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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 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"><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" class="active"><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>
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                        <h1 id="armv7a"><a class="header" href="#armv7a">armv7a</a></h1>
<p>keywords: arm, armv7, abi</p>
<ul>
<li>ISA type: <code>RISC</code></li>
<li>Endianness: <code>little</code>, <code>big</code></li>
</ul>
<h2 id="registers"><a class="header" href="#registers">Registers</a></h2>
<h3 id="general-purpose-registers"><a class="header" href="#general-purpose-registers">General purpose registers</a></h3>
<pre><code class="language-markdown">bytes
[3:0]     alt     desc
---------------------------------------------
r0-r12            general purpose registers
r11       fp
r13       sp      stack pointer
r14       lr      link register
r15       pc      program counter
</code></pre>
<h3 id="special-registers"><a class="header" href="#special-registers">Special registers</a></h3>
<pre><code class="language-markdown">bytes
[3:0]             desc
---------------------------------------------
cpsr              current program status register
</code></pre>
<h3 id="cpsr-register"><a class="header" href="#cpsr-register">CPSR register</a></h3>
<pre><code class="language-markdown">cpsr
bits  desc
-----------------------------
 [31]  N negative flag
 [30]  Z zero flag
 [29]  C carry flag
 [28]  V overflow flag
 [27]  Q cummulative saturation (sticky)
  [9]  E load/store endianness
  [8]  A disable asynchronous aborts
  [7]  I disable IRQ
  [6]  F disable FIQ
  [5]  T indicate Thumb state
[4:0]  M process mode (USR, FIQ, IRQ, SVC, ABT, UND, SYS)
</code></pre>
<h2 id="instructions-cheatsheet"><a class="header" href="#instructions-cheatsheet">Instructions cheatsheet</a></h2>
<h3 id="accessing-system-registers"><a class="header" href="#accessing-system-registers">Accessing system registers</a></h3>
<p>Reading from system registers:</p>
<pre><code class="language-armasm">mrs r0, cpsr      // move cpsr into r0
</code></pre>
<p>Writing to system registers:</p>
<pre><code class="language-armasm">msr cpsr, r0      // move r0 into cpsr
</code></pre>
<h3 id="control-flow"><a class="header" href="#control-flow">Control Flow</a></h3>
<pre><code class="language-armasm">b &lt;lable&gt;     // relative forward/back branch
bl &lt;lable&gt;    // relative forward/back branch &amp; link return addr in r14 (LR)

// branch &amp; exchange (can change between ARM &amp; Thumb instruction set)
//   bit Rm[0] == 0 -&gt; ARM
//   bit Rm[0] == 1 -&gt; Thumb
bx &lt;Rm&gt;       // absolute branch to address in register Rm
blx &lt;Rm&gt;      // absolute branch to address in register Rm &amp;
              // link return addr in r14 (LR)
</code></pre>
<h3 id="loadstore"><a class="header" href="#loadstore">Load/Store</a></h3>
<p>Different addressing modes.</p>
<pre><code class="language-armasm">ldr r1, [r0]                // r1 = [r0]
ldr r1, [r0, #4]            // r1 = [r0+4]

ldr r1, [r0, #4]!           // pre-inc : r0+=4; r1 = [r0]
ldr r1, [r0], #4            // post-inc: [r0] = r1; r0+=4

ldr r0, [r1, r2, lsl #3]    // r0 = [r1 + (r2&lt;&lt;3)]
</code></pre>
<p>Load/store multiple registers full-descending.</p>
<pre><code class="language-armasm">stmfd r0!, {r1-r2, r5}    // r0-=4; [r0]=r5
                          // r0-=4; [r0]=r2
                          // r0-=4; [r0]=r1
ldmfd r0!, {r1-r2, r5}    // r1=[r0]; r0+=4
                          // r2=[r0]; r0+=4
                          // r5=[r0]; r0+=4
</code></pre>
<blockquote>
<p><code>!</code> is optional but has the effect to update the base pointer register <code>r0</code> here.</p>
</blockquote>
<p>Push/Pop</p>
<pre><code class="language-armasm">push {r0-r2}    // effectively stmfd sp!, {r0-r2}
pop {r0-r2}     // effectively ldmfd sp!, {r0-r2}
</code></pre>
<h2 id="procedure-call-standard-arm-aapcs32"><a class="header" href="#procedure-call-standard-arm-aapcs32">Procedure Call Standard ARM (<a href="https://github.com/ARM-software/abi-aa/blob/master/aapcs32/aapcs32.rst"><code>aapcs32</code></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
-----------
r0        1
..       ..
r3        4
</code></pre>
</li>
<li>a double word (64bit) is passed in two consecutive registers (eg <code>r1+r2</code>)</li>
<li>additional arguments are passed on the stack. Arguments are pushed
<code>right-to-left (RTL)</code>, meaning next arguments are closer to current <code>sp</code>.
<pre><code class="language-markdown">void take(..., int a5, int a6);
                   |       |   | ... |       Hi
                   |       +--&gt;| a6  |       |
                   +----------&gt;| a5  | &lt;-SP  |
                               +-----+       v
                               | ... |       Lo
</code></pre>
</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
-----------------
r0         32 bit
r0+r1      64 bit
</code></pre>
</li>
</ul>
<h3 id="callee-saved-registers"><a class="header" href="#callee-saved-registers">Callee saved registers</a></h3>
<ul>
<li><code>r4</code> - <code>r11</code></li>
<li><code>sp</code></li>
</ul>
<h3 id="stack"><a class="header" href="#stack">Stack</a></h3>
<ul>
<li>full descending
<ul>
<li>full: <code>sp</code> points to the last used location (valid item)</li>
<li>descending: stack grows downwards</li>
</ul>
</li>
<li><code>sp</code> must be 4byte aligned (word boundary) at all time</li>
<li><code>sp</code> must be 8byte aligned on public interface interfaces</li>
</ul>
<h3 id="frame-chain"><a class="header" href="#frame-chain">Frame chain</a></h3>
<ul>
<li>not strictly required by each platform</li>
<li>linked list of stack-frames</li>
<li>each frame links to the frame of its caller by a <code>frame record</code>
<ul>
<li>a frame record is described as a <code>(FP,LR)</code> pair (2x32bit)</li>
</ul>
</li>
<li><code>r11 (FP)</code> must point to the frame record of the current stack-frame
<pre><code class="language-markdown">      +------+                   Hi
      |   0  |     frame0        |
   +-&gt;|   0  |                   |
   |  |  ... |                   |
   |  +------+                   |
   |  |  LR  |     frame1        |
   +--|  FP  |&lt;-+                |
      | ...  |  |                |
      +------+  |                |
      |  LR  |  |  current       |
r11 -&gt;|  FP  |--+  frame         v
      | ...  |                   Lo
</code></pre>
</li>
<li>end of the frame chain is indicated by following frame record <code>(0,-)</code></li>
<li>location of the frame record in the stack frame is not specified</li>
<li><code>r11</code> is not updated before the new frame record is fully constructed</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-armasm">push {fp, lr}
mov fp, sp              // FP points to frame record
</code></pre>
</li>
<li>epilogue
<pre><code class="language-armasm">pop {fp, pc}            // pop LR directly into PC
</code></pre>
</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>
</ul>
<pre><code class="language-armasm">// file: greet.S

#include &lt;asm/unistd.h&gt;      // syscall NRs

    .arch armv7-a

    .section .text, &quot;ax&quot;
    .balign 4

    // Emit `arm` instructions, same as `.arm` directive.
    .code 32
    .global _start
_start:
    // Branch with link and exchange instruction set.
    blx _do_greet

    mov r0, #0               // exit code
    mov r7, #__NR_exit       // exit(2) syscall
    swi 0x0

    // Emit `thumb` instructions, same as `.thumb` directive.
    .code 16
    .thumb_func
_do_greet:
    mov r0, #2               // fd
    ldr r1, =greeting        // buf
    ldr r2, =greeting_len    // &amp;len
    ldr r2, [r2]             // len
    mov r7, #__NR_write      // write(2) syscall
    swi 0x0

    // Branch and exchange instruction set.
    bx lr

    .balign 8                // align data on 8byte boundary
    .section .rodata, &quot;a&quot;
greeting:
    .asciz &quot;Hi ASM-World!\n&quot;
greeting_len:
    .int .-greeting
</code></pre>
<blockquote>
<p>man gcc: <code>file.S</code> assembler code that must be preprocessed.</p>
</blockquote>
<p>To cross-compile and run:</p>
<pre><code class="language-bash">&gt; arm-linux-gnueabi-gcc -o greet greet.S -nostartfiles -nostdlib  \
    -Wl,--dynamic-linker=/usr/arm-linux-gnueabi/lib/ld-linux.so.3 \
  &amp;&amp; qemu-arm ./greet
Hi ASM-World!
</code></pre>
<blockquote>
<p>Cross-compiling on <code>Ubuntu 20.04 (x86_64)</code>, paths might differ on other
distributions. Explicitly specifying the dynamic linker should not be
required when compiling natively on arm.</p>
</blockquote>
<h2 id="references"><a class="header" href="#references">References</a></h2>
<ul>
<li><a href="https://github.com/ARM-software/abi-aa/blob/master/aapcs32/aapcs32.rst">Procedure Call Standard ARM</a></li>
<li><a href="https://developer.arm.com/documentation/den0013/latest">ARMv7-A Programmer's Guide</a></li>
<li><a href="https://developer.arm.com/documentation/ddi0406/latest">ARMv7-A Architecture Reference Manual</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/ARM_002dDependent.html">GNU Assembler <code>ARM</code> dependent features</a></li>
</ul>

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                        <h1 id="armv7a"><a class="header" href="#armv7a">armv7a</a></h1>
<p>keywords: arm, armv7, abi</p>
<ul>
<li>ISA type: <code>RISC</code></li>
<li>Endianness: <code>little</code>, <code>big</code></li>
</ul>
<h2 id="registers"><a class="header" href="#registers">Registers</a></h2>
<h3 id="general-purpose-registers"><a class="header" href="#general-purpose-registers">General purpose registers</a></h3>
<pre><code class="language-markdown">bytes
[3:0]     alt     desc
---------------------------------------------
r0-r12            general purpose registers
r11       fp
r13       sp      stack pointer
r14       lr      link register
r15       pc      program counter
</code></pre>
<h3 id="special-registers"><a class="header" href="#special-registers">Special registers</a></h3>
<pre><code class="language-markdown">bytes
[3:0]             desc
---------------------------------------------
cpsr              current program status register
</code></pre>
<h3 id="cpsr-register"><a class="header" href="#cpsr-register">CPSR register</a></h3>
<pre><code class="language-markdown">cpsr
bits  desc
-----------------------------
 [31]  N negative flag
 [30]  Z zero flag
 [29]  C carry flag
 [28]  V overflow flag
 [27]  Q cummulative saturation (sticky)
  [9]  E load/store endianness
  [8]  A disable asynchronous aborts
  [7]  I disable IRQ
  [6]  F disable FIQ
  [5]  T indicate Thumb state
[4:0]  M process mode (USR, FIQ, IRQ, SVC, ABT, UND, SYS)
</code></pre>
<h2 id="instructions-cheatsheet"><a class="header" href="#instructions-cheatsheet">Instructions cheatsheet</a></h2>
<h3 id="accessing-system-registers"><a class="header" href="#accessing-system-registers">Accessing system registers</a></h3>
<p>Reading from system registers:</p>
<pre><code class="language-armasm">mrs r0, cpsr      // move cpsr into r0
</code></pre>
<p>Writing to system registers:</p>
<pre><code class="language-armasm">msr cpsr, r0      // move r0 into cpsr
</code></pre>
<h3 id="control-flow"><a class="header" href="#control-flow">Control Flow</a></h3>
<pre><code class="language-armasm">b &lt;lable&gt;     // relative forward/back branch
bl &lt;lable&gt;    // relative forward/back branch &amp; link return addr in r14 (LR)

// branch &amp; exchange (can change between ARM &amp; Thumb instruction set)
//   bit Rm[0] == 0 -&gt; ARM
//   bit Rm[0] == 1 -&gt; Thumb
bx &lt;Rm&gt;       // absolute branch to address in register Rm
blx &lt;Rm&gt;      // absolute branch to address in register Rm &amp;
              // link return addr in r14 (LR)
</code></pre>
<h3 id="loadstore"><a class="header" href="#loadstore">Load/Store</a></h3>
<p>Different addressing modes.</p>
<pre><code class="language-armasm">ldr r1, [r0]                // r1 = [r0]
ldr r1, [r0, #4]            // r1 = [r0+4]

ldr r1, [r0, #4]!           // pre-inc : r0+=4; r1 = [r0]
ldr r1, [r0], #4            // post-inc: [r0] = r1; r0+=4

ldr r0, [r1, r2, lsl #3]    // r0 = [r1 + (r2&lt;&lt;3)]
</code></pre>
<p>Load/store multiple registers full-descending.</p>
<pre><code class="language-armasm">stmfd r0!, {r1-r2, r5}    // r0-=4; [r0]=r5
                          // r0-=4; [r0]=r2
                          // r0-=4; [r0]=r1
ldmfd r0!, {r1-r2, r5}    // r1=[r0]; r0+=4
                          // r2=[r0]; r0+=4
                          // r5=[r0]; r0+=4
</code></pre>
<blockquote>
<p><code>!</code> is optional but has the effect to update the base pointer register <code>r0</code> here.</p>
</blockquote>
<p>Push/Pop</p>
<pre><code class="language-armasm">push {r0-r2}    // effectively stmfd sp!, {r0-r2}
pop {r0-r2}     // effectively ldmfd sp!, {r0-r2}
</code></pre>
<h2 id="procedure-call-standard-arm-aapcs32"><a class="header" href="#procedure-call-standard-arm-aapcs32">Procedure Call Standard ARM (<a href="https://github.com/ARM-software/abi-aa/blob/master/aapcs32/aapcs32.rst"><code>aapcs32</code></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
-----------
r0        1
..       ..
r3        4
</code></pre>
</li>
<li>a double word (64bit) is passed in two consecutive registers (eg <code>r1+r2</code>)</li>
<li>additional arguments are passed on the stack. Arguments are pushed
<code>right-to-left (RTL)</code>, meaning next arguments are closer to current <code>sp</code>.
<pre><code class="language-markdown">void take(..., int a5, int a6);
                   |       |   | ... |       Hi
                   |       +--&gt;| a6  |       |
                   +----------&gt;| a5  | &lt;-SP  |
                               +-----+       v
                               | ... |       Lo
</code></pre>
</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
-----------------
r0         32 bit
r0+r1      64 bit
</code></pre>
</li>
</ul>
<h3 id="callee-saved-registers"><a class="header" href="#callee-saved-registers">Callee saved registers</a></h3>
<ul>
<li><code>r4</code> - <code>r11</code></li>
<li><code>sp</code></li>
</ul>
<h3 id="stack"><a class="header" href="#stack">Stack</a></h3>
<ul>
<li>full descending
<ul>
<li>full: <code>sp</code> points to the last used location (valid item)</li>
<li>descending: stack grows downwards</li>
</ul>
</li>
<li><code>sp</code> must be 4byte aligned (word boundary) at all time</li>
<li><code>sp</code> must be 8byte aligned on public interface interfaces</li>
</ul>
<h3 id="frame-chain"><a class="header" href="#frame-chain">Frame chain</a></h3>
<ul>
<li>not strictly required by each platform</li>
<li>linked list of stack-frames</li>
<li>each frame links to the frame of its caller by a <code>frame record</code>
<ul>
<li>a frame record is described as a <code>(FP,LR)</code> pair (2x32bit)</li>
</ul>
</li>
<li><code>r11 (FP)</code> must point to the frame record of the current stack-frame
<pre><code class="language-markdown">      +------+                   Hi
      |   0  |     frame0        |
   +-&gt;|   0  |                   |
   |  |  ... |                   |
   |  +------+                   |
   |  |  LR  |     frame1        |
   +--|  FP  |&lt;-+                |
      | ...  |  |                |
      +------+  |                |
      |  LR  |  |  current       |
r11 -&gt;|  FP  |--+  frame         v
      | ...  |                   Lo
</code></pre>
</li>
<li>end of the frame chain is indicated by following frame record <code>(0,-)</code></li>
<li>location of the frame record in the stack frame is not specified</li>
<li><code>r11</code> is not updated before the new frame record is fully constructed</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-armasm">push {fp, lr}
mov fp, sp              // FP points to frame record
</code></pre>
</li>
<li>epilogue
<pre><code class="language-armasm">pop {fp, pc}            // pop LR directly into PC
</code></pre>
</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>
</ul>
<pre><code class="language-armasm">// file: greet.S

#include &lt;asm/unistd.h&gt;      // syscall NRs

    .arch armv7-a

    .section .text, &quot;ax&quot;
    .balign 4

    // Emit `arm` instructions, same as `.arm` directive.
    .code 32
    .global _start
_start:
    // Branch with link and exchange instruction set.
    blx _do_greet

    mov r0, #0               // exit code
    mov r7, #__NR_exit       // exit(2) syscall
    swi 0x0

    // Emit `thumb` instructions, same as `.thumb` directive.
    .code 16
    .thumb_func
_do_greet:
    mov r0, #2               // fd
    ldr r1, =greeting        // buf
    ldr r2, =greeting_len    // &amp;len
    ldr r2, [r2]             // len
    mov r7, #__NR_write      // write(2) syscall
    swi 0x0

    // Branch and exchange instruction set.
    bx lr

    .balign 8                // align data on 8byte boundary
    .section .rodata, &quot;a&quot;
greeting:
    .asciz &quot;Hi ASM-World!\n&quot;
greeting_len:
    .int .-greeting
</code></pre>
<blockquote>
<p>man gcc: <code>file.S</code> assembler code that must be preprocessed.</p>
</blockquote>
<p>To cross-compile and run:</p>
<pre><code class="language-bash">&gt; arm-linux-gnueabi-gcc -o greet greet.S -nostartfiles -nostdlib  \
    -Wl,--dynamic-linker=/usr/arm-linux-gnueabi/lib/ld-linux.so.3 \
  &amp;&amp; qemu-arm ./greet
Hi ASM-World!
</code></pre>
<blockquote>
<p>Cross-compiling on <code>Ubuntu 20.04 (x86_64)</code>, paths might differ on other
distributions. Explicitly specifying the dynamic linker should not be
required when compiling natively on arm.</p>
</blockquote>
<h2 id="references"><a class="header" href="#references">References</a></h2>
<ul>
<li><a href="https://github.com/ARM-software/abi-aa/blob/master/aapcs32/aapcs32.rst">Procedure Call Standard ARM</a></li>
<li><a href="https://developer.arm.com/documentation/den0013/latest">ARMv7-A Programmer's Guide</a></li>
<li><a href="https://developer.arm.com/documentation/ddi0406/latest">ARMv7-A Architecture Reference Manual</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/ARM_002dDependent.html">GNU Assembler <code>ARM</code> dependent features</a></li>
</ul>

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