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<h1><a class="header" href="#armv7a" id="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><a class="header" href="#registers" id="registers">Registers</a></h2>
<h3><a class="header" href="#general-purpose-registers" id="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><a class="header" href="#special-registers" id="special-registers">Special registers</a></h3>
<pre><code class="language-markdown">bytes
[3:0] desc
---------------------------------------------
cpsr current program status register
</code></pre>
<h3><a class="header" href="#cpsr-register" id="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><a class="header" href="#instructions-cheatsheet" id="instructions-cheatsheet">Instructions cheatsheet</a></h2>
<h3><a class="header" href="#accessing-system-registers" id="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><a class="header" href="#control-flow" id="control-flow">Control Flow</a></h3>
<pre><code class="language-armasm">b <lable> // relative forward/back branch
bl <lable> // relative forward/back branch & link return addr in r14 (LR)
// branch & exchange (can change between ARM & Thumb instruction set)
// bit Rm[0] == 0 -> ARM
// bit Rm[0] == 1 -> Thumb
bx <Rm> // absolute branch to address in register Rm
blx <Rm> // absolute branch to address in register Rm &
// link return addr in r14 (LR)
</code></pre>
<h3><a class="header" href="#loadstore" id="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<<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><a class="header" href="#procedure-call-standard-arm-a-hrefhttpsgithubcomarm-softwareabi-aablobmasteraapcs32aapcs32rstaapcs32a" id="procedure-call-standard-arm-a-hrefhttpsgithubcomarm-softwareabi-aablobmasteraapcs32aapcs32rstaapcs32a">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><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
-----------
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
| +-->| a6 | |
+---------->| a5 | <-SP |
+-----+ v
| ... | Lo
</code></pre>
</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
-----------------
r0 32 bit
r0+r1 64 bit
</code></pre>
</li>
</ul>
<h3><a class="header" href="#callee-saved-registers" id="callee-saved-registers">Callee saved registers</a></h3>
<ul>
<li><code>r4</code> - <code>r11</code></li>
<li><code>sp</code></li>
</ul>
<h3><a class="header" href="#stack" id="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><a class="header" href="#frame-chain" id="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 |
+->| 0 | |
| | ... | |
| +------+ |
| | LR | frame1 |
+--| FP |<-+ |
| ... | | |
+------+ | |
| LR | | current |
r11 ->| 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><a class="header" href="#function-prologue--epilogue" id="function-prologue--epilogue">Function prologue & 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><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>
</ul>
<pre><code class="language-armasm">// file: greet.S
#include <asm/unistd.h> // syscall NRs
.arch armv7-a
.section .text, "ax"
.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 // &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, "a"
greeting:
.asciz "Hi ASM-World!\n"
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">> arm-linux-gnueabi-gcc -o greet greet.S -nostartfiles -nostdlib \
-Wl,--dynamic-linker=/usr/arm-linux-gnueabi/lib/ld-linux.so.3 \
&& 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><a class="header" href="#references" id="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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