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<code><nobr>__SECT__</nobr></code> macro when it is invoked.
<p><code><nobr>STRUC</nobr></code> and <code><nobr>ENDSTRUC</nobr></code>
are defined as macros which use <code><nobr>ABSOLUTE</nobr></code> (and
also <code><nobr>__SECT__</nobr></code>).
<p><code><nobr>ABSOLUTE</nobr></code> doesn't have to take an absolute
constant as an argument: it can take an expression (actually, a critical
expression: see <a href="nasmdoc3.html#section-3.8">section 3.8</a>) and it
can be a value in a segment. For example, a TSR can re-use its setup code
as run-time BSS like this:
<p><pre>
        org     100h               ; it's a .COM program 

        jmp     setup              ; setup code comes last 

        ; the resident part of the TSR goes here 
setup: 
        ; now write the code that installs the TSR here 

absolute setup 

runtimevar1     resw    1 
runtimevar2     resd    20 

tsr_end:
</pre>
<p>This defines some variables `on top of' the setup code, so that after
the setup has finished running, the space it took up can be re-used as data
storage for the running TSR. The symbol `tsr_end' can be used to calculate
the total size of the part of the TSR that needs to be made resident.
<h3><a name="section-5.4">5.4 <code><nobr>EXTERN</nobr></code>: Importing Symbols from Other Modules</a></h3>
<p><code><nobr>EXTERN</nobr></code> is similar to the MASM directive
<code><nobr>EXTRN</nobr></code> and the C keyword
<code><nobr>extern</nobr></code>: it is used to declare a symbol which is
not defined anywhere in the module being assembled, but is assumed to be
defined in some other module and needs to be referred to by this one. Not
every object-file format can support external variables: the
<code><nobr>bin</nobr></code> format cannot.
<p>The <code><nobr>EXTERN</nobr></code> directive takes as many arguments
as you like. Each argument is the name of a symbol:
<p><pre>
extern  _printf 
extern  _sscanf,_fscanf
</pre>
<p>Some object-file formats provide extra features to the
<code><nobr>EXTERN</nobr></code> directive. In all cases, the extra
features are used by suffixing a colon to the symbol name followed by
object-format specific text. For example, the <code><nobr>obj</nobr></code>
format allows you to declare that the default segment base of an external
should be the group <code><nobr>dgroup</nobr></code> by means of the
directive
<p><pre>
extern  _variable:wrt dgroup
</pre>
<p>The primitive form of <code><nobr>EXTERN</nobr></code> differs from the
user-level form only in that it can take only one argument at a time: the
support for multiple arguments is implemented at the preprocessor level.
<p>You can declare the same variable as <code><nobr>EXTERN</nobr></code>
more than once: NASM will quietly ignore the second and later
redeclarations. You can't declare a variable as
<code><nobr>EXTERN</nobr></code> as well as something else, though.
<h3><a name="section-5.5">5.5 <code><nobr>GLOBAL</nobr></code>: Exporting Symbols to Other Modules</a></h3>
<p><code><nobr>GLOBAL</nobr></code> is the other end of
<code><nobr>EXTERN</nobr></code>: if one module declares a symbol as
<code><nobr>EXTERN</nobr></code> and refers to it, then in order to prevent
linker errors, some other module must actually <em>define</em> the symbol
and declare it as <code><nobr>GLOBAL</nobr></code>. Some assemblers use the
name <code><nobr>PUBLIC</nobr></code> for this purpose.
<p>The <code><nobr>GLOBAL</nobr></code> directive applying to a symbol must
appear <em>before</em> the definition of the symbol.
<p><code><nobr>GLOBAL</nobr></code> uses the same syntax as
<code><nobr>EXTERN</nobr></code>, except that it must refer to symbols
which <em>are</em> defined in the same module as the
<code><nobr>GLOBAL</nobr></code> directive. For example:
<p><pre>
global _main 
_main: 
        ; some code
</pre>
<p><code><nobr>GLOBAL</nobr></code>, like <code><nobr>EXTERN</nobr></code>,
allows object formats to define private extensions by means of a colon. The
<code><nobr>elf</nobr></code> object format, for example, lets you specify
whether global data items are functions or data:
<p><pre>
global  hashlookup:function, hashtable:data
</pre>
<p>Like <code><nobr>EXTERN</nobr></code>, the primitive form of
<code><nobr>GLOBAL</nobr></code> differs from the user-level form only in
that it can take only one argument at a time.
<h3><a name="section-5.6">5.6 <code><nobr>COMMON</nobr></code>: Defining Common Data Areas</a></h3>
<p>The <code><nobr>COMMON</nobr></code> directive is used to declare
<em>common variables</em>. A common variable is much like a global variable
declared in the uninitialised data section, so that
<p><pre>
common  intvar  4
</pre>
<p>is similar in function to
<p><pre>
global  intvar 
section .bss 

intvar  resd    1
</pre>
<p>The difference is that if more than one module defines the same common
variable, then at link time those variables will be <em>merged</em>, and
references to <code><nobr>intvar</nobr></code> in all modules will point at
the same piece of memory.
<p>Like <code><nobr>GLOBAL</nobr></code> and
<code><nobr>EXTERN</nobr></code>, <code><nobr>COMMON</nobr></code> supports
object-format specific extensions. For example, the
<code><nobr>obj</nobr></code> format allows common variables to be NEAR or
FAR, and the <code><nobr>elf</nobr></code> format allows you to specify the
alignment requirements of a common variable:
<p><pre>
common  commvar  4:near  ; works in OBJ 
common  intarray 100:4   ; works in ELF: 4 byte aligned
</pre>
<p>Once again, like <code><nobr>EXTERN</nobr></code> and
<code><nobr>GLOBAL</nobr></code>, the primitive form of
<code><nobr>COMMON</nobr></code> differs from the user-level form only in
that it can take only one argument at a time.
<h3><a name="section-5.7">5.7 <code><nobr>CPU</nobr></code>: Defining CPU Dependencies</a></h3>
<p>The <code><nobr>CPU</nobr></code> directive restricts assembly to those
instructions which are available on the specified CPU.
<p>Options are:
<ul>
<li><code><nobr>CPU 8086</nobr></code> Assemble only 8086 instruction set
<li><code><nobr>CPU 186</nobr></code> Assemble instructions up to the 80186
instruction set
<li><code><nobr>CPU 286</nobr></code> Assemble instructions up to the 286
instruction set
<li><code><nobr>CPU 386</nobr></code> Assemble instructions up to the 386
instruction set
<li><code><nobr>CPU 486</nobr></code> 486 instruction set
<li><code><nobr>CPU 586</nobr></code> Pentium instruction set
<li><code><nobr>CPU PENTIUM</nobr></code> Same as 586
<li><code><nobr>CPU 686</nobr></code> P6 instruction set
<li><code><nobr>CPU PPRO</nobr></code> Same as 686
<li><code><nobr>CPU P2</nobr></code> Same as 686
<li><code><nobr>CPU P3</nobr></code> Pentium III (Katmai) instruction sets
<li><code><nobr>CPU KATMAI</nobr></code> Same as P3
<li><code><nobr>CPU P4</nobr></code> Pentium 4 (Willamette) instruction set
<li><code><nobr>CPU WILLAMETTE</nobr></code> Same as P4
<li><code><nobr>CPU PRESCOTT</nobr></code> Prescott instruction set
<li><code><nobr>CPU IA64</nobr></code> IA64 CPU (in x86 mode) instruction
set
</ul>
<p>All options are case insensitive. All instructions will be selected only
if they apply to the selected CPU or lower. By default, all instructions
are available.
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