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<h1><a class="header" href="#x86_64" id="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><a class="header" href="#registers" id="registers">Registers</a></h2>
<h3><a class="header" href="#general-purpose-register" id="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><a class="header" href="#special-register" id="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><a class="header" href="#flags-register" id="flags-register">FLAGS register</a></h3>
<pre><code class="language-markdown">rflags
bits desc
-----------------------------
[11] OF overflow flag
[10] DF direction flag
[7] SF sign flag
[6] ZF zero flag
[4] AF auxiliary carry flag
[2] PF parity flag
[0] CF carry flag
</code></pre>
<h2><a class="header" href="#addressing" id="addressing">Addressing</a></h2>
<pre><code class="language-asm">movw [rax], rbx // save val in rbx at [rax]
movw [imm], rbx // save val in rbx at [imm]
movw rax, [rbx+4*rcx] // load val at [rbx+4*rcx] into rax
</code></pre>
<p><code>rip</code> relative addressing:</p>
<pre><code class="language-asm">lea rax, [rip+.my_str] // load addr of .my_str into rax
...
.my_str:
.asciz "Foo"
</code></pre>
<h2><a class="header" href="#size-directives" id="size-directives">Size directives</a></h2>
<p>Explicitly specify size of the operation.</p>
<pre><code class="language-nasm">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><a class="header" href="#a-hrefhttpswwwuclibcorgdocspsabi-x86_64pdfsysv-x86_64-abia" id="a-hrefhttpswwwuclibcorgdocspsabi-x86_64pdfsysv-x86_64-abia"><a href="https://www.uclibc.org/docs/psABI-x86_64.pdf">SysV x86_64 ABI</a></a></h2>
<h3><a class="header" href="#passing-arguments-to-functions" id="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 7
</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><a class="header" href="#return-values-from-functions" id="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><a class="header" href="#caller-saved-registers" id="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><a class="header" href="#callee-saved-registers" id="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><a class="header" href="#stack" id="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 |
| +------------+ <--- 16 byte aligned (X & ~0xf)
| [rbp+8] | saved RIP |
| [rbp] | saved RBP |
| [rbp-8] | func stack |
| | ... |
v +------------+
LO ADDR
</code></pre>
</li>
</ul>
<h2><a class="header" href="#asm-skeleton" id="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, "ax", @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, 1 # exit(2) syscall nr
syscall
.section .rdonly, "a", @progbits
greeting:
.asciz "Hi ASM-World!\n"
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">> gcc -o greet greet.s -nostartfiles -nostdlib && ./greet
Hi ASM-World!
</code></pre>
<h2><a class="header" href="#references" id="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 & 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>
</ul>
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