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<h1>NASM - The Netwide Assembler</h1>
<span class="subtitle">version 2.13.03</span>
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<h2 id="chapter-6">Chapter 6: Assembler Directives</h2>
<p>NASM, though it attempts to avoid the bureaucracy of assemblers like
MASM and TASM, is nevertheless forced to support a <em>few</em> directives.
These are described in this chapter.</p>
<p>NASM's directives come in two types: <em>user-level</em> directives and
<em>primitive</em> directives. Typically, each directive has a user-level
form and a primitive form. In almost all cases, we recommend that users use
the user-level forms of the directives, which are implemented as macros
which call the primitive forms.</p>
<p>Primitive directives are enclosed in square brackets; user-level
directives are not.</p>
<p>In addition to the universal directives described in this chapter, each
object file format can optionally supply extra directives in order to
control particular features of that file format. These
<em>format-specific</em> directives are documented along with the formats
that implement them, in <a href="nasmdoc7.html">chapter 7</a>.</p>
<h3 id="section-6.1">6.1 <code>BITS</code>: Specifying Target Processor Mode</h3>
<p>The <code>BITS</code> directive specifies whether NASM should generate
code designed to run on a processor operating in 16-bit mode, 32-bit mode
or 64-bit mode. The syntax is <code>BITS XX</code>, where XX is 16, 32 or
64.</p>
<p>In most cases, you should not need to use <code>BITS</code> explicitly.
The <code>aout</code>, <code>coff</code>, <code>elf</code>,
<code>macho</code>, <code>win32</code> and <code>win64</code> object
formats, which are designed for use in 32-bit or 64-bit operating systems,
all cause NASM to select 32-bit or 64-bit mode, respectively, by default.
The <code>obj</code> object format allows you to specify each segment you
define as either <code>USE16</code> or <code>USE32</code>, and NASM will
set its operating mode accordingly, so the use of the <code>BITS</code>
directive is once again unnecessary.</p>
<p>The most likely reason for using the <code>BITS</code> directive is to
write 32-bit or 64-bit code in a flat binary file; this is because the
<code>bin</code> output format defaults to 16-bit mode in anticipation of
it being used most frequently to write DOS <code>.COM</code> programs, DOS
<code>.SYS</code> device drivers and boot loader software.</p>
<p>The <code>BITS</code> directive can also be used to generate code for a
different mode than the standard one for the output format.</p>
<p>You do <em>not</em> need to specify <code>BITS 32</code> merely in order
to use 32-bit instructions in a 16-bit DOS program; if you do, the
assembler will generate incorrect code because it will be writing code
targeted at a 32-bit platform, to be run on a 16-bit one.</p>
<p>When NASM is in <code>BITS 16</code> mode, instructions which use 32-bit
data are prefixed with an 0x66 byte, and those referring to 32-bit
addresses have an 0x67 prefix. In <code>BITS 32</code> mode, the reverse is
true: 32-bit instructions require no prefixes, whereas instructions using
16-bit data need an 0x66 and those working on 16-bit addresses need an
0x67.</p>
<p>When NASM is in <code>BITS 64</code> mode, most instructions operate the
same as they do for <code>BITS 32</code> mode. However, there are 8 more
general and SSE registers, and 16-bit addressing is no longer supported.</p>
<p>The default address size is 64 bits; 32-bit addressing can be selected
with the 0x67 prefix. The default operand size is still 32 bits, however,
and the 0x66 prefix selects 16-bit operand size. The <code>REX</code>
prefix is used both to select 64-bit operand size, and to access the new
registers. NASM automatically inserts REX prefixes when necessary.</p>
<p>When the <code>REX</code> prefix is used, the processor does not know
how to address the AH, BH, CH or DH (high 8-bit legacy) registers. Instead,
it is possible to access the the low 8-bits of the SP, BP SI and DI
registers as SPL, BPL, SIL and DIL, respectively; but only when the REX
prefix is used.</p>
<p>The <code>BITS</code> directive has an exactly equivalent primitive
form, <code>[BITS 16]</code>, <code>[BITS 32]</code> and
<code>[BITS 64]</code>. The user-level form is a macro which has no
function other than to call the primitive form.</p>
<p>Note that the space is neccessary, e.g. <code>BITS32</code> will
<em>not</em> work!</p>
<h4 id="section-6.1.1">6.1.1 <code>USE16</code> & <code>USE32</code>: Aliases for BITS</h4>
<p>The `<code>USE16</code>' and `<code>USE32</code>' directives can be used
in place of `<code>BITS 16</code>' and `<code>BITS 32</code>', for
compatibility with other assemblers.</p>
<h3 id="section-6.2">6.2 <code>DEFAULT</code>: Change the assembler defaults</h3>
<p>The <code>DEFAULT</code> directive changes the assembler defaults.
Normally, NASM defaults to a mode where the programmer is expected to
explicitly specify most features directly. However, this is occasionally
obnoxious, as the explicit form is pretty much the only one one wishes to
use.</p>
<p>Currently, <code>DEFAULT</code> can set <code>REL</code> &
<code>ABS</code> and <code>BND</code> & <code>NOBND</code>.</p>
<h4 id="section-6.2.1">6.2.1 <code>REL</code> & <code>ABS</code>: RIP-relative addressing</h4>
<p>This sets whether registerless instructions in 64-bit mode are
<code>RIP</code>–relative or not. By default, they are absolute
unless overridden with the <code>REL</code> specifier (see
<a href="nasmdoc3.html#section-3.3">section 3.3</a>). However, if
<code>DEFAULT REL</code> is specified, <code>REL</code> is default, unless
overridden with the <code>ABS</code> specifier, <em>except when used with
an FS or GS segment override</em>.</p>
<p>The special handling of <code>FS</code> and <code>GS</code> overrides
are due to the fact that these registers are generally used as thread
pointers or other special functions in 64-bit mode, and generating
<code>RIP</code>–relative addresses would be extremely confusing.</p>
<p><code>DEFAULT REL</code> is disabled with <code>DEFAULT ABS</code>.</p>
<h4 id="section-6.2.2">6.2.2 <code>BND</code> & <code>NOBND</code>: <code>BND</code> prefix</h4>
<p>If <code>DEFAULT BND</code> is set, all bnd-prefix available
instructions following this directive are prefixed with bnd. To override
it, <code>NOBND</code> prefix can be used.</p>
<pre>
DEFAULT BND
call foo ; BND will be prefixed
nobnd call foo ; BND will NOT be prefixed
</pre>
<p><code>DEFAULT NOBND</code> can disable <code>DEFAULT BND</code> and then
<code>BND</code> prefix will be added only when explicitly specified in
code.</p>
<p><code>DEFAULT BND</code> is expected to be the normal configuration for
writing MPX-enabled code.</p>
<h3 id="section-6.3">6.3 <code>SECTION</code> or <code>SEGMENT</code>: Changing and Defining Sections</h3>
<p>The <code>SECTION</code> directive (<code>SEGMENT</code> is an exactly
equivalent synonym) changes which section of the output file the code you
write will be assembled into. In some object file formats, the number and
names of sections are fixed; in others, the user may make up as many as
they wish. Hence <code>SECTION</code> may sometimes give an error message,
or may define a new section, if you try to switch to a section that does
not (yet) exist.</p>
<p>The Unix object formats, and the <code>bin</code> object format (but see
<a href="nasmdoc7.html#section-7.1.3">section 7.1.3</a>), all support the
standardized section names <code>.text</code>, <code>.data</code> and
<code>.bss</code> for the code, data and uninitialized-data sections. The
<code>obj</code> format, by contrast, does not recognize these section
names as being special, and indeed will strip off the leading period of any
section name that has one.</p>
<h4 id="section-6.3.1">6.3.1 The <code>__SECT__</code> Macro</h4>
<p>The <code>SECTION</code> directive is unusual in that its user-level
form functions differently from its primitive form. The primitive form,
<code>[SECTION xyz]</code>, simply switches the current target section to
the one given. The user-level form, <code>SECTION xyz</code>, however,
first defines the single-line macro <code>__SECT__</code> to be the
primitive <code>[SECTION]</code> directive which it is about to issue, and
then issues it. So the user-level directive</p>
<pre>
SECTION .text
</pre>
<p>expands to the two lines</p>
<pre>
%define __SECT__ [SECTION .text]
[SECTION .text]
</pre>
<p>Users may find it useful to make use of this in their own macros. For
example, the <code>writefile</code> macro defined in
<a href="nasmdoc4.html#section-4.3.3">section 4.3.3</a> can be usefully
rewritten in the following more sophisticated form:</p>
<pre>
%macro writefile 2+
[section .data]
%%str: db %2
%%endstr:
__SECT__
mov dx,%%str
mov cx,%%endstr-%%str
mov bx,%1
mov ah,0x40
int 0x21
%endmacro
</pre>
<p>This form of the macro, once passed a string to output, first switches
temporarily to the data section of the file, using the primitive form of
the <code>SECTION</code> directive so as not to modify
<code>__SECT__</code>. It then declares its string in the data section, and
then invokes <code>__SECT__</code> to switch back to <em>whichever</em>
section the user was previously working in. It thus avoids the need, in the
previous version of the macro, to include a <code>JMP</code> instruction to
jump over the data, and also does not fail if, in a complicated
<code>OBJ</code> format module, the user could potentially be assembling
the code in any of several separate code sections.</p>
<h3 id="section-6.4">6.4 <code>ABSOLUTE</code>: Defining Absolute Labels</h3>
<p>The <code>ABSOLUTE</code> directive can be thought of as an alternative
form of <code>SECTION</code>: it causes the subsequent code to be directed
at no physical section, but at the hypothetical section starting at the
given absolute address. The only instructions you can use in this mode are
the <code>RESB</code> family.</p>
<p><code>ABSOLUTE</code> is used as follows:</p>
<pre>
absolute 0x1A
kbuf_chr resw 1
kbuf_free resw 1
kbuf resw 16
</pre>
<p>This example describes a section of the PC BIOS data area, at segment
address 0x40: the above code defines <code>kbuf_chr</code> to be 0x1A,
<code>kbuf_free</code> to be 0x1C, and <code>kbuf</code> to be 0x1E.</p>
<p>The user-level form of <code>ABSOLUTE</code>, like that of
<code>SECTION</code>, redefines the <code>__SECT__</code> macro when it is
invoked.</p>
<p><code>STRUC</code> and <code>ENDSTRUC</code> are defined as macros which
use <code>ABSOLUTE</code> (and also <code>__SECT__</code>).</p>
<p><code>ABSOLUTE</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.</p>
<h3 id="section-6.5">6.5 <code>EXTERN</code>: Importing Symbols from Other Modules</h3>
<p><code>EXTERN</code> is similar to the MASM directive <code>EXTRN</code>
and the C keyword <code>extern</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>bin</code> format cannot.</p>
<p>The <code>EXTERN</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>EXTERN</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>obj</code> format allows you to declare that
the default segment base of an external should be the group
<code>dgroup</code> by means of the directive</p>
<pre>
extern _variable:wrt dgroup
</pre>
<p>The primitive form of <code>EXTERN</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>
<p>You can declare the same variable as <code>EXTERN</code> more than once:
NASM will quietly ignore the second and later redeclarations. You can't
declare a variable as <code>EXTERN</code> as well as something else,
though.</p>
<h3 id="section-6.6">6.6 <code>GLOBAL</code>: Exporting Symbols to Other Modules</h3>
<p><code>GLOBAL</code> is the other end of <code>EXTERN</code>: if one
module declares a symbol as <code>EXTERN</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>GLOBAL</code>. Some
assemblers use the name <code>PUBLIC</code> for this purpose.</p>
<p>The <code>GLOBAL</code> directive applying to a symbol must appear
<em>before</em> the definition of the symbol.</p>
<p><code>GLOBAL</code> uses the same syntax as <code>EXTERN</code>, except
that it must refer to symbols which <em>are</em> defined in the same module
as the <code>GLOBAL</code> directive. For example:</p>
<pre>
global _main
_main:
; some code
</pre>
<p><code>GLOBAL</code>, like <code>EXTERN</code>, allows object formats to
define private extensions by means of a colon. The <code>elf</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>EXTERN</code>, the primitive form of <code>GLOBAL</code>
differs from the user-level form only in that it can take only one argument
at a time.</p>
<h3 id="section-6.7">6.7 <code>COMMON</code>: Defining Common Data Areas</h3>
<p>The <code>COMMON</code> directive is used to declare <em>common
variables</em>. A common variable is much like a global variable declared
in the uninitialized 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>intvar</code> in all modules will point at the same
piece of memory.</p>
<p>Like <code>GLOBAL</code> and <code>EXTERN</code>, <code>COMMON</code>
supports object-format specific extensions. For example, the
<code>obj</code> format allows common variables to be NEAR or FAR, and the
<code>elf</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>EXTERN</code> and <code>GLOBAL</code>, the
primitive form of <code>COMMON</code> differs from the user-level form only
in that it can take only one argument at a time.</p>
<h3 id="section-6.8">6.8 <code>CPU</code>: Defining CPU Dependencies</h3>
<p>The <code>CPU</code> directive restricts assembly to those instructions
which are available on the specified CPU.</p>
<p>Options are:</p>
<ul>
<li>
<p><code>CPU 8086</code> Assemble only 8086 instruction set</p>
</li>
<li>
<p><code>CPU 186</code> Assemble instructions up to the 80186 instruction
set</p>
</li>
<li>
<p><code>CPU 286</code> Assemble instructions up to the 286 instruction set</p>
</li>
<li>
<p><code>CPU 386</code> Assemble instructions up to the 386 instruction set</p>
</li>
<li>
<p><code>CPU 486</code> 486 instruction set</p>
</li>
<li>
<p><code>CPU 586</code> Pentium instruction set</p>
</li>
<li>
<p><code>CPU PENTIUM</code> Same as 586</p>
</li>
<li>
<p><code>CPU 686</code> P6 instruction set</p>
</li>
<li>
<p><code>CPU PPRO</code> Same as 686</p>
</li>
<li>
<p><code>CPU P2</code> Same as 686</p>
</li>
<li>
<p><code>CPU P3</code> Pentium III (Katmai) instruction sets</p>
</li>
<li>
<p><code>CPU KATMAI</code> Same as P3</p>
</li>
<li>
<p><code>CPU P4</code> Pentium 4 (Willamette) instruction set</p>
</li>
<li>
<p><code>CPU WILLAMETTE</code> Same as P4</p>
</li>
<li>
<p><code>CPU PRESCOTT</code> Prescott instruction set</p>
</li>
<li>
<p><code>CPU X64</code> x86-64 (x64/AMD64/Intel 64) instruction set</p>
</li>
<li>
<p><code>CPU IA64</code> IA64 CPU (in x86 mode) instruction set</p>
</li>
</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.</p>
<h3 id="section-6.9">6.9 <code>FLOAT</code>: Handling of floating-point constants</h3>
<p>By default, floating-point constants are rounded to nearest, and IEEE
denormals are supported. The following options can be set to alter this
behaviour:</p>
<ul>
<li>
<p><code>FLOAT DAZ</code> Flush denormals to zero</p>
</li>
<li>
<p><code>FLOAT NODAZ</code> Do not flush denormals to zero (default)</p>
</li>
<li>
<p><code>FLOAT NEAR</code> Round to nearest (default)</p>
</li>
<li>
<p><code>FLOAT UP</code> Round up (toward +Infinity)</p>
</li>
<li>
<p><code>FLOAT DOWN</code> Round down (toward –Infinity)</p>
</li>
<li>
<p><code>FLOAT ZERO</code> Round toward zero</p>
</li>
<li>
<p><code>FLOAT DEFAULT</code> Restore default settings</p>
</li>
</ul>
<p>The standard macros <code>__FLOAT_DAZ__</code>,
<code>__FLOAT_ROUND__</code>, and <code>__FLOAT__</code> contain the
current state, as long as the programmer has avoided the use of the
brackeded primitive form, (<code>[FLOAT]</code>).</p>
<p><code>__FLOAT__</code> contains the full set of floating-point settings;
this value can be saved away and invoked later to restore the setting.</p>
<h3 id="section-6.10">6.10 <code>[WARNING]</code>: Enable or disable warnings</h3>
<p>The <code>[WARNING]</code> directive can be used to enable or disable
classes of warnings in the same way as the <code>-w</code> option, see
<a href="nasmdoc2.html#section-2.1.25">section 2.1.25</a> for more details
about warning classes.</p>
<ul>
<li>
<p><code>[warning +</code><em>warning-class</em><code>]</code> enables
warnings for <em>warning-class</em>.</p>
</li>
<li>
<p><code>[warning -</code><em>warning-class</em><code>]</code> disables
warnings for <em>warning-class</em>.</p>
</li>
<li>
<p><code>[warning *</code><em>warning-class</em><code>]</code> restores
<em>warning-class</em> to the original value, either the default value or
as specified on the command line.</p>
</li>
</ul>
<p>The <code>[WARNING]</code> directive also accepts the <code>all</code>,
<code>error</code> and <code>error=</code><em>warning-class</em>
specifiers.</p>
<p>No "user form" (without the brackets) currently exists.</p>
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