your code with calls to <code>notdeadyet</code> until
you find the crash point.</p>
<h4 id="section-4.11.5">4.11.5 <code>__BITS__</code>: Current BITS Mode</h4>
<p>The <code>__BITS__</code> standard macro is updated every time that the
BITS mode is set using the <code>BITS XX</code> or <code>[BITS XX]</code>
directive, where XX is a valid mode number of 16, 32 or 64.
<code>__BITS__</code> receives the specified mode number and makes it
globally available. This can be very useful for those who utilize
mode-dependent macros.</p>
<h4 id="section-4.11.6">4.11.6 <code>__OUTPUT_FORMAT__</code>: Current Output Format</h4>
<p>The <code>__OUTPUT_FORMAT__</code> standard macro holds the current
output format name, as given by the <code>-f</code> option or NASM's
default. Type <code>nasm -hf</code> for a list.</p>
<pre>
%ifidn __OUTPUT_FORMAT__, win32
%define NEWLINE 13, 10
%elifidn __OUTPUT_FORMAT__, elf32
%define NEWLINE 10
%endif
</pre>
<h4 id="section-4.11.7">4.11.7 <code>__DEBUG_FORMAT__</code>: Current Debug Format</h4>
<p>If debugging information generation is enabled, The
<code>__DEBUG_FORMAT__</code> standard macro holds the current debug format
name as specified by the <code>-F</code> or <code>-g</code> option or the
output format default. Type <code>nasm -f</code> <em>output</em>
<code>y</code> for a list.</p>
<p><code>__DEBUG_FORMAT__</code> is not defined if debugging is not
enabled, or if the debug format specified is <code>null</code>.</p>
<h4 id="section-4.11.8">4.11.8 Assembly Date and Time Macros</h4>
<p>NASM provides a variety of macros that represent the timestamp of the
assembly session.</p>
<ul>
<li>
<p>The <code>__DATE__</code> and <code>__TIME__</code> macros give the
assembly date and time as strings, in ISO 8601 format
(<code>"YYYY-MM-DD"</code> and <code>"HH:MM:SS"</code>, respectively.)</p>
</li>
<li>
<p>The <code>__DATE_NUM__</code> and <code>__TIME_NUM__</code> macros give
the assembly date and time in numeric form; in the format
<code>YYYYMMDD</code> and <code>HHMMSS</code> respectively.</p>
</li>
<li>
<p>The <code>__UTC_DATE__</code> and <code>__UTC_TIME__</code> macros give
the assembly date and time in universal time (UTC) as strings, in ISO 8601
format (<code>"YYYY-MM-DD"</code> and <code>"HH:MM:SS"</code>,
respectively.) If the host platform doesn't provide UTC time, these macros
are undefined.</p>
</li>
<li>
<p>The <code>__UTC_DATE_NUM__</code> and <code>__UTC_TIME_NUM__</code>
macros give the assembly date and time universal time (UTC) in numeric
form; in the format <code>YYYYMMDD</code> and <code>HHMMSS</code>
respectively. If the host platform doesn't provide UTC time, these macros
are undefined.</p>
</li>
<li>
<p>The <code>__POSIX_TIME__</code> macro is defined as a number containing
the number of seconds since the POSIX epoch, 1 January 1970 00:00:00 UTC;
excluding any leap seconds. This is computed using UTC time if available on
the host platform, otherwise it is computed using the local time as if it
was UTC.</p>
</li>
</ul>
<p>All instances of time and date macros in the same assembly session
produce consistent output. For example, in an assembly session started at
42 seconds after midnight on January 1, 2010 in Moscow (timezone UTC+3)
these macros would have the following values, assuming, of course, a
properly configured environment with a correct clock:</p>
<pre>
__DATE__ "2010-01-01"
__TIME__ "00:00:42"
__DATE_NUM__ 20100101
__TIME_NUM__ 000042
__UTC_DATE__ "2009-12-31"
__UTC_TIME__ "21:00:42"
__UTC_DATE_NUM__ 20091231
__UTC_TIME_NUM__ 210042
__POSIX_TIME__ 1262293242
</pre>
<h4 id="section-4.11.9">4.11.9 <code>__USE_</code><em>package</em><code>__</code>: Package Include Test</h4>
<p>When a standard macro package (see <a href="nasmdoc5.html">chapter
5</a>) is included with the <code>%use</code> directive (see
<a href="#section-4.6.4">section 4.6.4</a>), a single-line macro of the
form <code>__USE_</code><em>package</em><code>__</code> is automatically
defined. This allows testing if a particular package is invoked or not.</p>
<p>For example, if the <code>altreg</code> package is included (see
<a href="nasmdoc5.html#section-5.1">section 5.1</a>), then the macro
<code>__USE_ALTREG__</code> is defined.</p>
<h4 id="section-4.11.10">4.11.10 <code>__PASS__</code>: Assembly Pass</h4>
<p>The macro <code>__PASS__</code> is defined to be <code>1</code> on
preparatory passes, and <code>2</code> on the final pass. In
preprocess-only mode, it is set to <code>3</code>, and when running only to
generate dependencies (due to the <code>-M</code> or <code>-MG</code>
option, see <a href="nasmdoc2.html#section-2.1.4">section 2.1.4</a>) it is
set to <code>0</code>.</p>
<p><em>Avoid using this macro if at all possible. It is tremendously easy
to generate very strange errors by misusing it, and the semantics may
change in future versions of NASM.</em></p>
<h4 id="section-4.11.11">4.11.11 <code>STRUC</code> and <code>ENDSTRUC</code>: Declaring Structure Data Types</h4>
<p>The core of NASM contains no intrinsic means of defining data
structures; instead, the preprocessor is sufficiently powerful that data
structures can be implemented as a set of macros. The macros
<code>STRUC</code> and <code>ENDSTRUC</code> are used to define a structure
data type.</p>
<p><code>STRUC</code> takes one or two parameters. The first parameter is
the name of the data type. The second, optional parameter is the base
offset of the structure. The name of the data type is defined as a symbol
with the value of the base offset, and the name of the data type with the
suffix <code>_size</code> appended to it is defined as an <code>EQU</code>
giving the size of the structure. Once <code>STRUC</code> has been issued,
you are defining the structure, and should define fields using the
<code>RESB</code> family of pseudo-instructions, and then invoke
<code>ENDSTRUC</code> to finish the definition.</p>
<p>For example, to define a structure called <code>mytype</code> containing
a longword, a word, a byte and a string of bytes, you might code</p>
<pre>
struc mytype
mt_long: resd 1
mt_word: resw 1
mt_byte: resb 1
mt_str: resb 32
endstruc
</pre>
<p>The above code defines six symbols: <code>mt_long</code> as 0 (the
offset from the beginning of a <code>mytype</code> structure to the
longword field), <code>mt_word</code> as 4, <code>mt_byte</code> as 6,
<code>mt_str</code> as 7, <code>mytype_size</code> as 39, and
<code>mytype</code> itself as zero.</p>
<p>The reason why the structure type name is defined at zero by default is
a side effect of allowing structures to work with the local label
mechanism: if your structure members tend to have the same names in more
than one structure, you can define the above structure like this:</p>
<pre>
struc mytype
.long: resd 1
.word: resw 1
.byte: resb 1
.str: resb 32
endstruc
</pre>
<p>This defines the offsets to the structure fields as
<code>mytype.long</code>, <code>mytype.word</code>,
<code>mytype.byte</code> and <code>mytype.str</code>.</p>
<p>NASM, since it has no <em>intrinsic</em> structure support, does not
support any form of period notation to refer to the elements of a structure
once you have one (except the above local-label notation), so code such as
<code>mov ax,[mystruc.mt_word]</code> is not valid. <code>mt_word</code> is
a constant just like any other constant, so the correct syntax is
<code>mov ax,[mystruc+mt_word]</code> or
<code>mov ax,[mystruc+mytype.word]</code>.</p>
<p>Sometimes you only have the address of the structure displaced by an
offset. For example, consider this standard stack frame setup:</p>
<pre>
push ebp
mov ebp, esp
sub esp, 40
</pre>
<p>In this case, you could access an element by subtracting the offset:</p>
<pre>
mov [ebp - 40 + mytype.word], ax
</pre>
<p>However, if you do not want to repeat this offset, you can use –40
as a base offset:</p>
<pre>
struc mytype, -40
</pre>
<p>And access an element this way:</p>
<pre>
mov [ebp + mytype.word], ax
</pre>
<h4 id="section-4.11.12">4.11.12 <code>ISTRUC</code>, <code>AT</code> and <code>IEND</code>: Declaring Instances of Structures</h4>
<p>Having defined a structure type, the next thing you typically want to do
is to declare instances of that structure in your data segment. NASM
provides an easy way to do this in the <code>ISTRUC</code> mechanism. To
declare a structure of type <code>mytype</code> in a program, you code
something like this:</p>
<pre>
mystruc:
istruc mytype
at mt_long, dd 123456
at mt_word, dw 1024
at mt_byte, db 'x'
at mt_str, db 'hello, world', 13, 10, 0
iend
</pre>
<p>The function of the <code>AT</code> macro is to make use of the
<code>TIMES</code> prefix to advance the assembly position to the correct
point for the specified structure field, and then to declare the specified
data. Therefore the structure fields must be declared in the same order as
they were specified in the structure definition.</p>
<p>If the data to go in a structure field requires more than one source
line to specify, the remaining source lines can easily come after the
<code>AT</code> line. For example:</p>
<pre>
at mt_str, db 123,134,145,156,167,178,189
db 190,100,0
</pre>
<p>Depending on personal taste, you can also omit the code part of the
<code>AT</code> line completely, and start the structure field on the next
line:</p>
<pre>
at mt_str
db 'hello, world'
db 13,10,0
</pre>
<h4 id="section-4.11.13">4.11.13 <code>ALIGN</code> and <code>ALIGNB</code>: Data Alignment</h4>
<p>The <code>ALIGN</code> and <code>ALIGNB</code> macros provides a
convenient way to align code or data on a word, longword, paragraph or
other boundary. (Some assemblers call this directive <code>EVEN</code>.)
The syntax of the <code>ALIGN</code> and <code>ALIGNB</code> macros is</p>
<pre>
align 4 ; align on 4-byte boundary
align 16 ; align on 16-byte boundary
align 8,db 0 ; pad with 0s rather than NOPs
align 4,resb 1 ; align to 4 in the BSS
alignb 4 ; equivalent to previous line
</pre>
<p>Both macros require their first argument to be a power of two; they both
compute the number of additional bytes required to bring the length of the
current section up to a multiple of that power of two, and then apply the
<code>TIMES</code> prefix to their second argument to perform the
alignment.</p>
<p>If the second argument is not specified, the default for
<code>ALIGN</code> is <code>NOP</code>, and the default for
<code>ALIGNB</code> is <code>RESB 1</code>. So if the second argument is
specified, the two macros are equivalent. Normally, you can just use
<code>ALIGN</code> in code and data sections and <code>ALIGNB</code> in BSS
sections, and never need the second argument except for special purposes.</p>
<p><code>ALIGN</code> and <code>ALIGNB</code>, being simple macros, perform
no error checking: they cannot warn you if their first argument fails to be
a power of two, or if their second argument generates more than one byte of
code. In each of these cases they will silently do the wrong thing.</p>
<p><code>ALIGNB</code> (or <code>ALIGN</code> with a second argument of
<code>RESB 1</code>) can be used within structure definitions:</p>
<pre>
struc mytype2
mt_byte:
resb 1
alignb 2
mt_word:
resw 1
alignb 4
mt_long:
resd 1
mt_str:
resb 32
endstruc
</pre>
<p>This will ensure that the structure members are sensibly aligned
relative to the base of the structure.</p>
<p>A final caveat: <code>ALIGN</code> and <code>ALIGNB</code> work relative
to the beginning of the <em>section</em>, not the beginning of the address
space in the final executable. Aligning to a 16-byte boundary when the
section you're in is only guaranteed to be aligned to a 4-byte boundary,
for example, is a waste of effort. Again, NASM does not check that the
section's alignment characteristics are sensible for the use of
<code>ALIGN</code> or <code>ALIGNB</code>.</p>
<p>Both <code>ALIGN</code> and <code>ALIGNB</code> do call
<code>SECTALIGN</code> macro implicitly. See
<a href="#section-4.11.14">section 4.11.14</a> for details.</p>
<p>See also the <code>smartalign</code> standard macro package,
<a href="nasmdoc5.html#section-5.2">section 5.2</a>.</p>
<h4 id="section-4.11.14">4.11.14 <code>SECTALIGN</code>: Section Alignment</h4>
<p>The <code>SECTALIGN</code> macros provides a way to modify alignment
attribute of output file section. Unlike the <code>align=</code> attribute
(which is allowed at section definition only) the <code>SECTALIGN</code>
macro may be used at any time.</p>
<p>For example the directive</p>
<pre>
SECTALIGN 16
</pre>
<p>sets the section alignment requirements to 16 bytes. Once increased it
can not be decreased, the magnitude may grow only.</p>
<p>Note that <code>ALIGN</code> (see <a href="#section-4.11.13">section
4.11.13</a>) calls the <code>SECTALIGN</code> macro implicitly so the
active section alignment requirements may be updated. This is by default
behaviour, if for some reason you want the <code>ALIGN</code> do not call
<code>SECTALIGN</code> at all use the directive</p>
<pre>
SECTALIGN OFF
</pre>
<p>It is still possible to turn in on again by</p>
<pre>
SECTALIGN ON
</pre>
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