<html><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8"><title>Hello, world!</title><meta name="generator" content="DocBook XSL Stylesheets V1.76.1"><meta name="keywords" content="assembly, assembler, asm, inline, 32-bit, IA-32, i386, x86, nasm, gas, as, as86, yasm, fasm, shasm, osimpa, OS, Linux, Unix, kernel, system, libc, glibc, system call, interrupt, small, fast, embedded, hardware, port, macroprocessor, metaprogramming, preprocessor"><link rel="home" href="Assembly-HOWTO.html" title="Linux Assembly HOWTO"><link rel="up" href="quickstart.html" title="Chapter 6. Quick start"><link rel="prev" href="quickstart.html" title="Chapter 6. Quick start"><link rel="next" href="build.html" title="Building an executable"></head><body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Hello, world!</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="quickstart.html">Prev</a> </td><th width="60%" align="center">Chapter 6. Quick start</th><td width="20%" align="right"> <a accesskey="n" href="build.html">Next</a></td></tr></table><hr></div><div class="section" title="Hello, world!"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="idp472144"></a>Hello, world!</h2></div></div></div>
<div class="section" title="Program layout"><div class="titlepage"><div><div><h3 class="title"><a name="idp475040"></a>Program layout</h3></div></div></div>
<p>
Linux is 32-bit, runs in protected mode, has flat memory model, and uses the
ELF format for binaries.
</p>
<p>
A program can be divided into sections: <code class="literal">.text</code> for your code
(read-only), <code class="literal">.data</code> for your data (read-write),
<code class="literal">.bss</code> for uninitialized data (read-write); there can actually
be a few other standard sections, as well as some user-defined sections, but
there's rare need to use them and they are out of our interest here. A program
must have at least <code class="literal">.text</code> section.
</p>
<p>
Now we will write our first program. Here is sample code:
</p>
</div>
<div class="section" title="NASM (hello.asm)"><div class="titlepage"><div><div><h3 class="title"><a name="idp479584"></a>NASM (hello.asm)</h3></div></div></div>
<p>
</p><pre class="programlisting">
section .text ;section declaration
;we must export the entry point to the ELF linker or
global _start ;loader. They conventionally recognize _start as their
;entry point. Use ld -e foo to override the default.
_start:
;write our string to stdout
mov edx,len ;third argument: message length
mov ecx,msg ;second argument: pointer to message to write
mov ebx,1 ;first argument: file handle (stdout)
mov eax,4 ;system call number (sys_write)
int 0x80 ;call kernel
;and exit
mov ebx,0 ;first syscall argument: exit code
mov eax,1 ;system call number (sys_exit)
int 0x80 ;call kernel
section .data ;section declaration
msg db "Hello, world!",0xa ;our dear string
len equ $ - msg ;length of our dear string
</pre><p>
</p>
</div>
<div class="section" title="GAS (hello.S)"><div class="titlepage"><div><div><h3 class="title"><a name="idp482592"></a>GAS (hello.S)</h3></div></div></div>
<p>
</p><pre class="programlisting">
.text # section declaration
# we must export the entry point to the ELF linker or
.global _start # loader. They conventionally recognize _start as their
# entry point. Use ld -e foo to override the default.
_start:
# write our string to stdout
movl $len,%edx # third argument: message length
movl $msg,%ecx # second argument: pointer to message to write
movl $1,%ebx # first argument: file handle (stdout)
movl $4,%eax # system call number (sys_write)
int $0x80 # call kernel
# and exit
movl $0,%ebx # first argument: exit code
movl $1,%eax # system call number (sys_exit)
int $0x80 # call kernel
.data # section declaration
msg:
.ascii "Hello, world!\n" # our dear string
len = . - msg # length of our dear string
</pre><p>
</p>
</div>
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