aboutsummaryrefslogtreecommitdiffhomepage
path: root/arch/armv7.html
blob: df97c2bb86fa2f7c42e3b05e909dbf6daf200ef1 (plain) (blame)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
<!DOCTYPE HTML>
<html lang="en" class="light" dir="ltr">
    <head>
        <!-- Book generated using mdBook -->
        <meta charset="UTF-8">
        <title>armv7 - Notes</title>


        <!-- Custom HTML head -->
        
        <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 class="sidebar-visible no-js">
    <div id="body-container">
        <!-- Provide site root to javascript -->
        <script>
            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>
            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>
            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('light')
            html.classList.add(theme);
            var body = document.querySelector('body');
            body.classList.remove('no-js')
            body.classList.add('js');
        </script>

        <input type="checkbox" id="sidebar-toggle-anchor" class="hidden">

        <!-- Hide / unhide sidebar before it is displayed -->
        <script>
            var body = document.querySelector('body');
            var sidebar = null;
            var sidebar_toggle = document.getElementById("sidebar-toggle-anchor");
            if (document.body.clientWidth >= 1080) {
                try { sidebar = localStorage.getItem('mdbook-sidebar'); } catch(e) { }
                sidebar = sidebar || 'visible';
            } else {
                sidebar = 'hidden';
            }
            sidebar_toggle.checked = sidebar === 'visible';
            body.classList.remove('sidebar-visible');
            body.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="../shells/index.html"><strong aria-hidden="true">1.</strong> Shells</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../shells/zsh.html"><strong aria-hidden="true">1.1.</strong> zsh</a></li><li class="chapter-item expanded "><a href="../shells/bash.html"><strong aria-hidden="true">1.2.</strong> bash</a></li><li class="chapter-item expanded "><a href="../shells/fish.html"><strong aria-hidden="true">1.3.</strong> fish</a></li></ol></li><li class="chapter-item expanded "><a href="../cli/index.html"><strong aria-hidden="true">2.</strong> CLI foo</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../cli/awk.html"><strong aria-hidden="true">2.1.</strong> awk</a></li><li class="chapter-item expanded "><a href="../cli/sed.html"><strong aria-hidden="true">2.2.</strong> sed</a></li><li class="chapter-item expanded "><a href="../cli/column.html"><strong aria-hidden="true">2.3.</strong> column</a></li><li class="chapter-item expanded "><a href="../cli/sort.html"><strong aria-hidden="true">2.4.</strong> sort</a></li><li class="chapter-item expanded "><a href="../cli/tr.html"><strong aria-hidden="true">2.5.</strong> tr</a></li><li class="chapter-item expanded "><a href="../cli/tac.html"><strong aria-hidden="true">2.6.</strong> tac</a></li><li class="chapter-item expanded "><a href="../cli/paste.html"><strong aria-hidden="true">2.7.</strong> paste</a></li></ol></li><li class="chapter-item expanded "><a href="../tools/index.html"><strong aria-hidden="true">3.</strong> Tools</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../tools/tmux.html"><strong aria-hidden="true">3.1.</strong> tmux</a></li><li class="chapter-item expanded "><a href="../tools/screen.html"><strong aria-hidden="true">3.2.</strong> screen</a></li><li class="chapter-item expanded "><a href="../tools/emacs.html"><strong aria-hidden="true">3.3.</strong> emacs</a></li><li class="chapter-item expanded "><a href="../tools/gpg.html"><strong aria-hidden="true">3.4.</strong> gpg</a></li><li class="chapter-item expanded "><a href="../tools/radare2.html"><strong aria-hidden="true">3.5.</strong> radare2</a></li><li class="chapter-item expanded "><a href="../tools/qemu.html"><strong aria-hidden="true">3.6.</strong> qemu</a></li><li class="chapter-item expanded "><a href="../tools/pacman.html"><strong aria-hidden="true">3.7.</strong> pacman</a></li><li class="chapter-item expanded "><a href="../tools/dot.html"><strong aria-hidden="true">3.8.</strong> dot</a></li><li class="chapter-item expanded "><a href="../tools/ffmpeg.html"><strong aria-hidden="true">3.9.</strong> ffmpeg</a></li><li class="chapter-item expanded "><a href="../tools/gnuplot.html"><strong aria-hidden="true">3.10.</strong> gnuplot</a></li><li class="chapter-item expanded "><a href="../tools/restic.html"><strong aria-hidden="true">3.11.</strong> restic</a></li><li class="chapter-item expanded "><a href="../tools/qrencode.html"><strong aria-hidden="true">3.12.</strong> qrencode</a></li></ol></li><li class="chapter-item expanded "><a href="../process/index.html"><strong aria-hidden="true">4.</strong> Process management & inspection</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../process/lsof.html"><strong aria-hidden="true">4.1.</strong> lsof</a></li><li class="chapter-item expanded "><a href="../process/pidstat.html"><strong aria-hidden="true">4.2.</strong> pidstat</a></li><li class="chapter-item expanded "><a href="../process/pgrep.html"><strong aria-hidden="true">4.3.</strong> pgrep</a></li><li class="chapter-item expanded "><a href="../process/ps.html"><strong aria-hidden="true">4.4.</strong> ps</a></li><li class="chapter-item expanded "><a href="../process/pmap.html"><strong aria-hidden="true">4.5.</strong> pmap</a></li><li class="chapter-item expanded "><a href="../process/pstack.html"><strong aria-hidden="true">4.6.</strong> pstack</a></li><li class="chapter-item expanded "><a href="../process/taskset.html"><strong aria-hidden="true">4.7.</strong> taskset</a></li><li class="chapter-item expanded "><a href="../process/nice.html"><strong aria-hidden="true">4.8.</strong> nice</a></li></ol></li><li class="chapter-item expanded "><a href="../trace_profile/index.html"><strong aria-hidden="true">5.</strong> Trace and Profile</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../trace_profile/time.html"><strong aria-hidden="true">5.1.</strong> time</a></li><li class="chapter-item expanded "><a href="../trace_profile/strace.html"><strong aria-hidden="true">5.2.</strong> strace</a></li><li class="chapter-item expanded "><a href="../trace_profile/ltrace.html"><strong aria-hidden="true">5.3.</strong> ltrace</a></li><li class="chapter-item expanded "><a href="../trace_profile/perf.html"><strong aria-hidden="true">5.4.</strong> perf</a></li><li class="chapter-item expanded "><a href="../trace_profile/oprofile.html"><strong aria-hidden="true">5.5.</strong> OProfile</a></li><li class="chapter-item expanded "><a href="../trace_profile/callgrind.html"><strong aria-hidden="true">5.6.</strong> callgrind</a></li><li class="chapter-item expanded "><a href="../trace_profile/valgrind.html"><strong aria-hidden="true">5.7.</strong> valgrind</a></li></ol></li><li class="chapter-item expanded "><a href="../debug/index.html"><strong aria-hidden="true">6.</strong> Debug</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../debug/gdb.html"><strong aria-hidden="true">6.1.</strong> gdb</a></li><li class="chapter-item expanded "><a href="../debug/gdbserver.html"><strong aria-hidden="true">6.2.</strong> gdbserver</a></li></ol></li><li class="chapter-item expanded "><a href="../binary/index.html"><strong aria-hidden="true">7.</strong> Binary</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../binary/od.html"><strong aria-hidden="true">7.1.</strong> od</a></li><li class="chapter-item expanded "><a href="../binary/xxd.html"><strong aria-hidden="true">7.2.</strong> xxd</a></li><li class="chapter-item expanded "><a href="../binary/readelf.html"><strong aria-hidden="true">7.3.</strong> readelf</a></li><li class="chapter-item expanded "><a href="../binary/objdump.html"><strong aria-hidden="true">7.4.</strong> objdump</a></li><li class="chapter-item expanded "><a href="../binary/nm.html"><strong aria-hidden="true">7.5.</strong> nm</a></li></ol></li><li class="chapter-item expanded "><a href="../development/index.html"><strong aria-hidden="true">8.</strong> Development</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../development/c++filt.html"><strong aria-hidden="true">8.1.</strong> c++filt</a></li><li class="chapter-item expanded "><a href="../development/c++.html"><strong aria-hidden="true">8.2.</strong> c++</a></li><li class="chapter-item expanded "><a href="../development/glibc.html"><strong aria-hidden="true">8.3.</strong> glibc</a></li><li class="chapter-item expanded "><a href="../development/gcc.html"><strong aria-hidden="true">8.4.</strong> gcc</a></li><li class="chapter-item expanded "><a href="../development/git.html"><strong aria-hidden="true">8.5.</strong> git</a></li><li class="chapter-item expanded "><a href="../development/cmake.html"><strong aria-hidden="true">8.6.</strong> cmake</a></li><li class="chapter-item expanded "><a href="../development/make.html"><strong aria-hidden="true">8.7.</strong> make</a></li><li class="chapter-item expanded "><a href="../development/ld.so.html"><strong aria-hidden="true">8.8.</strong> ld.so</a></li><li class="chapter-item expanded "><a href="../development/symbolver.html"><strong aria-hidden="true">8.9.</strong> symbol versioning</a></li><li class="chapter-item expanded "><a href="../development/python.html"><strong aria-hidden="true">8.10.</strong> python</a></li><li class="chapter-item expanded "><a href="../development/gcov.html"><strong aria-hidden="true">8.11.</strong> gcov</a></li><li class="chapter-item expanded "><a href="../development/pgo.html"><strong aria-hidden="true">8.12.</strong> pgo</a></li></ol></li><li class="chapter-item expanded "><a href="../linux/index.html"><strong aria-hidden="true">9.</strong> Linux</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../linux/systemd.html"><strong aria-hidden="true">9.1.</strong> systemd</a></li><li class="chapter-item expanded "><a href="../linux/coredump.html"><strong aria-hidden="true">9.2.</strong> coredump</a></li><li class="chapter-item expanded "><a href="../linux/ptrace_scope.html"><strong aria-hidden="true">9.3.</strong> ptrace_scope</a></li><li class="chapter-item expanded "><a href="../linux/cryptsetup.html"><strong aria-hidden="true">9.4.</strong> cryptsetup</a></li><li class="chapter-item expanded "><a href="../linux/swap.html"><strong aria-hidden="true">9.5.</strong> swap</a></li><li class="chapter-item expanded "><a href="../linux/input.html"><strong aria-hidden="true">9.6.</strong> input</a></li><li class="chapter-item expanded "><a href="../linux/acl.html"><strong aria-hidden="true">9.7.</strong> acl</a></li><li class="chapter-item expanded "><a href="../linux/zfs.html"><strong aria-hidden="true">9.8.</strong> zfs</a></li><li class="chapter-item expanded "><a href="../linux/cpufreq.html"><strong aria-hidden="true">9.9.</strong> cpufreq</a></li><li class="chapter-item expanded "><a href="../linux/cups.html"><strong aria-hidden="true">9.10.</strong> cups</a></li></ol></li><li class="chapter-item expanded "><a href="../network/index.html"><strong aria-hidden="true">10.</strong> Network</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../network/ssh.html"><strong aria-hidden="true">10.1.</strong> ssh</a></li><li class="chapter-item expanded "><a href="../network/ss.html"><strong aria-hidden="true">10.2.</strong> ss</a></li><li class="chapter-item expanded "><a href="../network/tcpdump.html"><strong aria-hidden="true">10.3.</strong> tcpdump</a></li><li class="chapter-item expanded "><a href="../network/tshark.html"><strong aria-hidden="true">10.4.</strong> tshark</a></li><li class="chapter-item expanded "><a href="../network/firewall-cmd.html"><strong aria-hidden="true">10.5.</strong> firewall-cmd</a></li><li class="chapter-item expanded "><a href="../network/nftables.html"><strong aria-hidden="true">10.6.</strong> nftables</a></li></ol></li><li class="chapter-item expanded "><a href="../web/index.html"><strong aria-hidden="true">11.</strong> Web</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../web/html.html"><strong aria-hidden="true">11.1.</strong> html</a></li><li class="chapter-item expanded "><a href="../web/css.html"><strong aria-hidden="true">11.2.</strong> css</a></li><li class="chapter-item expanded "><a href="../web/chartjs.html"><strong aria-hidden="true">11.3.</strong> chartjs</a></li><li class="chapter-item expanded "><a href="../web/plotly.html"><strong aria-hidden="true">11.4.</strong> plotly</a></li></ol></li><li class="chapter-item expanded "><a href="../arch/index.html"><strong aria-hidden="true">12.</strong> Arch</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../arch/x86_64.html"><strong aria-hidden="true">12.1.</strong> x86_64</a></li><li class="chapter-item expanded "><a href="../arch/armv8.html"><strong aria-hidden="true">12.2.</strong> armv8</a></li><li class="chapter-item expanded "><a href="../arch/arm64.html"><strong aria-hidden="true">12.3.</strong> arm64</a></li><li class="chapter-item expanded "><a href="../arch/armv7.html" class="active"><strong aria-hidden="true">12.4.</strong> armv7</a></li><li class="chapter-item expanded "><a href="../arch/riscv.html"><strong aria-hidden="true">12.5.</strong> riscv</a></li></ol></li></ol>
            </div>
            <div id="sidebar-resize-handle" class="sidebar-resize-handle">
                <div class="sidebar-resize-indicator"></div>
            </div>
        </nav>

        <!-- Track and set sidebar scroll position -->
        <script>
            var sidebarScrollbox = document.querySelector('#sidebar .sidebar-scrollbox');
            sidebarScrollbox.addEventListener('click', function(e) {
                if (e.target.tagName === 'A') {
                    sessionStorage.setItem('sidebar-scroll', sidebarScrollbox.scrollTop);
                }
            }, { passive: true });
            var sidebarScrollTop = sessionStorage.getItem('sidebar-scroll');
            sessionStorage.removeItem('sidebar-scroll');
            if (sidebarScrollTop) {
                // preserve sidebar scroll position when navigating via links within sidebar
                sidebarScrollbox.scrollTop = sidebarScrollTop;
            } else {
                // scroll sidebar to current active section when navigating via "next/previous chapter" buttons
                var activeSection = document.querySelector('#sidebar .active');
                if (activeSection) {
                    activeSection.scrollIntoView({ block: 'center' });
                }
            }
        </script>

        <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">
                    <div class="left-buttons">
                        <label id="sidebar-toggle" class="icon-button" for="sidebar-toggle-anchor" title="Toggle Table of Contents" aria-label="Toggle Table of Contents" aria-controls="sidebar">
                            <i class="fa fa-bars"></i>
                        </label>
                        <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</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>
                    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="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, "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    // &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, "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">&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>

                    </main>

                    <nav class="nav-wrapper" aria-label="Page navigation">
                        <!-- Mobile navigation buttons -->
                            <a rel="prev" href="../arch/arm64.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 prefetch" href="../arch/riscv.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/arm64.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 prefetch" href="../arch/riscv.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>
            window.playground_copyable = true;
        </script>


        <script src="../elasticlunr.min.js"></script>
        <script src="../mark.min.js"></script>
        <script src="../searcher.js"></script>

        <script src="../clipboard.min.js"></script>
        <script src="../highlight.js"></script>
        <script src="../book.js"></script>

        <!-- Custom JS scripts -->


    </div>
    </body>
</html>