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                <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 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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 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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="../arch/index.html"><strong aria-hidden="true">8.</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">8.1.</strong> x86_64</a></li><li class="chapter-item expanded "><a href="../arch/arm64.html"><strong aria-hidden="true">8.2.</strong> arm64</a></li><li class="chapter-item expanded "><a href="../arch/armv7.html"><strong aria-hidden="true">8.3.</strong> armv7</a></li><li class="chapter-item expanded "><a href="../arch/riscv.html"><strong aria-hidden="true">8.4.</strong> riscv</a></li></ol></li></ol>
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                        <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>

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                        <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>

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