path: root/arch/x86_64.html



<!DOCTYPE HTML>
<html lang="en" class="sidebar-visible no-js light">
    <head>
        <!-- Book generated using mdBook -->
        <meta charset="UTF-8">
        <title>x86_64 - Notes</title>
        <!-- Custom HTML head -->
        <meta content="text/html; charset=utf-8" http-equiv="Content-Type">
        <meta name="description" content="">
        <meta name="viewport" content="width=device-width, initial-scale=1">
        <meta name="theme-color" content="#ffffff" />

        <link rel="icon" href="../favicon.svg">
        <link rel="shortcut icon" href="../favicon.png">
        <link rel="stylesheet" href="../css/variables.css">
        <link rel="stylesheet" href="../css/general.css">
        <link rel="stylesheet" href="../css/chrome.css">
        <link rel="stylesheet" href="../css/print.css" media="print">
        <!-- Fonts -->
        <link rel="stylesheet" href="../FontAwesome/css/font-awesome.css">
        <link rel="stylesheet" href="../fonts/fonts.css">
        <!-- Highlight.js Stylesheets -->
        <link rel="stylesheet" href="../highlight.css">
        <link rel="stylesheet" href="../tomorrow-night.css">
        <link rel="stylesheet" href="../ayu-highlight.css">

        <!-- Custom theme stylesheets -->
    </head>
    <body>
        <!-- Provide site root to javascript -->
        <script type="text/javascript">
            var path_to_root = "../";
            var default_theme = window.matchMedia("(prefers-color-scheme: dark)").matches ? "navy" : "light";
        </script>

        <!-- Work around some values being stored in localStorage wrapped in quotes -->
        <script type="text/javascript">
            try {
                var theme = localStorage.getItem('mdbook-theme');
                var sidebar = localStorage.getItem('mdbook-sidebar');

                if (theme.startsWith('"') && theme.endsWith('"')) {
                    localStorage.setItem('mdbook-theme', theme.slice(1, theme.length - 1));
                }

                if (sidebar.startsWith('"') && sidebar.endsWith('"')) {
                    localStorage.setItem('mdbook-sidebar', sidebar.slice(1, sidebar.length - 1));
                }
            } catch (e) { }
        </script>

        <!-- Set the theme before any content is loaded, prevents flash -->
        <script type="text/javascript">
            var theme;
            try { theme = localStorage.getItem('mdbook-theme'); } catch(e) { }
            if (theme === null || theme === undefined) { theme = default_theme; }
            var html = document.querySelector('html');
            html.classList.remove('no-js')
            html.classList.remove('light')
            html.classList.add(theme);
            html.classList.add('js');
        </script>

        <!-- Hide / unhide sidebar before it is displayed -->
        <script type="text/javascript">
            var html = document.querySelector('html');
            var sidebar = 'hidden';
            if (document.body.clientWidth >= 1080) {
                try { sidebar = localStorage.getItem('mdbook-sidebar'); } catch(e) { }
                sidebar = sidebar || 'visible';
            }
            html.classList.remove('sidebar-visible');
            html.classList.add("sidebar-" + sidebar);
        </script>

        <nav id="sidebar" class="sidebar" aria-label="Table of contents">
            <div class="sidebar-scrollbox">
                <ol class="chapter"><li class="chapter-item expanded affix "><a href="../intro.html">Introduction</a></li><li class="chapter-item expanded "><a href="../tools/index.html"><strong aria-hidden="true">1.</strong> Tools</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../tools/zsh.html"><strong aria-hidden="true">1.1.</strong> zsh</a></li><li class="chapter-item expanded "><a href="../tools/bash.html"><strong aria-hidden="true">1.2.</strong> bash</a></li><li class="chapter-item expanded "><a href="../tools/fish.html"><strong aria-hidden="true">1.3.</strong> fish</a></li><li class="chapter-item expanded "><a href="../tools/tmux.html"><strong aria-hidden="true">1.4.</strong> tmux</a></li><li class="chapter-item expanded "><a href="../tools/git.html"><strong aria-hidden="true">1.5.</strong> git</a></li><li class="chapter-item expanded "><a href="../tools/awk.html"><strong aria-hidden="true">1.6.</strong> awk</a></li><li class="chapter-item expanded "><a href="../tools/emacs.html"><strong aria-hidden="true">1.7.</strong> emacs</a></li><li class="chapter-item expanded "><a href="../tools/gpg.html"><strong aria-hidden="true">1.8.</strong> gpg</a></li><li class="chapter-item expanded "><a href="../tools/gdb.html"><strong aria-hidden="true">1.9.</strong> gdb</a></li><li class="chapter-item expanded "><a href="../tools/gdbserver.html"><strong aria-hidden="true">1.10.</strong> gdbserver</a></li><li class="chapter-item expanded "><a href="../tools/radare2.html"><strong aria-hidden="true">1.11.</strong> radare2</a></li><li class="chapter-item expanded "><a href="../tools/qemu.html"><strong aria-hidden="true">1.12.</strong> qemu</a></li><li class="chapter-item expanded "><a href="../tools/pacman.html"><strong aria-hidden="true">1.13.</strong> pacman</a></li><li class="chapter-item expanded "><a href="../tools/dot.html"><strong aria-hidden="true">1.14.</strong> dot</a></li></ol></li><li class="chapter-item expanded "><a href="../monitor/index.html"><strong aria-hidden="true">2.</strong> Resource analysis & monitor</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../monitor/lsof.html"><strong aria-hidden="true">2.1.</strong> lsof</a></li><li class="chapter-item expanded "><a href="../monitor/ss.html"><strong aria-hidden="true">2.2.</strong> ss</a></li><li class="chapter-item expanded "><a href="../monitor/pidstat.html"><strong aria-hidden="true">2.3.</strong> pidstat</a></li><li class="chapter-item expanded "><a href="../monitor/pgrep.html"><strong aria-hidden="true">2.4.</strong> pgrep</a></li><li class="chapter-item expanded "><a href="../monitor/pmap.html"><strong aria-hidden="true">2.5.</strong> pmap</a></li><li class="chapter-item expanded "><a href="../monitor/pstack.html"><strong aria-hidden="true">2.6.</strong> pstack</a></li></ol></li><li class="chapter-item expanded "><a href="../trace_profile/index.html"><strong aria-hidden="true">3.</strong> Trace and Profile</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../trace_profile/strace.html"><strong aria-hidden="true">3.1.</strong> strace</a></li><li class="chapter-item expanded "><a href="../trace_profile/ltrace.html"><strong aria-hidden="true">3.2.</strong> ltrace</a></li><li class="chapter-item expanded "><a href="../trace_profile/perf.html"><strong aria-hidden="true">3.3.</strong> perf</a></li><li class="chapter-item expanded "><a href="../trace_profile/oprofile.html"><strong aria-hidden="true">3.4.</strong> OProfile</a></li><li class="chapter-item expanded "><a href="../trace_profile/time.html"><strong aria-hidden="true">3.5.</strong> time</a></li></ol></li><li class="chapter-item expanded "><a href="../binary/index.html"><strong aria-hidden="true">4.</strong> Binary</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../binary/od.html"><strong aria-hidden="true">4.1.</strong> od</a></li><li class="chapter-item expanded "><a href="../binary/xxd.html"><strong aria-hidden="true">4.2.</strong> xxd</a></li><li class="chapter-item expanded "><a href="../binary/readelf.html"><strong aria-hidden="true">4.3.</strong> readelf</a></li><li class="chapter-item expanded "><a href="../binary/objdump.html"><strong aria-hidden="true">4.4.</strong> objdump</a></li><li class="chapter-item expanded "><a href="../binary/nm.html"><strong aria-hidden="true">4.5.</strong> nm</a></li></ol></li><li class="chapter-item expanded "><a href="../development/index.html"><strong aria-hidden="true">5.</strong> Development</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../development/c++filt.html"><strong aria-hidden="true">5.1.</strong> c++filt</a></li><li class="chapter-item expanded "><a href="../development/c++.html"><strong aria-hidden="true">5.2.</strong> c++</a></li><li class="chapter-item expanded "><a href="../development/glibc.html"><strong aria-hidden="true">5.3.</strong> glibc</a></li><li class="chapter-item expanded "><a href="../development/gcc.html"><strong aria-hidden="true">5.4.</strong> gcc</a></li><li class="chapter-item expanded "><a href="../development/make.html"><strong aria-hidden="true">5.5.</strong> make</a></li><li class="chapter-item expanded "><a href="../development/ld.so.html"><strong aria-hidden="true">5.6.</strong> ld.so</a></li><li class="chapter-item expanded "><a href="../development/symbolver.html"><strong aria-hidden="true">5.7.</strong> symbol versioning</a></li><li class="chapter-item expanded "><a href="../development/python.html"><strong aria-hidden="true">5.8.</strong> python</a></li></ol></li><li class="chapter-item expanded "><a href="../linux/index.html"><strong aria-hidden="true">6.</strong> Linux</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../linux/systemd.html"><strong aria-hidden="true">6.1.</strong> systemd</a></li><li class="chapter-item expanded "><a href="../linux/coredump.html"><strong aria-hidden="true">6.2.</strong> coredump</a></li><li class="chapter-item expanded "><a href="../linux/ptrace_scope.html"><strong aria-hidden="true">6.3.</strong> ptrace_scope</a></li><li class="chapter-item expanded "><a href="../linux/cryptsetup.html"><strong aria-hidden="true">6.4.</strong> cryptsetup</a></li><li class="chapter-item expanded "><a href="../linux/swap.html"><strong aria-hidden="true">6.5.</strong> swap</a></li></ol></li><li class="chapter-item expanded "><a href="../network/index.html"><strong aria-hidden="true">7.</strong> Network</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../network/tcpdump.html"><strong aria-hidden="true">7.1.</strong> tcpdump</a></li></ol></li><li class="chapter-item expanded "><a href="../web/index.html"><strong aria-hidden="true">8.</strong> Web</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../web/html.html"><strong aria-hidden="true">8.1.</strong> html</a></li></ol></li><li class="chapter-item expanded "><a href="../arch/index.html"><strong aria-hidden="true">9.</strong> Arch</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../arch/x86_64.html" class="active"><strong aria-hidden="true">9.1.</strong> x86_64</a></li><li class="chapter-item expanded "><a href="../arch/arm64.html"><strong aria-hidden="true">9.2.</strong> arm64</a></li><li class="chapter-item expanded "><a href="../arch/armv7.html"><strong aria-hidden="true">9.3.</strong> armv7</a></li><li class="chapter-item expanded "><a href="../arch/riscv.html"><strong aria-hidden="true">9.4.</strong> riscv</a></li></ol></li></ol>
            </div>
            <div id="sidebar-resize-handle" class="sidebar-resize-handle"></div>
        </nav>

        <div id="page-wrapper" class="page-wrapper">

            <div class="page">
                <div id="menu-bar-hover-placeholder"></div>
                <div id="menu-bar" class="menu-bar sticky bordered">
                    <div class="left-buttons">
                        <button id="sidebar-toggle" class="icon-button" type="button" title="Toggle Table of Contents" aria-label="Toggle Table of Contents" aria-controls="sidebar">
                            <i class="fa fa-bars"></i>
                        </button>
                        <button id="theme-toggle" class="icon-button" type="button" title="Change theme" aria-label="Change theme" aria-haspopup="true" aria-expanded="false" aria-controls="theme-list">
                            <i class="fa fa-paint-brush"></i>
                        </button>
                        <ul id="theme-list" class="theme-popup" aria-label="Themes" role="menu">
                            <li role="none"><button role="menuitem" class="theme" id="light">Light (default)</button></li>
                            <li role="none"><button role="menuitem" class="theme" id="rust">Rust</button></li>
                            <li role="none"><button role="menuitem" class="theme" id="coal">Coal</button></li>
                            <li role="none"><button role="menuitem" class="theme" id="navy">Navy</button></li>
                            <li role="none"><button role="menuitem" class="theme" id="ayu">Ayu</button></li>
                        </ul>
                        <button id="search-toggle" class="icon-button" type="button" title="Search. (Shortkey: s)" aria-label="Toggle Searchbar" aria-expanded="false" aria-keyshortcuts="S" aria-controls="searchbar">
                            <i class="fa fa-search"></i>
                        </button>
                    </div>

                    <h1 class="menu-title">Notes</h1>

                    <div class="right-buttons">
                        <a href="../print.html" title="Print this book" aria-label="Print this book">
                            <i id="print-button" class="fa fa-print"></i>
                        </a>
                        <a href="https://github.com/johannst/notes" title="Git repository" aria-label="Git repository">
                            <i id="git-repository-button" class="fa fa-github"></i>
                        </a>
                    </div>
                </div>

                <div id="search-wrapper" class="hidden">
                    <form id="searchbar-outer" class="searchbar-outer">
                        <input type="search" id="searchbar" name="searchbar" placeholder="Search this book ..." aria-controls="searchresults-outer" aria-describedby="searchresults-header">
                    </form>
                    <div id="searchresults-outer" class="searchresults-outer hidden">
                        <div id="searchresults-header" class="searchresults-header"></div>
                        <ul id="searchresults">
                        </ul>
                    </div>
                </div>
                <!-- Apply ARIA attributes after the sidebar and the sidebar toggle button are added to the DOM -->
                <script type="text/javascript">
                    document.getElementById('sidebar-toggle').setAttribute('aria-expanded', sidebar === 'visible');
                    document.getElementById('sidebar').setAttribute('aria-hidden', sidebar !== 'visible');
                    Array.from(document.querySelectorAll('#sidebar a')).forEach(function(link) {
                        link.setAttribute('tabIndex', sidebar === 'visible' ? 0 : -1);
                    });
                </script>

                <div id="content" class="content">
                    <main>
                        <h1 id="x86_64"><a class="header" href="#x86_64">x86_64</a></h1>
<p>keywords: x86_64, x86, abi</p>
<ul>
<li>64bit synonyms: <code>x86_64</code>, <code>x64</code>, <code>amd64</code>, <code>intel 64</code></li>
<li>32bit synonyms: <code>x86</code>, <code>ia32</code>, <code>i386</code></li>
<li>ISA type: <code>CISC</code></li>
<li>Endianness: <code>little</code></li>
</ul>
<h2 id="registers"><a class="header" href="#registers">Registers</a></h2>
<h3 id="general-purpose-register"><a class="header" href="#general-purpose-register">General purpose register</a></h3>
<pre><code class="language-markdown">bytes
[7:0]      [3:0]   [1:0]   [1]   [0]     desc
----------------------------------------------------------
rax        eax     ax      ah    al      accumulator
rbx        ebx     bx      bh    bl      base register
rcx        ecx     cx      ch    cl      counter
rdx        edx     dx      dh    dl      data register
rsi        esi     si      -     sil     source index
rdi        edi     di      -     dil     destination index
rbp        ebp     bp      -     bpl     base pointer
rsp        esp     sp      -     spl     stack pointer
r8-15      rNd     rNw     -     rNb
</code></pre>
<h3 id="special-register"><a class="header" href="#special-register">Special register</a></h3>
<pre><code class="language-markdown">bytes
[7:0]      [3:0]     [1:0]      desc
---------------------------------------------------
rflags     eflags    flags      flags register
rip        eip       ip         instruction pointer
</code></pre>
<h3 id="flags-register"><a class="header" href="#flags-register">FLAGS register</a></h3>
<pre><code class="language-markdown">rflags
bits    desc                            instr        comment
--------------------------------------------------------------------------------------------------------------
   [21]   ID   identification                        ability to set/clear -&gt; indicates support for CPUID instr
   [18]   AC   alignment check                       alignment exception for PL 3 (user), requires CR0.AM
[13:12] IOPL   io privilege level
   [11]   OF   overflow flag
   [10]   DF   direction flag           cld/std      increment (0) or decrement (1) registers in string operations
    [9]   IF   interrupt enable         cli/sti
    [7]   SF   sign flag
    [6]   ZF   zero flag
    [4]   AF   auxiliary carry flag
    [2]   PF   parity flag
    [0]   CF   carry flag
</code></pre>
<p>Change flag bits with <code>pushf</code> / <code>popf</code> instructions:</p>
<pre><code class="language-x86asm">pushfd                          // push flags (4bytes) onto stack
or dword ptr [esp], (1 &lt;&lt; 18)   // enable AC flag
popfd                           // pop flags (4byte) from stack
</code></pre>
<blockquote>
<p>There is also <code>pushfq</code> / <code>popfq</code> to push and pop all 8 bytes of <code>rflags</code>.</p>
</blockquote>
<h3 id="model-specific-register-msr"><a class="header" href="#model-specific-register-msr">Model Specific Register (MSR)</a></h3>
<pre><code class="language-x86asm">rdmsr     // Read MSR register, effectively does EDX:EAX &lt;- MSR[ECX]
wrmsr     // Write MSR register, effectively does MSR[ECX] &lt;- EDX:EAX
</code></pre>
<h2 id="size-directives"><a class="header" href="#size-directives">Size directives</a></h2>
<p>Explicitly specify size of the operation.</p>
<pre><code class="language-x86asm">mov  byte ptr [rax], 0xff    // save 1 byte(s) at [rax]
mov  word ptr [rax], 0xff    // save 2 byte(s) at [rax]
mov dword ptr [rax], 0xff    // save 4 byte(s) at [rax]
mov qword ptr [rax], 0xff    // save 8 byte(s) at [rax]
</code></pre>
<h2 id="addressing"><a class="header" href="#addressing">Addressing</a></h2>
<pre><code class="language-x86asm">mov qword ptr [rax], rbx         // save val in rbx at [rax]
mov qword ptr [imm], rbx         // save val in rbx at [imm]
mov rax, qword ptr [rbx+4*rcx]   // load val at [rbx+4*rcx] into rax
</code></pre>
<p><code>rip</code> relative addressing:</p>
<pre><code class="language-x86asm">lea rax, [rip+.my_str]       // load addr of .my_str into rax
...
.my_str:
.asciz &quot;Foo&quot;
</code></pre>
<h2 id="string-instructions"><a class="header" href="#string-instructions">String instructions</a></h2>
<p>The operand size of a string instruction is defined by the instruction suffix
<code>b | w | d | q</code>.</p>
<p>Source and destination registers are modified according to the <code>direction flag (DF)</code> in the <code>flags</code> register</p>
<ul>
<li><code>DF=0</code> increment src/dest registers</li>
<li><code>DF=1</code> decrement src/dest registers</li>
</ul>
<p>Following explanation assumes <code>byte</code> operands with <code>DF=0</code>:</p>
<pre><code class="language-x86asm">movsb   // move data from string to string
        // ES:[DI] &lt;- DS:[SI]
        // DI &lt;- DI + 1
        // SI &lt;- SI + 1

lodsb   // load string
        // AL &lt;- DS:[SI]
        // SI &lt;- SI + 1

stosb   // store string
        // ES:[DI] &lt;- AL
        // DI &lt;- DI + 1

cmpsb   // compare string operands
        // DS:[SI] - ES:[DI]    ; set status flag (eg ZF)
        // SI &lt;- SI + 1
        // DI &lt;- DI + 1

scasb   // scan string
        // AL - ES:[DI]         ; set status flag (eg ZF)
        // DI &lt;- DI + 1
</code></pre>
<p>String operations can be repeated:</p>
<pre><code class="language-x86asm">rep     // repeat until rcx = 0
repz    // repeat until rcx = 0 or while ZF = 0
repnz   // repeat until rcx = 0 or while ZF = 1
</code></pre>
<h3 id="example-simple-memset"><a class="header" href="#example-simple-memset">Example: Simple <code>memset</code></a></h3>
<pre><code class="language-x86asm">// memset (dest, 0xaa /* char */, 0x10 /* len */)

lea di, [dest]
mov al, 0xaa
mov cx, 0x10
rep stosb
</code></pre>
<h2 id="sysv-x86_64-abi"><a class="header" href="#sysv-x86_64-abi"><a href="https://www.uclibc.org/docs/psABI-x86_64.pdf">SysV x86_64 ABI</a></a></h2>
<h3 id="passing-arguments-to-functions"><a class="header" href="#passing-arguments-to-functions">Passing arguments to functions</a></h3>
<ul>
<li>Integer/Pointer arguments
<pre><code class="language-markdown">reg     arg
-----------
rdi       1
rsi       2
rdx       3
rcx       4
r8        5
r9        6
</code></pre>
</li>
<li>Floating point arguments
<pre><code class="language-markdown">reg     arg
-----------
xmm0      1
  ..     ..
xmm7      8
</code></pre>
</li>
<li>Additional arguments are passed on the stack. Arguments are pushed
right-to-left (RTL), meaning next arguments are closer to current <code>rsp</code>.</li>
</ul>
<h3 id="return-values-from-functions"><a class="header" href="#return-values-from-functions">Return values from functions</a></h3>
<ul>
<li>Integer/Pointer return values
<pre><code class="language-markdown">reg          size
-----------------
rax        64 bit
rax+rdx   128 bit
</code></pre>
</li>
<li>Floating point return values:
<pre><code class="language-markdown">reg            size
-------------------
xmm0         64 bit
xmm0+xmm1   128 bit
</code></pre>
</li>
</ul>
<h3 id="caller-saved-registers"><a class="header" href="#caller-saved-registers">Caller saved registers</a></h3>
<p>Caller must save these registers if they should be preserved across function
calls.</p>
<ul>
<li><code>rax</code></li>
<li><code>rcx</code></li>
<li><code>rdx</code></li>
<li><code>rsi</code></li>
<li><code>rdi</code></li>
<li><code>rsp</code></li>
<li><code>r8</code> - <code>r11</code></li>
</ul>
<h3 id="callee-saved-registers"><a class="header" href="#callee-saved-registers">Callee saved registers</a></h3>
<p>Caller can expect these registers to be preserved across function calls. Callee
must must save these registers in case they are used.</p>
<ul>
<li><code>rbx</code></li>
<li><code>rbp</code></li>
<li><code>r12</code> – <code>r15</code></li>
</ul>
<h3 id="stack"><a class="header" href="#stack">Stack</a></h3>
<ul>
<li>grows downwards</li>
<li>frames aligned on 16 byte boundary
<pre><code class="language-text">Hi ADDR
 |                +------------+
 |                | prev frame |
 |                +------------+ &lt;--- 16 byte aligned (X &amp; ~0xf)
 |       [rbp+8]  | saved RIP  |
 |       [rbp]    | saved RBP  |
 |       [rbp-8]  | func stack |
 |                | ...        |
 v                +------------+
Lo ADDR
</code></pre>
</li>
</ul>
<h3 id="function-prologue--epilogue"><a class="header" href="#function-prologue--epilogue">Function prologue &amp; epilogue</a></h3>
<ul>
<li>prologue
<pre><code class="language-x86asm">push rbp        // save caller base pointer
mov rbp, rsp    // save caller stack pointer
</code></pre>
</li>
<li>epilogue
<pre><code class="language-x86asm">mov rsp, rbp    // restore caller stack pointer
pop rbp         // restore caller base pointer
</code></pre>
<blockquote>
<p>Equivalent to <code>leave</code> instruction.</p>
</blockquote>
</li>
</ul>
<h2 id="asm-skeleton"><a class="header" href="#asm-skeleton">ASM skeleton</a></h2>
<p>Small assembler skeleton, ready to use with following properties:</p>
<ul>
<li>use raw Linux syscalls (<code>man 2 syscall</code> for ABI)</li>
<li>no <code>C runtime (crt)</code></li>
<li>gnu assembler <a href="https://sourceware.org/binutils/docs/as"><code>gas</code></a></li>
<li>intel syntax</li>
</ul>
<pre><code class="language-x86asm"># file: greet.s

    .intel_syntax noprefix

    .section .text, &quot;ax&quot;, @progbits
    .global _start
_start:
    mov rdi, 1                      # fd
    lea rsi, [rip + greeting]       # buf
    mov rdx, [rip + greeting_len]   # count
    mov rax, 1                      # write(2) syscall nr
    syscall

    mov rdi, 0                      # exit code
    mov rax, 60                     # exit(2) syscall nr
    syscall

    .section .rdonly, &quot;a&quot;, @progbits
greeting:
    .asciz &quot;Hi ASM-World!\n&quot;
greeting_len:
    .int .-greeting
</code></pre>
<blockquote>
<p>Syscall numbers are defined in <code>/usr/include/asm/unistd.h</code>.</p>
</blockquote>
<p>To compile and run:</p>
<pre><code class="language-bash">&gt; gcc -o greet greet.s -nostartfiles -nostdlib &amp;&amp; ./greet
Hi ASM-World!
</code></pre>
<h2 id="references"><a class="header" href="#references">References</a></h2>
<ul>
<li><a href="https://www.uclibc.org/docs/psABI-x86_64.pdf">SystemV AMD64 ABI</a></li>
<li><a href="https://www.amd.com/system/files/TechDocs/24592.pdf">AMD64 Vol1: Application Programming</a></li>
<li><a href="https://www.amd.com/system/files/TechDocs/24593.pdf">AMD64 Vol2: System Programming</a></li>
<li><a href="https://www.amd.com/system/files/TechDocs/24594.pdf">AMD64 Vol3: General-Purpose &amp; System Instructions</a></li>
<li><a href="https://cs.brown.edu/courses/cs033/docs/guides/x64_cheatsheet.pdf">X86_64 Cheat-Sheet</a></li>
<li><a href="https://software.intel.com/content/www/us/en/develop/download/intel-64-and-ia-32-architectures-software-developers-manual-volume-1-basic-architecture.html">Intel 64 Vol1: Basic Architecture</a></li>
<li><a href="https://software.intel.com/content/www/us/en/develop/download/intel-64-and-ia-32-architectures-sdm-combined-volumes-2a-2b-2c-and-2d-instruction-set-reference-a-z.html">Intel 64 Vol2: Instruction Set Reference</a></li>
<li><a href="https://software.intel.com/content/www/us/en/develop/download/intel-64-and-ia-32-architectures-sdm-combined-volumes-3a-3b-3c-and-3d-system-programming-guide.html">Intel 64 Vol3: System Programming Guide</a></li>
<li><a href="https://sourceware.org/binutils/docs/as">GNU Assembler</a></li>
<li><a href="https://sourceware.org/binutils/docs/as/Pseudo-Ops.html#Pseudo-Ops">GNU Assembler Directives</a></li>
<li><a href="https://sourceware.org/binutils/docs/as/i386_002dDependent.html">GNU Assembler <code>x86_64</code> dependent features</a></li>
</ul>

                    </main>

                    <nav class="nav-wrapper" aria-label="Page navigation">
                        <!-- Mobile navigation buttons -->
                            <a rel="prev" href="../arch/index.html" class="mobile-nav-chapters previous" title="Previous chapter" aria-label="Previous chapter" aria-keyshortcuts="Left">
                                <i class="fa fa-angle-left"></i>
                            </a>
                            <a rel="next" href="../arch/arm64.html" class="mobile-nav-chapters next" title="Next chapter" aria-label="Next chapter" aria-keyshortcuts="Right">
                                <i class="fa fa-angle-right"></i>
                            </a>
                        <div style="clear: both"></div>
                    </nav>
                </div>
            </div>

            <nav class="nav-wide-wrapper" aria-label="Page navigation">
                    <a rel="prev" href="../arch/index.html" class="nav-chapters previous" title="Previous chapter" aria-label="Previous chapter" aria-keyshortcuts="Left">
                        <i class="fa fa-angle-left"></i>
                    </a>
                    <a rel="next" href="../arch/arm64.html" class="nav-chapters next" title="Next chapter" aria-label="Next chapter" aria-keyshortcuts="Right">
                        <i class="fa fa-angle-right"></i>
                    </a>
            </nav>

        </div>

        <script type="text/javascript">
            window.playground_copyable = true;
        </script>
        <script src="../elasticlunr.min.js" type="text/javascript" charset="utf-8"></script>
        <script src="../mark.min.js" type="text/javascript" charset="utf-8"></script>
        <script src="../searcher.js" type="text/javascript" charset="utf-8"></script>
        <script src="../clipboard.min.js" type="text/javascript" charset="utf-8"></script>
        <script src="../highlight.js" type="text/javascript" charset="utf-8"></script>
        <script src="../book.js" type="text/javascript" charset="utf-8"></script>

        <!-- Custom JS scripts -->
    </body>
</html>
<!DOCTYPE HTML>
<html lang="en" class="sidebar-visible no-js light">
    <head>
        <!-- Book generated using mdBook -->
        <meta charset="UTF-8">
        <title>x86_64 - Notes</title>
        <!-- Custom HTML head -->
        <meta content="text/html; charset=utf-8" http-equiv="Content-Type">
        <meta name="description" content="">
        <meta name="viewport" content="width=device-width, initial-scale=1">
        <meta name="theme-color" content="#ffffff" />

        <link rel="icon" href="../favicon.svg">
        <link rel="shortcut icon" href="../favicon.png">
        <link rel="stylesheet" href="../css/variables.css">
        <link rel="stylesheet" href="../css/general.css">
        <link rel="stylesheet" href="../css/chrome.css">
        <link rel="stylesheet" href="../css/print.css" media="print">
        <!-- Fonts -->
        <link rel="stylesheet" href="../FontAwesome/css/font-awesome.css">
        <link rel="stylesheet" href="../fonts/fonts.css">
        <!-- Highlight.js Stylesheets -->
        <link rel="stylesheet" href="../highlight.css">
        <link rel="stylesheet" href="../tomorrow-night.css">
        <link rel="stylesheet" href="../ayu-highlight.css">

        <!-- Custom theme stylesheets -->
    </head>
    <body>
        <!-- Provide site root to javascript -->
        <script type="text/javascript">
            var path_to_root = "../";
            var default_theme = window.matchMedia("(prefers-color-scheme: dark)").matches ? "navy" : "light";
        </script>

        <!-- Work around some values being stored in localStorage wrapped in quotes -->
        <script type="text/javascript">
            try {
                var theme = localStorage.getItem('mdbook-theme');
                var sidebar = localStorage.getItem('mdbook-sidebar');

                if (theme.startsWith('"') && theme.endsWith('"')) {
                    localStorage.setItem('mdbook-theme', theme.slice(1, theme.length - 1));
                }

                if (sidebar.startsWith('"') && sidebar.endsWith('"')) {
                    localStorage.setItem('mdbook-sidebar', sidebar.slice(1, sidebar.length - 1));
                }
            } catch (e) { }
        </script>

        <!-- Set the theme before any content is loaded, prevents flash -->
        <script type="text/javascript">
            var theme;
            try { theme = localStorage.getItem('mdbook-theme'); } catch(e) { }
            if (theme === null || theme === undefined) { theme = default_theme; }
            var html = document.querySelector('html');
            html.classList.remove('no-js')
            html.classList.remove('light')
            html.classList.add(theme);
            html.classList.add('js');
        </script>

        <!-- Hide / unhide sidebar before it is displayed -->
        <script type="text/javascript">
            var html = document.querySelector('html');
            var sidebar = 'hidden';
            if (document.body.clientWidth >= 1080) {
                try { sidebar = localStorage.getItem('mdbook-sidebar'); } catch(e) { }
                sidebar = sidebar || 'visible';
            }
            html.classList.remove('sidebar-visible');
            html.classList.add("sidebar-" + sidebar);
        </script>

        <nav id="sidebar" class="sidebar" aria-label="Table of contents">
            <div class="sidebar-scrollbox">
                <ol class="chapter"><li class="chapter-item expanded affix "><a href="../intro.html">Introduction</a></li><li class="chapter-item expanded "><a href="../tools/index.html"><strong aria-hidden="true">1.</strong> Tools</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../tools/zsh.html"><strong aria-hidden="true">1.1.</strong> zsh</a></li><li class="chapter-item expanded "><a href="../tools/bash.html"><strong aria-hidden="true">1.2.</strong> bash</a></li><li class="chapter-item expanded "><a href="../tools/fish.html"><strong aria-hidden="true">1.3.</strong> fish</a></li><li class="chapter-item expanded "><a href="../tools/tmux.html"><strong aria-hidden="true">1.4.</strong> tmux</a></li><li class="chapter-item expanded "><a href="../tools/git.html"><strong aria-hidden="true">1.5.</strong> git</a></li><li class="chapter-item expanded "><a href="../tools/awk.html"><strong aria-hidden="true">1.6.</strong> awk</a></li><li class="chapter-item expanded "><a href="../tools/emacs.html"><strong aria-hidden="true">1.7.</strong> emacs</a></li><li class="chapter-item expanded "><a href="../tools/gpg.html"><strong aria-hidden="true">1.8.</strong> gpg</a></li><li class="chapter-item expanded "><a href="../tools/gdb.html"><strong aria-hidden="true">1.9.</strong> gdb</a></li><li class="chapter-item expanded "><a href="../tools/gdbserver.html"><strong aria-hidden="true">1.10.</strong> gdbserver</a></li><li class="chapter-item expanded "><a href="../tools/radare2.html"><strong aria-hidden="true">1.11.</strong> radare2</a></li><li class="chapter-item expanded "><a href="../tools/qemu.html"><strong aria-hidden="true">1.12.</strong> qemu</a></li><li class="chapter-item expanded "><a href="../tools/pacman.html"><strong aria-hidden="true">1.13.</strong> pacman</a></li><li class="chapter-item expanded "><a href="../tools/dot.html"><strong aria-hidden="true">1.14.</strong> dot</a></li></ol></li><li class="chapter-item expanded "><a href="../monitor/index.html"><strong aria-hidden="true">2.</strong> Resource analysis & monitor</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../monitor/lsof.html"><strong aria-hidden="true">2.1.</strong> lsof</a></li><li class="chapter-item expanded "><a href="../monitor/ss.html"><strong aria-hidden="true">2.2.</strong> ss</a></li><li class="chapter-item expanded "><a href="../monitor/pidstat.html"><strong aria-hidden="true">2.3.</strong> pidstat</a></li><li class="chapter-item expanded "><a href="../monitor/pgrep.html"><strong aria-hidden="true">2.4.</strong> pgrep</a></li><li class="chapter-item expanded "><a href="../monitor/pmap.html"><strong aria-hidden="true">2.5.</strong> pmap</a></li><li class="chapter-item expanded "><a href="../monitor/pstack.html"><strong aria-hidden="true">2.6.</strong> pstack</a></li></ol></li><li class="chapter-item expanded "><a href="../trace_profile/index.html"><strong aria-hidden="true">3.</strong> Trace and Profile</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../trace_profile/strace.html"><strong aria-hidden="true">3.1.</strong> strace</a></li><li class="chapter-item expanded "><a href="../trace_profile/ltrace.html"><strong aria-hidden="true">3.2.</strong> ltrace</a></li><li class="chapter-item expanded "><a href="../trace_profile/perf.html"><strong aria-hidden="true">3.3.</strong> perf</a></li><li class="chapter-item expanded "><a href="../trace_profile/oprofile.html"><strong aria-hidden="true">3.4.</strong> OProfile</a></li><li class="chapter-item expanded "><a href="../trace_profile/time.html"><strong aria-hidden="true">3.5.</strong> time</a></li></ol></li><li class="chapter-item expanded "><a href="../binary/index.html"><strong aria-hidden="true">4.</strong> Binary</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../binary/od.html"><strong aria-hidden="true">4.1.</strong> od</a></li><li class="chapter-item expanded "><a href="../binary/xxd.html"><strong aria-hidden="true">4.2.</strong> xxd</a></li><li class="chapter-item expanded "><a href="../binary/readelf.html"><strong aria-hidden="true">4.3.</strong> readelf</a></li><li class="chapter-item expanded "><a href="../binary/objdump.html"><strong aria-hidden="true">4.4.</strong> objdump</a></li><li class="chapter-item expanded "><a href="../binary/nm.html"><strong aria-hidden="true">4.5.</strong> nm</a></li></ol></li><li class="chapter-item expanded "><a href="../development/index.html"><strong aria-hidden="true">5.</strong> Development</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../development/c++filt.html"><strong aria-hidden="true">5.1.</strong> c++filt</a></li><li class="chapter-item expanded "><a href="../development/c++.html"><strong aria-hidden="true">5.2.</strong> c++</a></li><li class="chapter-item expanded "><a href="../development/glibc.html"><strong aria-hidden="true">5.3.</strong> glibc</a></li><li class="chapter-item expanded "><a href="../development/gcc.html"><strong aria-hidden="true">5.4.</strong> gcc</a></li><li class="chapter-item expanded "><a href="../development/make.html"><strong aria-hidden="true">5.5.</strong> make</a></li><li class="chapter-item expanded "><a href="../development/ld.so.html"><strong aria-hidden="true">5.6.</strong> ld.so</a></li><li class="chapter-item expanded "><a href="../development/symbolver.html"><strong aria-hidden="true">5.7.</strong> symbol versioning</a></li><li class="chapter-item expanded "><a href="../development/python.html"><strong aria-hidden="true">5.8.</strong> python</a></li></ol></li><li class="chapter-item expanded "><a href="../linux/index.html"><strong aria-hidden="true">6.</strong> Linux</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../linux/systemd.html"><strong aria-hidden="true">6.1.</strong> systemd</a></li><li class="chapter-item expanded "><a href="../linux/coredump.html"><strong aria-hidden="true">6.2.</strong> coredump</a></li><li class="chapter-item expanded "><a href="../linux/ptrace_scope.html"><strong aria-hidden="true">6.3.</strong> ptrace_scope</a></li><li class="chapter-item expanded "><a href="../linux/cryptsetup.html"><strong aria-hidden="true">6.4.</strong> cryptsetup</a></li><li class="chapter-item expanded "><a href="../linux/swap.html"><strong aria-hidden="true">6.5.</strong> swap</a></li></ol></li><li class="chapter-item expanded "><a href="../network/index.html"><strong aria-hidden="true">7.</strong> Network</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../network/tcpdump.html"><strong aria-hidden="true">7.1.</strong> tcpdump</a></li></ol></li><li class="chapter-item expanded "><a href="../web/index.html"><strong aria-hidden="true">8.</strong> Web</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../web/html.html"><strong aria-hidden="true">8.1.</strong> html</a></li></ol></li><li class="chapter-item expanded "><a href="../arch/index.html"><strong aria-hidden="true">9.</strong> Arch</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../arch/x86_64.html" class="active"><strong aria-hidden="true">9.1.</strong> x86_64</a></li><li class="chapter-item expanded "><a href="../arch/arm64.html"><strong aria-hidden="true">9.2.</strong> arm64</a></li><li class="chapter-item expanded "><a href="../arch/armv7.html"><strong aria-hidden="true">9.3.</strong> armv7</a></li><li class="chapter-item expanded "><a href="../arch/riscv.html"><strong aria-hidden="true">9.4.</strong> riscv</a></li></ol></li></ol>
            </div>
            <div id="sidebar-resize-handle" class="sidebar-resize-handle"></div>
        </nav>

        <div id="page-wrapper" class="page-wrapper">

            <div class="page">
                <div id="menu-bar-hover-placeholder"></div>
                <div id="menu-bar" class="menu-bar sticky bordered">
                    <div class="left-buttons">
                        <button id="sidebar-toggle" class="icon-button" type="button" title="Toggle Table of Contents" aria-label="Toggle Table of Contents" aria-controls="sidebar">
                            <i class="fa fa-bars"></i>
                        </button>
                        <button id="theme-toggle" class="icon-button" type="button" title="Change theme" aria-label="Change theme" aria-haspopup="true" aria-expanded="false" aria-controls="theme-list">
                            <i class="fa fa-paint-brush"></i>
                        </button>
                        <ul id="theme-list" class="theme-popup" aria-label="Themes" role="menu">
                            <li role="none"><button role="menuitem" class="theme" id="light">Light (default)</button></li>
                            <li role="none"><button role="menuitem" class="theme" id="rust">Rust</button></li>
                            <li role="none"><button role="menuitem" class="theme" id="coal">Coal</button></li>
                            <li role="none"><button role="menuitem" class="theme" id="navy">Navy</button></li>
                            <li role="none"><button role="menuitem" class="theme" id="ayu">Ayu</button></li>
                        </ul>
                        <button id="search-toggle" class="icon-button" type="button" title="Search. (Shortkey: s)" aria-label="Toggle Searchbar" aria-expanded="false" aria-keyshortcuts="S" aria-controls="searchbar">
                            <i class="fa fa-search"></i>
                        </button>
                    </div>

                    <h1 class="menu-title">Notes</h1>

                    <div class="right-buttons">
                        <a href="../print.html" title="Print this book" aria-label="Print this book">
                            <i id="print-button" class="fa fa-print"></i>
                        </a>
                        <a href="https://github.com/johannst/notes" title="Git repository" aria-label="Git repository">
                            <i id="git-repository-button" class="fa fa-github"></i>
                        </a>
                    </div>
                </div>

                <div id="search-wrapper" class="hidden">
                    <form id="searchbar-outer" class="searchbar-outer">
                        <input type="search" id="searchbar" name="searchbar" placeholder="Search this book ..." aria-controls="searchresults-outer" aria-describedby="searchresults-header">
                    </form>
                    <div id="searchresults-outer" class="searchresults-outer hidden">
                        <div id="searchresults-header" class="searchresults-header"></div>
                        <ul id="searchresults">
                        </ul>
                    </div>
                </div>
                <!-- Apply ARIA attributes after the sidebar and the sidebar toggle button are added to the DOM -->
                <script type="text/javascript">
                    document.getElementById('sidebar-toggle').setAttribute('aria-expanded', sidebar === 'visible');
                    document.getElementById('sidebar').setAttribute('aria-hidden', sidebar !== 'visible');
                    Array.from(document.querySelectorAll('#sidebar a')).forEach(function(link) {
                        link.setAttribute('tabIndex', sidebar === 'visible' ? 0 : -1);
                    });
                </script>

                <div id="content" class="content">
                    <main>
                        <h1 id="x86_64"><a class="header" href="#x86_64">x86_64</a></h1>
<p>keywords: x86_64, x86, abi</p>
<ul>
<li>64bit synonyms: <code>x86_64</code>, <code>x64</code>, <code>amd64</code>, <code>intel 64</code></li>
<li>32bit synonyms: <code>x86</code>, <code>ia32</code>, <code>i386</code></li>
<li>ISA type: <code>CISC</code></li>
<li>Endianness: <code>little</code></li>
</ul>
<h2 id="registers"><a class="header" href="#registers">Registers</a></h2>
<h3 id="general-purpose-register"><a class="header" href="#general-purpose-register">General purpose register</a></h3>
<pre><code class="language-markdown">bytes
[7:0]      [3:0]   [1:0]   [1]   [0]     desc
----------------------------------------------------------
rax        eax     ax      ah    al      accumulator
rbx        ebx     bx      bh    bl      base register
rcx        ecx     cx      ch    cl      counter
rdx        edx     dx      dh    dl      data register
rsi        esi     si      -     sil     source index
rdi        edi     di      -     dil     destination index
rbp        ebp     bp      -     bpl     base pointer
rsp        esp     sp      -     spl     stack pointer
r8-15      rNd     rNw     -     rNb
</code></pre>
<h3 id="special-register"><a class="header" href="#special-register">Special register</a></h3>
<pre><code class="language-markdown">bytes
[7:0]      [3:0]     [1:0]      desc
---------------------------------------------------
rflags     eflags    flags      flags register
rip        eip       ip         instruction pointer
</code></pre>
<h3 id="flags-register"><a class="header" href="#flags-register">FLAGS register</a></h3>
<pre><code class="language-markdown">rflags
bits    desc                            instr        comment
--------------------------------------------------------------------------------------------------------------
   [21]   ID   identification                        ability to set/clear -&gt; indicates support for CPUID instr
   [18]   AC   alignment check                       alignment exception for PL 3 (user), requires CR0.AM
[13:12] IOPL   io privilege level
   [11]   OF   overflow flag
   [10]   DF   direction flag           cld/std      increment (0) or decrement (1) registers in string operations
    [9]   IF   interrupt enable         cli/sti
    [7]   SF   sign flag
    [6]   ZF   zero flag
    [4]   AF   auxiliary carry flag
    [2]   PF   parity flag
    [0]   CF   carry flag
</code></pre>
<p>Change flag bits with <code>pushf</code> / <code>popf</code> instructions:</p>
<pre><code class="language-x86asm">pushfd                          // push flags (4bytes) onto stack
or dword ptr [esp], (1 &lt;&lt; 18)   // enable AC flag
popfd                           // pop flags (4byte) from stack
</code></pre>
<blockquote>
<p>There is also <code>pushfq</code> / <code>popfq</code> to push and pop all 8 bytes of <code>rflags</code>.</p>
</blockquote>
<h3 id="model-specific-register-msr"><a class="header" href="#model-specific-register-msr">Model Specific Register (MSR)</a></h3>
<pre><code class="language-x86asm">rdmsr     // Read MSR register, effectively does EDX:EAX &lt;- MSR[ECX]
wrmsr     // Write MSR register, effectively does MSR[ECX] &lt;- EDX:EAX
</code></pre>
<h2 id="size-directives"><a class="header" href="#size-directives">Size directives</a></h2>
<p>Explicitly specify size of the operation.</p>
<pre><code class="language-x86asm">mov  byte ptr [rax], 0xff    // save 1 byte(s) at [rax]
mov  word ptr [rax], 0xff    // save 2 byte(s) at [rax]
mov dword ptr [rax], 0xff    // save 4 byte(s) at [rax]
mov qword ptr [rax], 0xff    // save 8 byte(s) at [rax]
</code></pre>
<h2 id="addressing"><a class="header" href="#addressing">Addressing</a></h2>
<pre><code class="language-x86asm">mov qword ptr [rax], rbx         // save val in rbx at [rax]
mov qword ptr [imm], rbx         // save val in rbx at [imm]
mov rax, qword ptr [rbx+4*rcx]   // load val at [rbx+4*rcx] into rax
</code></pre>
<p><code>rip</code> relative addressing:</p>
<pre><code class="language-x86asm">lea rax, [rip+.my_str]       // load addr of .my_str into rax
...
.my_str:
.asciz &quot;Foo&quot;
</code></pre>
<h2 id="string-instructions"><a class="header" href="#string-instructions">String instructions</a></h2>
<p>The operand size of a string instruction is defined by the instruction suffix
<code>b | w | d | q</code>.</p>
<p>Source and destination registers are modified according to the <code>direction flag (DF)</code> in the <code>flags</code> register</p>
<ul>
<li><code>DF=0</code> increment src/dest registers</li>
<li><code>DF=1</code> decrement src/dest registers</li>
</ul>
<p>Following explanation assumes <code>byte</code> operands with <code>DF=0</code>:</p>
<pre><code class="language-x86asm">movsb   // move data from string to string
        // ES:[DI] &lt;- DS:[SI]
        // DI &lt;- DI + 1
        // SI &lt;- SI + 1

lodsb   // load string
        // AL &lt;- DS:[SI]
        // SI &lt;- SI + 1

stosb   // store string
        // ES:[DI] &lt;- AL
        // DI &lt;- DI + 1

cmpsb   // compare string operands
        // DS:[SI] - ES:[DI]    ; set status flag (eg ZF)
        // SI &lt;- SI + 1
        // DI &lt;- DI + 1

scasb   // scan string
        // AL - ES:[DI]         ; set status flag (eg ZF)
        // DI &lt;- DI + 1
</code></pre>
<p>String operations can be repeated:</p>
<pre><code class="language-x86asm">rep     // repeat until rcx = 0
repz    // repeat until rcx = 0 or while ZF = 0
repnz   // repeat until rcx = 0 or while ZF = 1
</code></pre>
<h3 id="example-simple-memset"><a class="header" href="#example-simple-memset">Example: Simple <code>memset</code></a></h3>
<pre><code class="language-x86asm">// memset (dest, 0xaa /* char */, 0x10 /* len */)

lea di, [dest]
mov al, 0xaa
mov cx, 0x10
rep stosb
</code></pre>
<h2 id="sysv-x86_64-abi"><a class="header" href="#sysv-x86_64-abi"><a href="https://www.uclibc.org/docs/psABI-x86_64.pdf">SysV x86_64 ABI</a></a></h2>
<h3 id="passing-arguments-to-functions"><a class="header" href="#passing-arguments-to-functions">Passing arguments to functions</a></h3>
<ul>
<li>Integer/Pointer arguments
<pre><code class="language-markdown">reg     arg
-----------
rdi       1
rsi       2
rdx       3
rcx       4
r8        5
r9        6
</code></pre>
</li>
<li>Floating point arguments
<pre><code class="language-markdown">reg     arg
-----------
xmm0      1
  ..     ..
xmm7      8
</code></pre>
</li>
<li>Additional arguments are passed on the stack. Arguments are pushed
right-to-left (RTL), meaning next arguments are closer to current <code>rsp</code>.</li>
</ul>
<h3 id="return-values-from-functions"><a class="header" href="#return-values-from-functions">Return values from functions</a></h3>
<ul>
<li>Integer/Pointer return values
<pre><code class="language-markdown">reg          size
-----------------
rax        64 bit
rax+rdx   128 bit
</code></pre>
</li>
<li>Floating point return values:
<pre><code class="language-markdown">reg            size
-------------------
xmm0         64 bit
xmm0+xmm1   128 bit
</code></pre>
</li>
</ul>
<h3 id="caller-saved-registers"><a class="header" href="#caller-saved-registers">Caller saved registers</a></h3>
<p>Caller must save these registers if they should be preserved across function
calls.</p>
<ul>
<li><code>rax</code></li>
<li><code>rcx</code></li>
<li><code>rdx</code></li>
<li><code>rsi</code></li>
<li><code>rdi</code></li>
<li><code>rsp</code></li>
<li><code>r8</code> - <code>r11</code></li>
</ul>
<h3 id="callee-saved-registers"><a class="header" href="#callee-saved-registers">Callee saved registers</a></h3>
<p>Caller can expect these registers to be preserved across function calls. Callee
must must save these registers in case they are used.</p>
<ul>
<li><code>rbx</code></li>
<li><code>rbp</code></li>
<li><code>r12</code> – <code>r15</code></li>
</ul>
<h3 id="stack"><a class="header" href="#stack">Stack</a></h3>
<ul>
<li>grows downwards</li>
<li>frames aligned on 16 byte boundary
<pre><code class="language-text">Hi ADDR
 |                +------------+
 |                | prev frame |
 |                +------------+ &lt;--- 16 byte aligned (X &amp; ~0xf)
 |       [rbp+8]  | saved RIP  |
 |       [rbp]    | saved RBP  |
 |       [rbp-8]  | func stack |
 |                | ...        |
 v                +------------+
Lo ADDR
</code></pre>
</li>
</ul>
<h3 id="function-prologue--epilogue"><a class="header" href="#function-prologue--epilogue">Function prologue &amp; epilogue</a></h3>
<ul>
<li>prologue
<pre><code class="language-x86asm">push rbp        // save caller base pointer
mov rbp, rsp    // save caller stack pointer
</code></pre>
</li>
<li>epilogue
<pre><code class="language-x86asm">mov rsp, rbp    // restore caller stack pointer
pop rbp         // restore caller base pointer
</code></pre>
<blockquote>
<p>Equivalent to <code>leave</code> instruction.</p>
</blockquote>
</li>
</ul>
<h2 id="asm-skeleton"><a class="header" href="#asm-skeleton">ASM skeleton</a></h2>
<p>Small assembler skeleton, ready to use with following properties:</p>
<ul>
<li>use raw Linux syscalls (<code>man 2 syscall</code> for ABI)</li>
<li>no <code>C runtime (crt)</code></li>
<li>gnu assembler <a href="https://sourceware.org/binutils/docs/as"><code>gas</code></a></li>
<li>intel syntax</li>
</ul>
<pre><code class="language-x86asm"># file: greet.s

    .intel_syntax noprefix

    .section .text, &quot;ax&quot;, @progbits
    .global _start
_start:
    mov rdi, 1                      # fd
    lea rsi, [rip + greeting]       # buf
    mov rdx, [rip + greeting_len]   # count
    mov rax, 1                      # write(2) syscall nr
    syscall

    mov rdi, 0                      # exit code
    mov rax, 60                     # exit(2) syscall nr
    syscall

    .section .rdonly, &quot;a&quot;, @progbits
greeting:
    .asciz &quot;Hi ASM-World!\n&quot;
greeting_len:
    .int .-greeting
</code></pre>
<blockquote>
<p>Syscall numbers are defined in <code>/usr/include/asm/unistd.h</code>.</p>
</blockquote>
<p>To compile and run:</p>
<pre><code class="language-bash">&gt; gcc -o greet greet.s -nostartfiles -nostdlib &amp;&amp; ./greet
Hi ASM-World!
</code></pre>
<h2 id="references"><a class="header" href="#references">References</a></h2>
<ul>
<li><a href="https://www.uclibc.org/docs/psABI-x86_64.pdf">SystemV AMD64 ABI</a></li>
<li><a href="https://www.amd.com/system/files/TechDocs/24592.pdf">AMD64 Vol1: Application Programming</a></li>
<li><a href="https://www.amd.com/system/files/TechDocs/24593.pdf">AMD64 Vol2: System Programming</a></li>
<li><a href="https://www.amd.com/system/files/TechDocs/24594.pdf">AMD64 Vol3: General-Purpose &amp; System Instructions</a></li>
<li><a href="https://cs.brown.edu/courses/cs033/docs/guides/x64_cheatsheet.pdf">X86_64 Cheat-Sheet</a></li>
<li><a href="https://software.intel.com/content/www/us/en/develop/download/intel-64-and-ia-32-architectures-software-developers-manual-volume-1-basic-architecture.html">Intel 64 Vol1: Basic Architecture</a></li>
<li><a href="https://software.intel.com/content/www/us/en/develop/download/intel-64-and-ia-32-architectures-sdm-combined-volumes-2a-2b-2c-and-2d-instruction-set-reference-a-z.html">Intel 64 Vol2: Instruction Set Reference</a></li>
<li><a href="https://software.intel.com/content/www/us/en/develop/download/intel-64-and-ia-32-architectures-sdm-combined-volumes-3a-3b-3c-and-3d-system-programming-guide.html">Intel 64 Vol3: System Programming Guide</a></li>
<li><a href="https://sourceware.org/binutils/docs/as">GNU Assembler</a></li>
<li><a href="https://sourceware.org/binutils/docs/as/Pseudo-Ops.html#Pseudo-Ops">GNU Assembler Directives</a></li>
<li><a href="https://sourceware.org/binutils/docs/as/i386_002dDependent.html">GNU Assembler <code>x86_64</code> dependent features</a></li>
</ul>

                    </main>

                    <nav class="nav-wrapper" aria-label="Page navigation">
                        <!-- Mobile navigation buttons -->
                            <a rel="prev" href="../arch/index.html" class="mobile-nav-chapters previous" title="Previous chapter" aria-label="Previous chapter" aria-keyshortcuts="Left">
                                <i class="fa fa-angle-left"></i>
                            </a>
                            <a rel="next" href="../arch/arm64.html" class="mobile-nav-chapters next" title="Next chapter" aria-label="Next chapter" aria-keyshortcuts="Right">
                                <i class="fa fa-angle-right"></i>
                            </a>
                        <div style="clear: both"></div>
                    </nav>
                </div>
            </div>

            <nav class="nav-wide-wrapper" aria-label="Page navigation">
                    <a rel="prev" href="../arch/index.html" class="nav-chapters previous" title="Previous chapter" aria-label="Previous chapter" aria-keyshortcuts="Left">
                        <i class="fa fa-angle-left"></i>
                    </a>
                    <a rel="next" href="../arch/arm64.html" class="nav-chapters next" title="Next chapter" aria-label="Next chapter" aria-keyshortcuts="Right">
                        <i class="fa fa-angle-right"></i>
                    </a>
            </nav>

        </div>

        <script type="text/javascript">
            window.playground_copyable = true;
        </script>
        <script src="../elasticlunr.min.js" type="text/javascript" charset="utf-8"></script>
        <script src="../mark.min.js" type="text/javascript" charset="utf-8"></script>
        <script src="../searcher.js" type="text/javascript" charset="utf-8"></script>
        <script src="../clipboard.min.js" type="text/javascript" charset="utf-8"></script>
        <script src="../highlight.js" type="text/javascript" charset="utf-8"></script>
        <script src="../book.js" type="text/javascript" charset="utf-8"></script>

        <!-- Custom JS scripts -->
    </body>
</html>