Editing X86 assembly language (section)

==="Hello world!" program for Linux in AT&T and NASM assembly===
{|class=wikitable
! width=30%|AT&T (GNU as) !! width=30%|Intel (NASM) !! Description
|-
|<syntaxhighlight lang="gas">
.data
</syntaxhighlight>
|<syntaxhighlight lang="nasm">
section .data
</syntaxhighlight>
|Like in the Windows example, <code>.data</code> is the section for initialized data.
|-
|<syntaxhighlight lang="gas">
str: .ascii "Hello, world!\n"
</syntaxhighlight>
|<syntaxhighlight lang="nasm">
str:     db 'Hello world!', 0Ah
</syntaxhighlight>
|Define a string of text containing "Hello, world!" and then a new line (<code>\n</code>, which is <code>0x0A</code>). Bind the label "str" to the address of the defined string.
|-
|<syntaxhighlight lang="gas">
str_len = . - str
</syntaxhighlight>
|<syntaxhighlight lang="nasm">
str_len: equ $ - str
</syntaxhighlight>
|Calculate the length of <code>str</code>. <code>.</code> means "here" in gas and <code>$</code> means the same in nasm. By subtracting "str" from "here", one gets the length of the previously-defined string.
|-
|<syntaxhighlight lang="gas">
.text
</syntaxhighlight>
|<syntaxhighlight lang=nasm>
section .text
</syntaxhighlight>
|Like in the Windows example, <code>.text</code> is the section for program code.
|-
|<syntaxhighlight lang="gas">.globl _start</syntaxhighlight>
|<syntaxhighlight lang=nasm>global _start</syntaxhighlight>
|export the _start function to the global scope for it to be "seen" by the linker
|-
|<syntaxhighlight lang="gas">_start:</syntaxhighlight>
|<syntaxhighlight lang="nasm">_start:</syntaxhighlight>
|Define a label called <code>_start</code>, to which we will write our subroutine. The name <code>_start</code>, by Linux convention, defines the entry point.
|-
|<syntaxhighlight lang="gas">
    movl $4, %eax
    movl $1, %ebx
    movl $str, %ecx
    movl $str_len, %edx
</syntaxhighlight>
|<syntaxhighlight lang="nasm">
	mov	eax, 4
	mov	ebx, 1
	mov	ecx, str
	mov	edx, str_len
</syntaxhighlight>
|Prepare a system call. EAX=4 requests the "sys_write" call on Linux x86. EBX=1 means "stdout" for sys_write. ECX holds the string to write, and EDX holds the number of bytes to write. The is equivalent to the libc-wrapped version <code>write(1, str, str_len)</code>.
|-
|<syntaxhighlight lang="gas">    int $0x80</syntaxhighlight>
|<syntaxhighlight lang="nasm">    int 80h</syntaxhighlight>
|On x86, the system interrupt "80h" is used for invoking a system call according to the values of eax, ebx, ecx, and edx.
|-
|<syntaxhighlight lang="gas">
    movl $1, %eax
    movl $0, %ebx
    int $0x80
</syntaxhighlight>
|<syntaxhighlight lang="nasm">
	mov	eax, 1
	mov	ebx, 0
	int	80h
</syntaxhighlight>
|Load another system call, then call it with INT 80h: EAX=1 is sys_exit, and EBX for sys_exit holds the return value. A return value of 0 means a normal exit. In C syntax, <code>_exit(0);</code>.
|}

Note for NASM:
<pre>
; This program runs in 32-bit protected mode.
;  build: nasm -f elf -F stabs name.asm
;  link:  ld -o name name.o
;
; In 64-bit long mode you can use 64-bit registers (e.g. rax instead of eax, rbx instead of ebx, etc.)
; Also change "-f elf " for "-f elf64" in build command.
; For 64-bit long mode, "lea rcx, str" would be the address of the message, note 64-bit register rcx.
</pre>