Editing X86 assembly language (section)

==Syntax==
x86 assembly language has two primary [[Syntax of programming languages|syntax]] branches: ''[[Intel]] syntax'' and ''[[AT&amp;T Corporation|AT&amp;T]] syntax''.<ref name="GASvsNASM">{{cite web|url=http://www.ibm.com/developerworks/linux/library/l-gas-nasm/index.html |title=Linux assemblers: A comparison of GAS and NASM |date=2007-10-17 |access-date=2008-07-02 |first=Ram |last=Narayam |website=[[IBM]] |url-status=dead |archive-url=https://web.archive.org/web/20131003180256/http://www.ibm.com/developerworks/linux/library/l-gas-nasm/index.html |archive-date=October 3, 2013 }}</ref> Intel syntax is dominant in the [[DOS]] and [[Microsoft Windows|Windows]] environments, while AT&amp;T syntax is dominant in [[Unix-like]] systems, as Unix was originally developed at [[AT&amp;T Bell Labs]].<ref>{{cite web|title=The Creation of Unix |url=http://www.bell-labs.com/history/unix/ |url-status=dead |archive-url=https://web.archive.org/web/20140402192351/http://www.bell-labs.com/history/unix/ |archive-date=April 2, 2014 }}</ref> Below is a summary of the main differences between ''Intel syntax'' and ''AT&amp;T syntax'':

{| class="wikitable"
!
! AT&amp;T
! Intel
|-
! scope="row" | Parameter order
| style="vertical-align:top;" | <syntaxhighlight lang="asm">movl $5, %eax</syntaxhighlight> Source before the destination.
| style="vertical-align:top;" | <syntaxhighlight lang="nasm">mov eax, 5</syntaxhighlight> Destination before source.
|-
! scope="row" | Parameter size
| style="vertical-align:top;" | <syntaxhighlight lang="asm">addl $0x24, %esp
movslq %ecx, %rax
paddd %xmm1, %xmm2</syntaxhighlight> Mnemonics are suffixed with a letter indicating the size of the operands: ''q'' for qword (64 bits), ''l'' for long (dword, 32 bits), ''w'' for word (16 bits), and ''b'' for byte (8 bits).<ref name="GASvsNASM"/>
<syntaxhighlight lang="asm"></syntaxhighlight>
| style="vertical-align:top;" | <syntaxhighlight lang="nasm">add esp, 24h
movsxd rax, ecx
paddd xmm2, xmm1</syntaxhighlight> Derived from the name of the register that is used (e.g. ''rax, eax, ax, al'' imply ''q, l, w, b'', respectively).

Width-based names may still appear in instructions when they define a different operation.
* MOVSXD refers to sign extension with dword input, unlike MOVSX.
* SIMD registers have width-named instructions that determine how to split up the register. AT&T tends to keep the names unchanged, so PADDD is not renamed to "paddl".
|-
! scope="row" | [[Sigil (computer programming)|Sigils]]
| style="vertical-align:top;" | [[Constant (programming)|Immediate values]] prefixed with a "$", registers prefixed with a "%".<ref name="GASvsNASM"/>
| style="vertical-align:top;" | The assembler automatically detects the type of symbols; i.e., whether they are registers, constants or something else.
|-
! scope="row" | Effective [[Memory address|addresses]]
| style="vertical-align:top;" | <syntaxhighlight lang="asm">movl offset(%ebx, %ecx, 4), %eax</syntaxhighlight> General syntax of <code>''displacement''(''base'', ''index'', ''scale'')</code>.
| style="vertical-align:top;" | <syntaxhighlight lang="nasm">mov eax, [ebx + ecx*4 + offset]</syntaxhighlight> Arithmetic expressions in square brackets; additionally, size keywords like ''byte'', ''word'', or ''dword'' have to be used if the size cannot be determined from the operands.<ref name="GASvsNASM"/>
|}

Many x86 assemblers use ''Intel syntax'', including [[FASM]], [[MASM]], [[Netwide Assembler|NASM]], [[TASM]], and [[YASM]]. The [[GNU Assembler]], which originally used ''AT&amp;T syntax'',  has supported both syntaxes since version 2.10 via the ''<code>.intel_syntax</code>'' directive.<ref name="GASvsNASM"/><ref name="WhichAsm">{{cite web|url=http://webster.cs.ucr.edu/AsmTools/WhichAsm.html|title=Which Assembler is the Best?|access-date=2008-05-18| first = Randall | last = Hyde }}</ref><ref>{{cite web|title = GNU Assembler News, v2.1 supports Intel syntax|url = http://sourceware.org/cgi-bin/cvsweb.cgi/src/gas/NEWS?rev=1.93&content-type=text/x-cvsweb-markup&cvsroot=src|date=2008-04-04|access-date=2008-07-02}}</ref> A quirk in the AT&amp;T syntax for x86 is that [[x87]] floating-point operands are reversed, an inherited bug from the original AT&T assembler.<ref>{{cite web |title=i386-Bugs (Using as) |url=https://sourceware.org/binutils/docs/as/i386_002dBugs.html |website=Binutils documentation |access-date=15 January 2020}}</ref>

The AT&T syntax is nearly universal across other architectures (retaining the same operand order for the {{code|mov}} instruction); it was originally designed for PDP-11 assembly and was inherited onto [[Unix-like]] systems. In contrast, the Intel syntax is specific to the [[x86 architecture]] and is the one used in the x86 platform's official documentation. The [[Intel 8080]], which predates the x86 architecture, also uses the "destination-first" order for {{code|mov}} instruction.<ref>{{cite web |title=Intel 8080 Assembly Language Programming Manual |url=https://altairclone.com/downloads/manuals/8080%20Programmers%20Manual.pdf |access-date=12 May 2023}}</ref>

=== Reserved words ===
In most x86 assembly languages, the [[reserved word]]s consist of two parts: mnemonics that translate to opcodes, and directives (or "pseudo-ops") that access features in the assembler program beyond the simple translation of opcodes. For a list of the former part, see [[x86 instruction listings]]. The latter part is highly assembler-dependent, with no such thing as a standard among Intel-syntax assemblers.<ref>{{cite web |title=NASM - The Netwide Assembler |url=https://www.nasm.us/xdoc/2.13/html/nasmdoc6.html |website=www.nasm.us}}</ref> AT&T-syntax assemblers share a common way of naming directives (all directives starts with a dot, like <code>.ascii</code>),<ref>{{cite web |title=Statements (Using as) |url=https://sourceware.org/binutils/docs/as/Statements.html |website=sourceware.org}}</ref> and a number of basic directives such as <code>.ascii</code> and <code>.string</code> are broadly supported.<ref>{{cite web |title=Pseudo Ops (Using as) ::  Assembler Directives |url=https://sourceware.org/binutils/docs/as/Pseudo-Ops.html |website=sourceware.org}}</ref><ref>{{cite web |title=Assembler Directives - x86 Assembly Language Reference Manual |url=https://docs.oracle.com/cd/E37838_01/html/E61064/eoiyg.html |website=docs.oracle.com}}</ref>