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1 | @c Copyright 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005, |
2 | @c 2006, 2007, 2008, 2009 Free Software Foundation, Inc. | |
252b5132 RH |
3 | @c This is part of the GAS manual. |
4 | @c For copying conditions, see the file as.texinfo. | |
5 | ||
6 | @ifset GENERIC | |
7 | @page | |
8 | @node ARM-Dependent | |
9 | @chapter ARM Dependent Features | |
10 | @end ifset | |
11 | ||
12 | @ifclear GENERIC | |
13 | @node Machine Dependencies | |
14 | @chapter ARM Dependent Features | |
15 | @end ifclear | |
16 | ||
17 | @cindex ARM support | |
18 | @cindex Thumb support | |
19 | @menu | |
20 | * ARM Options:: Options | |
21 | * ARM Syntax:: Syntax | |
22 | * ARM Floating Point:: Floating Point | |
23 | * ARM Directives:: ARM Machine Directives | |
24 | * ARM Opcodes:: Opcodes | |
6057a28f | 25 | * ARM Mapping Symbols:: Mapping Symbols |
7da4f750 | 26 | * ARM Unwinding Tutorial:: Unwinding |
252b5132 RH |
27 | @end menu |
28 | ||
29 | @node ARM Options | |
30 | @section Options | |
31 | @cindex ARM options (none) | |
32 | @cindex options for ARM (none) | |
adcf07e6 | 33 | |
252b5132 | 34 | @table @code |
adcf07e6 | 35 | |
03b1477f | 36 | @cindex @code{-mcpu=} command line option, ARM |
92081f48 | 37 | @item -mcpu=@var{processor}[+@var{extension}@dots{}] |
252b5132 RH |
38 | This option specifies the target processor. The assembler will issue an |
39 | error message if an attempt is made to assemble an instruction which | |
03b1477f RE |
40 | will not execute on the target processor. The following processor names are |
41 | recognized: | |
42 | @code{arm1}, | |
43 | @code{arm2}, | |
44 | @code{arm250}, | |
45 | @code{arm3}, | |
46 | @code{arm6}, | |
47 | @code{arm60}, | |
48 | @code{arm600}, | |
49 | @code{arm610}, | |
50 | @code{arm620}, | |
51 | @code{arm7}, | |
52 | @code{arm7m}, | |
53 | @code{arm7d}, | |
54 | @code{arm7dm}, | |
55 | @code{arm7di}, | |
56 | @code{arm7dmi}, | |
57 | @code{arm70}, | |
58 | @code{arm700}, | |
59 | @code{arm700i}, | |
60 | @code{arm710}, | |
61 | @code{arm710t}, | |
62 | @code{arm720}, | |
63 | @code{arm720t}, | |
64 | @code{arm740t}, | |
65 | @code{arm710c}, | |
66 | @code{arm7100}, | |
67 | @code{arm7500}, | |
68 | @code{arm7500fe}, | |
69 | @code{arm7t}, | |
70 | @code{arm7tdmi}, | |
1ff4677c | 71 | @code{arm7tdmi-s}, |
03b1477f RE |
72 | @code{arm8}, |
73 | @code{arm810}, | |
74 | @code{strongarm}, | |
75 | @code{strongarm1}, | |
76 | @code{strongarm110}, | |
77 | @code{strongarm1100}, | |
78 | @code{strongarm1110}, | |
79 | @code{arm9}, | |
80 | @code{arm920}, | |
81 | @code{arm920t}, | |
82 | @code{arm922t}, | |
83 | @code{arm940t}, | |
84 | @code{arm9tdmi}, | |
7fac0536 NC |
85 | @code{fa526} (Faraday FA526 processor), |
86 | @code{fa626} (Faraday FA626 processor), | |
03b1477f | 87 | @code{arm9e}, |
7de9afa2 | 88 | @code{arm926e}, |
1ff4677c | 89 | @code{arm926ej-s}, |
03b1477f RE |
90 | @code{arm946e-r0}, |
91 | @code{arm946e}, | |
db8ac8f9 | 92 | @code{arm946e-s}, |
03b1477f RE |
93 | @code{arm966e-r0}, |
94 | @code{arm966e}, | |
db8ac8f9 PB |
95 | @code{arm966e-s}, |
96 | @code{arm968e-s}, | |
03b1477f | 97 | @code{arm10t}, |
db8ac8f9 | 98 | @code{arm10tdmi}, |
03b1477f RE |
99 | @code{arm10e}, |
100 | @code{arm1020}, | |
101 | @code{arm1020t}, | |
7de9afa2 | 102 | @code{arm1020e}, |
db8ac8f9 | 103 | @code{arm1022e}, |
1ff4677c | 104 | @code{arm1026ej-s}, |
7fac0536 NC |
105 | @code{fa626te} (Faraday FA626TE processor), |
106 | @code{fa726te} (Faraday FA726TE processor), | |
1ff4677c RE |
107 | @code{arm1136j-s}, |
108 | @code{arm1136jf-s}, | |
db8ac8f9 PB |
109 | @code{arm1156t2-s}, |
110 | @code{arm1156t2f-s}, | |
0dd132b6 NC |
111 | @code{arm1176jz-s}, |
112 | @code{arm1176jzf-s}, | |
113 | @code{mpcore}, | |
114 | @code{mpcorenovfp}, | |
b38f9f31 | 115 | @code{cortex-a5}, |
62b3e311 | 116 | @code{cortex-a8}, |
15290f0a | 117 | @code{cortex-a9}, |
dbb1f804 | 118 | @code{cortex-a15}, |
62b3e311 | 119 | @code{cortex-r4}, |
307c948d | 120 | @code{cortex-r4f}, |
62b3e311 | 121 | @code{cortex-m3}, |
5b19eaba NC |
122 | @code{cortex-m1}, |
123 | @code{cortex-m0}, | |
03b1477f RE |
124 | @code{ep9312} (ARM920 with Cirrus Maverick coprocessor), |
125 | @code{i80200} (Intel XScale processor) | |
e16bb312 | 126 | @code{iwmmxt} (Intel(r) XScale processor with Wireless MMX(tm) technology coprocessor) |
03b1477f RE |
127 | and |
128 | @code{xscale}. | |
129 | The special name @code{all} may be used to allow the | |
130 | assembler to accept instructions valid for any ARM processor. | |
131 | ||
132 | In addition to the basic instruction set, the assembler can be told to | |
133 | accept various extension mnemonics that extend the processor using the | |
134 | co-processor instruction space. For example, @code{-mcpu=arm920+maverick} | |
135 | is equivalent to specifying @code{-mcpu=ep9312}. The following extensions | |
136 | are currently supported: | |
137 | @code{+maverick} | |
e16bb312 | 138 | @code{+iwmmxt} |
03b1477f RE |
139 | and |
140 | @code{+xscale}. | |
141 | ||
142 | @cindex @code{-march=} command line option, ARM | |
92081f48 | 143 | @item -march=@var{architecture}[+@var{extension}@dots{}] |
252b5132 RH |
144 | This option specifies the target architecture. The assembler will issue |
145 | an error message if an attempt is made to assemble an instruction which | |
03b1477f RE |
146 | will not execute on the target architecture. The following architecture |
147 | names are recognized: | |
148 | @code{armv1}, | |
149 | @code{armv2}, | |
150 | @code{armv2a}, | |
151 | @code{armv2s}, | |
152 | @code{armv3}, | |
153 | @code{armv3m}, | |
154 | @code{armv4}, | |
155 | @code{armv4xm}, | |
156 | @code{armv4t}, | |
157 | @code{armv4txm}, | |
158 | @code{armv5}, | |
159 | @code{armv5t}, | |
160 | @code{armv5txm}, | |
161 | @code{armv5te}, | |
09d92015 | 162 | @code{armv5texp}, |
c5f98204 | 163 | @code{armv6}, |
1ddd7f43 | 164 | @code{armv6j}, |
0dd132b6 NC |
165 | @code{armv6k}, |
166 | @code{armv6z}, | |
167 | @code{armv6zk}, | |
62b3e311 | 168 | @code{armv7}, |
c450d570 PB |
169 | @code{armv7-a}, |
170 | @code{armv7-r}, | |
171 | @code{armv7-m}, | |
9e3c6df6 | 172 | @code{armv7e-m}, |
e16bb312 | 173 | @code{iwmmxt} |
03b1477f RE |
174 | and |
175 | @code{xscale}. | |
176 | If both @code{-mcpu} and | |
177 | @code{-march} are specified, the assembler will use | |
178 | the setting for @code{-mcpu}. | |
179 | ||
180 | The architecture option can be extended with the same instruction set | |
181 | extension options as the @code{-mcpu} option. | |
182 | ||
183 | @cindex @code{-mfpu=} command line option, ARM | |
184 | @item -mfpu=@var{floating-point-format} | |
185 | ||
186 | This option specifies the floating point format to assemble for. The | |
187 | assembler will issue an error message if an attempt is made to assemble | |
188 | an instruction which will not execute on the target floating point unit. | |
189 | The following format options are recognized: | |
190 | @code{softfpa}, | |
191 | @code{fpe}, | |
bc89618b RE |
192 | @code{fpe2}, |
193 | @code{fpe3}, | |
03b1477f RE |
194 | @code{fpa}, |
195 | @code{fpa10}, | |
196 | @code{fpa11}, | |
197 | @code{arm7500fe}, | |
198 | @code{softvfp}, | |
199 | @code{softvfp+vfp}, | |
200 | @code{vfp}, | |
201 | @code{vfp10}, | |
202 | @code{vfp10-r0}, | |
203 | @code{vfp9}, | |
204 | @code{vfpxd}, | |
62f3b8c8 PB |
205 | @code{vfpv2}, |
206 | @code{vfpv3}, | |
207 | @code{vfpv3-fp16}, | |
208 | @code{vfpv3-d16}, | |
209 | @code{vfpv3-d16-fp16}, | |
210 | @code{vfpv3xd}, | |
211 | @code{vfpv3xd-d16}, | |
212 | @code{vfpv4}, | |
213 | @code{vfpv4-d16}, | |
f0cd0667 | 214 | @code{fpv4-sp-d16}, |
09d92015 MM |
215 | @code{arm1020t}, |
216 | @code{arm1020e}, | |
b1cc4aeb | 217 | @code{arm1136jf-s}, |
62f3b8c8 PB |
218 | @code{maverick}, |
219 | @code{neon}, | |
03b1477f | 220 | and |
62f3b8c8 | 221 | @code{neon-vfpv4}. |
03b1477f RE |
222 | |
223 | In addition to determining which instructions are assembled, this option | |
224 | also affects the way in which the @code{.double} assembler directive behaves | |
225 | when assembling little-endian code. | |
226 | ||
227 | The default is dependent on the processor selected. For Architecture 5 or | |
228 | later, the default is to assembler for VFP instructions; for earlier | |
229 | architectures the default is to assemble for FPA instructions. | |
adcf07e6 | 230 | |
252b5132 RH |
231 | @cindex @code{-mthumb} command line option, ARM |
232 | @item -mthumb | |
03b1477f RE |
233 | This option specifies that the assembler should start assembling Thumb |
234 | instructions; that is, it should behave as though the file starts with a | |
235 | @code{.code 16} directive. | |
adcf07e6 | 236 | |
252b5132 RH |
237 | @cindex @code{-mthumb-interwork} command line option, ARM |
238 | @item -mthumb-interwork | |
239 | This option specifies that the output generated by the assembler should | |
240 | be marked as supporting interworking. | |
adcf07e6 | 241 | |
52970753 NC |
242 | @cindex @code{-mimplicit-it} command line option, ARM |
243 | @item -mimplicit-it=never | |
244 | @itemx -mimplicit-it=always | |
245 | @itemx -mimplicit-it=arm | |
246 | @itemx -mimplicit-it=thumb | |
247 | The @code{-mimplicit-it} option controls the behavior of the assembler when | |
248 | conditional instructions are not enclosed in IT blocks. | |
249 | There are four possible behaviors. | |
250 | If @code{never} is specified, such constructs cause a warning in ARM | |
251 | code and an error in Thumb-2 code. | |
252 | If @code{always} is specified, such constructs are accepted in both | |
253 | ARM and Thumb-2 code, where the IT instruction is added implicitly. | |
254 | If @code{arm} is specified, such constructs are accepted in ARM code | |
255 | and cause an error in Thumb-2 code. | |
256 | If @code{thumb} is specified, such constructs cause a warning in ARM | |
257 | code and are accepted in Thumb-2 code. If you omit this option, the | |
258 | behavior is equivalent to @code{-mimplicit-it=arm}. | |
e07e6e58 | 259 | |
5a5829dd NS |
260 | @cindex @code{-mapcs-26} command line option, ARM |
261 | @cindex @code{-mapcs-32} command line option, ARM | |
262 | @item -mapcs-26 | |
263 | @itemx -mapcs-32 | |
264 | These options specify that the output generated by the assembler should | |
252b5132 RH |
265 | be marked as supporting the indicated version of the Arm Procedure. |
266 | Calling Standard. | |
adcf07e6 | 267 | |
077b8428 NC |
268 | @cindex @code{-matpcs} command line option, ARM |
269 | @item -matpcs | |
270 | This option specifies that the output generated by the assembler should | |
271 | be marked as supporting the Arm/Thumb Procedure Calling Standard. If | |
272 | enabled this option will cause the assembler to create an empty | |
273 | debugging section in the object file called .arm.atpcs. Debuggers can | |
274 | use this to determine the ABI being used by. | |
275 | ||
adcf07e6 | 276 | @cindex @code{-mapcs-float} command line option, ARM |
252b5132 | 277 | @item -mapcs-float |
1be59579 | 278 | This indicates the floating point variant of the APCS should be |
252b5132 | 279 | used. In this variant floating point arguments are passed in FP |
550262c4 | 280 | registers rather than integer registers. |
adcf07e6 NC |
281 | |
282 | @cindex @code{-mapcs-reentrant} command line option, ARM | |
252b5132 RH |
283 | @item -mapcs-reentrant |
284 | This indicates that the reentrant variant of the APCS should be used. | |
285 | This variant supports position independent code. | |
adcf07e6 | 286 | |
33a392fb PB |
287 | @cindex @code{-mfloat-abi=} command line option, ARM |
288 | @item -mfloat-abi=@var{abi} | |
289 | This option specifies that the output generated by the assembler should be | |
290 | marked as using specified floating point ABI. | |
291 | The following values are recognized: | |
292 | @code{soft}, | |
293 | @code{softfp} | |
294 | and | |
295 | @code{hard}. | |
296 | ||
d507cf36 PB |
297 | @cindex @code{-eabi=} command line option, ARM |
298 | @item -meabi=@var{ver} | |
299 | This option specifies which EABI version the produced object files should | |
300 | conform to. | |
b45619c0 | 301 | The following values are recognized: |
3a4a14e9 PB |
302 | @code{gnu}, |
303 | @code{4} | |
d507cf36 | 304 | and |
3a4a14e9 | 305 | @code{5}. |
d507cf36 | 306 | |
252b5132 RH |
307 | @cindex @code{-EB} command line option, ARM |
308 | @item -EB | |
309 | This option specifies that the output generated by the assembler should | |
310 | be marked as being encoded for a big-endian processor. | |
adcf07e6 | 311 | |
252b5132 RH |
312 | @cindex @code{-EL} command line option, ARM |
313 | @item -EL | |
314 | This option specifies that the output generated by the assembler should | |
315 | be marked as being encoded for a little-endian processor. | |
adcf07e6 | 316 | |
252b5132 RH |
317 | @cindex @code{-k} command line option, ARM |
318 | @cindex PIC code generation for ARM | |
319 | @item -k | |
a349d9dd PB |
320 | This option specifies that the output of the assembler should be marked |
321 | as position-independent code (PIC). | |
adcf07e6 | 322 | |
845b51d6 PB |
323 | @cindex @code{--fix-v4bx} command line option, ARM |
324 | @item --fix-v4bx | |
325 | Allow @code{BX} instructions in ARMv4 code. This is intended for use with | |
326 | the linker option of the same name. | |
327 | ||
278df34e NS |
328 | @cindex @code{-mwarn-deprecated} command line option, ARM |
329 | @item -mwarn-deprecated | |
330 | @itemx -mno-warn-deprecated | |
331 | Enable or disable warnings about using deprecated options or | |
332 | features. The default is to warn. | |
333 | ||
252b5132 RH |
334 | @end table |
335 | ||
336 | ||
337 | @node ARM Syntax | |
338 | @section Syntax | |
339 | @menu | |
cab7e4d9 | 340 | * ARM-Instruction-Set:: Instruction Set |
252b5132 RH |
341 | * ARM-Chars:: Special Characters |
342 | * ARM-Regs:: Register Names | |
b6895b4f | 343 | * ARM-Relocations:: Relocations |
99f1a7a7 | 344 | * ARM-Neon-Alignment:: NEON Alignment Specifiers |
252b5132 RH |
345 | @end menu |
346 | ||
cab7e4d9 NC |
347 | @node ARM-Instruction-Set |
348 | @subsection Instruction Set Syntax | |
349 | Two slightly different syntaxes are support for ARM and THUMB | |
350 | instructions. The default, @code{divided}, uses the old style where | |
351 | ARM and THUMB instructions had their own, separate syntaxes. The new, | |
352 | @code{unified} syntax, which can be selected via the @code{.syntax} | |
353 | directive, and has the following main features: | |
354 | ||
355 | @table @bullet | |
356 | @item | |
357 | Immediate operands do not require a @code{#} prefix. | |
358 | ||
359 | @item | |
360 | The @code{IT} instruction may appear, and if it does it is validated | |
361 | against subsequent conditional affixes. In ARM mode it does not | |
362 | generate machine code, in THUMB mode it does. | |
363 | ||
364 | @item | |
365 | For ARM instructions the conditional affixes always appear at the end | |
366 | of the instruction. For THUMB instructions conditional affixes can be | |
367 | used, but only inside the scope of an @code{IT} instruction. | |
368 | ||
369 | @item | |
370 | All of the instructions new to the V6T2 architecture (and later) are | |
371 | available. (Only a few such instructions can be written in the | |
372 | @code{divided} syntax). | |
373 | ||
374 | @item | |
375 | The @code{.N} and @code{.W} suffixes are recognized and honored. | |
376 | ||
377 | @item | |
378 | All instructions set the flags if and only if they have an @code{s} | |
379 | affix. | |
380 | @end table | |
381 | ||
252b5132 RH |
382 | @node ARM-Chars |
383 | @subsection Special Characters | |
384 | ||
385 | @cindex line comment character, ARM | |
386 | @cindex ARM line comment character | |
550262c4 NC |
387 | The presence of a @samp{@@} on a line indicates the start of a comment |
388 | that extends to the end of the current line. If a @samp{#} appears as | |
389 | the first character of a line, the whole line is treated as a comment. | |
390 | ||
391 | @cindex line separator, ARM | |
392 | @cindex statement separator, ARM | |
393 | @cindex ARM line separator | |
a349d9dd PB |
394 | The @samp{;} character can be used instead of a newline to separate |
395 | statements. | |
550262c4 NC |
396 | |
397 | @cindex immediate character, ARM | |
398 | @cindex ARM immediate character | |
399 | Either @samp{#} or @samp{$} can be used to indicate immediate operands. | |
252b5132 RH |
400 | |
401 | @cindex identifiers, ARM | |
402 | @cindex ARM identifiers | |
403 | *TODO* Explain about /data modifier on symbols. | |
404 | ||
405 | @node ARM-Regs | |
406 | @subsection Register Names | |
407 | ||
408 | @cindex ARM register names | |
409 | @cindex register names, ARM | |
410 | *TODO* Explain about ARM register naming, and the predefined names. | |
411 | ||
99f1a7a7 DG |
412 | @node ARM-Neon-Alignment |
413 | @subsection NEON Alignment Specifiers | |
414 | ||
415 | @cindex alignment for NEON instructions | |
416 | Some NEON load/store instructions allow an optional address | |
417 | alignment qualifier. | |
418 | The ARM documentation specifies that this is indicated by | |
419 | @samp{@@ @var{align}}. However GAS already interprets | |
420 | the @samp{@@} character as a "line comment" start, | |
421 | so @samp{: @var{align}} is used instead. For example: | |
422 | ||
423 | @smallexample | |
424 | vld1.8 @{q0@}, [r0, :128] | |
425 | @end smallexample | |
426 | ||
252b5132 RH |
427 | @node ARM Floating Point |
428 | @section Floating Point | |
429 | ||
430 | @cindex floating point, ARM (@sc{ieee}) | |
431 | @cindex ARM floating point (@sc{ieee}) | |
432 | The ARM family uses @sc{ieee} floating-point numbers. | |
433 | ||
b6895b4f PB |
434 | @node ARM-Relocations |
435 | @subsection ARM relocation generation | |
436 | ||
437 | @cindex data relocations, ARM | |
438 | @cindex ARM data relocations | |
439 | Specific data relocations can be generated by putting the relocation name | |
440 | in parentheses after the symbol name. For example: | |
441 | ||
442 | @smallexample | |
443 | .word foo(TARGET1) | |
444 | @end smallexample | |
445 | ||
446 | This will generate an @samp{R_ARM_TARGET1} relocation against the symbol | |
447 | @var{foo}. | |
448 | The following relocations are supported: | |
449 | @code{GOT}, | |
450 | @code{GOTOFF}, | |
451 | @code{TARGET1}, | |
452 | @code{TARGET2}, | |
453 | @code{SBREL}, | |
454 | @code{TLSGD}, | |
455 | @code{TLSLDM}, | |
456 | @code{TLSLDO}, | |
b43420e6 NC |
457 | @code{GOTTPOFF}, |
458 | @code{GOT_PREL} | |
b6895b4f PB |
459 | and |
460 | @code{TPOFF}. | |
461 | ||
462 | For compatibility with older toolchains the assembler also accepts | |
463 | @code{(PLT)} after branch targets. This will generate the deprecated | |
464 | @samp{R_ARM_PLT32} relocation. | |
465 | ||
466 | @cindex MOVW and MOVT relocations, ARM | |
467 | Relocations for @samp{MOVW} and @samp{MOVT} instructions can be generated | |
468 | by prefixing the value with @samp{#:lower16:} and @samp{#:upper16} | |
b45619c0 | 469 | respectively. For example to load the 32-bit address of foo into r0: |
252b5132 | 470 | |
b6895b4f PB |
471 | @smallexample |
472 | MOVW r0, #:lower16:foo | |
473 | MOVT r0, #:upper16:foo | |
474 | @end smallexample | |
252b5132 RH |
475 | |
476 | @node ARM Directives | |
477 | @section ARM Machine Directives | |
478 | ||
479 | @cindex machine directives, ARM | |
480 | @cindex ARM machine directives | |
481 | @table @code | |
482 | ||
4a6bc624 NS |
483 | @c AAAAAAAAAAAAAAAAAAAAAAAAA |
484 | ||
485 | @cindex @code{.2byte} directive, ARM | |
486 | @cindex @code{.4byte} directive, ARM | |
487 | @cindex @code{.8byte} directive, ARM | |
488 | @item .2byte @var{expression} [, @var{expression}]* | |
489 | @itemx .4byte @var{expression} [, @var{expression}]* | |
490 | @itemx .8byte @var{expression} [, @var{expression}]* | |
491 | These directives write 2, 4 or 8 byte values to the output section. | |
492 | ||
493 | @cindex @code{.align} directive, ARM | |
adcf07e6 NC |
494 | @item .align @var{expression} [, @var{expression}] |
495 | This is the generic @var{.align} directive. For the ARM however if the | |
496 | first argument is zero (ie no alignment is needed) the assembler will | |
497 | behave as if the argument had been 2 (ie pad to the next four byte | |
062b7c0c | 498 | boundary). This is for compatibility with ARM's own assembler. |
adcf07e6 | 499 | |
4a6bc624 NS |
500 | @cindex @code{.arch} directive, ARM |
501 | @item .arch @var{name} | |
502 | Select the target architecture. Valid values for @var{name} are the same as | |
503 | for the @option{-march} commandline option. | |
252b5132 | 504 | |
4a6bc624 NS |
505 | @cindex @code{.arm} directive, ARM |
506 | @item .arm | |
507 | This performs the same action as @var{.code 32}. | |
252b5132 | 508 | |
4a6bc624 NS |
509 | @anchor{arm_pad} |
510 | @cindex @code{.pad} directive, ARM | |
511 | @item .pad #@var{count} | |
512 | Generate unwinder annotations for a stack adjustment of @var{count} bytes. | |
513 | A positive value indicates the function prologue allocated stack space by | |
514 | decrementing the stack pointer. | |
0bbf2aa4 | 515 | |
4a6bc624 | 516 | @c BBBBBBBBBBBBBBBBBBBBBBBBBB |
0bbf2aa4 | 517 | |
4a6bc624 NS |
518 | @cindex @code{.bss} directive, ARM |
519 | @item .bss | |
520 | This directive switches to the @code{.bss} section. | |
0bbf2aa4 | 521 | |
4a6bc624 NS |
522 | @c CCCCCCCCCCCCCCCCCCCCCCCCCC |
523 | ||
524 | @cindex @code{.cantunwind} directive, ARM | |
525 | @item .cantunwind | |
526 | Prevents unwinding through the current function. No personality routine | |
527 | or exception table data is required or permitted. | |
528 | ||
529 | @cindex @code{.code} directive, ARM | |
530 | @item .code @code{[16|32]} | |
531 | This directive selects the instruction set being generated. The value 16 | |
532 | selects Thumb, with the value 32 selecting ARM. | |
533 | ||
534 | @cindex @code{.cpu} directive, ARM | |
535 | @item .cpu @var{name} | |
536 | Select the target processor. Valid values for @var{name} are the same as | |
537 | for the @option{-mcpu} commandline option. | |
538 | ||
539 | @c DDDDDDDDDDDDDDDDDDDDDDDDDD | |
540 | ||
541 | @cindex @code{.dn} and @code{.qn} directives, ARM | |
f467aa98 | 542 | @item @var{name} .dn @var{register name} [@var{.type}] [[@var{index}]] |
1f9bb1ca | 543 | @itemx @var{name} .qn @var{register name} [@var{.type}] [[@var{index}]] |
23753660 JB |
544 | |
545 | The @code{dn} and @code{qn} directives are used to create typed | |
546 | and/or indexed register aliases for use in Advanced SIMD Extension | |
547 | (Neon) instructions. The former should be used to create aliases | |
548 | of double-precision registers, and the latter to create aliases of | |
549 | quad-precision registers. | |
550 | ||
551 | If these directives are used to create typed aliases, those aliases can | |
552 | be used in Neon instructions instead of writing types after the mnemonic | |
553 | or after each operand. For example: | |
554 | ||
555 | @smallexample | |
556 | x .dn d2.f32 | |
557 | y .dn d3.f32 | |
558 | z .dn d4.f32[1] | |
559 | vmul x,y,z | |
560 | @end smallexample | |
561 | ||
562 | This is equivalent to writing the following: | |
563 | ||
564 | @smallexample | |
565 | vmul.f32 d2,d3,d4[1] | |
566 | @end smallexample | |
567 | ||
568 | Aliases created using @code{dn} or @code{qn} can be destroyed using | |
569 | @code{unreq}. | |
570 | ||
4a6bc624 | 571 | @c EEEEEEEEEEEEEEEEEEEEEEEEEE |
252b5132 | 572 | |
4a6bc624 NS |
573 | @cindex @code{.eabi_attribute} directive, ARM |
574 | @item .eabi_attribute @var{tag}, @var{value} | |
575 | Set the EABI object attribute @var{tag} to @var{value}. | |
252b5132 | 576 | |
4a6bc624 NS |
577 | The @var{tag} is either an attribute number, or one of the following: |
578 | @code{Tag_CPU_raw_name}, @code{Tag_CPU_name}, @code{Tag_CPU_arch}, | |
579 | @code{Tag_CPU_arch_profile}, @code{Tag_ARM_ISA_use}, | |
75375b3e | 580 | @code{Tag_THUMB_ISA_use}, @code{Tag_FP_arch}, @code{Tag_WMMX_arch}, |
4a6bc624 NS |
581 | @code{Tag_Advanced_SIMD_arch}, @code{Tag_PCS_config}, |
582 | @code{Tag_ABI_PCS_R9_use}, @code{Tag_ABI_PCS_RW_data}, | |
583 | @code{Tag_ABI_PCS_RO_data}, @code{Tag_ABI_PCS_GOT_use}, | |
584 | @code{Tag_ABI_PCS_wchar_t}, @code{Tag_ABI_FP_rounding}, | |
585 | @code{Tag_ABI_FP_denormal}, @code{Tag_ABI_FP_exceptions}, | |
586 | @code{Tag_ABI_FP_user_exceptions}, @code{Tag_ABI_FP_number_model}, | |
75375b3e | 587 | @code{Tag_ABI_align_needed}, @code{Tag_ABI_align_preserved}, |
4a6bc624 NS |
588 | @code{Tag_ABI_enum_size}, @code{Tag_ABI_HardFP_use}, |
589 | @code{Tag_ABI_VFP_args}, @code{Tag_ABI_WMMX_args}, | |
590 | @code{Tag_ABI_optimization_goals}, @code{Tag_ABI_FP_optimization_goals}, | |
591 | @code{Tag_compatibility}, @code{Tag_CPU_unaligned_access}, | |
75375b3e | 592 | @code{Tag_FP_HP_extension}, @code{Tag_ABI_FP_16bit_format}, |
cd21e546 | 593 | @code{Tag_MPextension_use}, @code{Tag_DIV_use}, |
4a6bc624 NS |
594 | @code{Tag_nodefaults}, @code{Tag_also_compatible_with}, |
595 | @code{Tag_conformance}, @code{Tag_T2EE_use}, | |
cd21e546 | 596 | @code{Tag_Virtualization_use} |
4a6bc624 NS |
597 | |
598 | The @var{value} is either a @code{number}, @code{"string"}, or | |
599 | @code{number, "string"} depending on the tag. | |
600 | ||
75375b3e MGD |
601 | Note - the following legacy values are also accepted by @var{tag}: |
602 | @code{Tag_VFP_arch}, @code{Tag_ABI_align8_needed}, | |
603 | @code{Tag_ABI_align8_preserved}, @code{Tag_VFP_HP_extension}, | |
604 | ||
4a6bc624 NS |
605 | @cindex @code{.even} directive, ARM |
606 | @item .even | |
607 | This directive aligns to an even-numbered address. | |
608 | ||
609 | @cindex @code{.extend} directive, ARM | |
610 | @cindex @code{.ldouble} directive, ARM | |
611 | @item .extend @var{expression} [, @var{expression}]* | |
612 | @itemx .ldouble @var{expression} [, @var{expression}]* | |
613 | These directives write 12byte long double floating-point values to the | |
614 | output section. These are not compatible with current ARM processors | |
615 | or ABIs. | |
616 | ||
617 | @c FFFFFFFFFFFFFFFFFFFFFFFFFF | |
618 | ||
619 | @anchor{arm_fnend} | |
620 | @cindex @code{.fnend} directive, ARM | |
621 | @item .fnend | |
622 | Marks the end of a function with an unwind table entry. The unwind index | |
623 | table entry is created when this directive is processed. | |
252b5132 | 624 | |
4a6bc624 NS |
625 | If no personality routine has been specified then standard personality |
626 | routine 0 or 1 will be used, depending on the number of unwind opcodes | |
627 | required. | |
628 | ||
629 | @anchor{arm_fnstart} | |
630 | @cindex @code{.fnstart} directive, ARM | |
631 | @item .fnstart | |
632 | Marks the start of a function with an unwind table entry. | |
633 | ||
634 | @cindex @code{.force_thumb} directive, ARM | |
252b5132 RH |
635 | @item .force_thumb |
636 | This directive forces the selection of Thumb instructions, even if the | |
637 | target processor does not support those instructions | |
638 | ||
4a6bc624 NS |
639 | @cindex @code{.fpu} directive, ARM |
640 | @item .fpu @var{name} | |
641 | Select the floating-point unit to assemble for. Valid values for @var{name} | |
642 | are the same as for the @option{-mfpu} commandline option. | |
252b5132 | 643 | |
4a6bc624 NS |
644 | @c GGGGGGGGGGGGGGGGGGGGGGGGGG |
645 | @c HHHHHHHHHHHHHHHHHHHHHHHHHH | |
e1da3f5b | 646 | |
4a6bc624 NS |
647 | @cindex @code{.handlerdata} directive, ARM |
648 | @item .handlerdata | |
649 | Marks the end of the current function, and the start of the exception table | |
650 | entry for that function. Anything between this directive and the | |
651 | @code{.fnend} directive will be added to the exception table entry. | |
652 | ||
653 | Must be preceded by a @code{.personality} or @code{.personalityindex} | |
654 | directive. | |
655 | ||
656 | @c IIIIIIIIIIIIIIIIIIIIIIIIII | |
c921be7d NC |
657 | |
658 | @cindex @code{.inst} directive, ARM | |
659 | @item .inst @var{opcode} [ , @dots{} ] | |
1f9bb1ca AS |
660 | @itemx .inst.n @var{opcode} [ , @dots{} ] |
661 | @itemx .inst.w @var{opcode} [ , @dots{} ] | |
c921be7d NC |
662 | Generates the instruction corresponding to the numerical value @var{opcode}. |
663 | @code{.inst.n} and @code{.inst.w} allow the Thumb instruction size to be | |
664 | specified explicitly, overriding the normal encoding rules. | |
665 | ||
4a6bc624 NS |
666 | @c JJJJJJJJJJJJJJJJJJJJJJJJJJ |
667 | @c KKKKKKKKKKKKKKKKKKKKKKKKKK | |
668 | @c LLLLLLLLLLLLLLLLLLLLLLLLLL | |
669 | ||
670 | @item .ldouble @var{expression} [, @var{expression}]* | |
671 | See @code{.extend}. | |
5395a469 | 672 | |
252b5132 RH |
673 | @cindex @code{.ltorg} directive, ARM |
674 | @item .ltorg | |
675 | This directive causes the current contents of the literal pool to be | |
676 | dumped into the current section (which is assumed to be the .text | |
677 | section) at the current location (aligned to a word boundary). | |
3d0c9500 NC |
678 | @code{GAS} maintains a separate literal pool for each section and each |
679 | sub-section. The @code{.ltorg} directive will only affect the literal | |
680 | pool of the current section and sub-section. At the end of assembly | |
681 | all remaining, un-empty literal pools will automatically be dumped. | |
682 | ||
683 | Note - older versions of @code{GAS} would dump the current literal | |
684 | pool any time a section change occurred. This is no longer done, since | |
685 | it prevents accurate control of the placement of literal pools. | |
252b5132 | 686 | |
4a6bc624 | 687 | @c MMMMMMMMMMMMMMMMMMMMMMMMMM |
252b5132 | 688 | |
4a6bc624 NS |
689 | @cindex @code{.movsp} directive, ARM |
690 | @item .movsp @var{reg} [, #@var{offset}] | |
691 | Tell the unwinder that @var{reg} contains an offset from the current | |
692 | stack pointer. If @var{offset} is not specified then it is assumed to be | |
693 | zero. | |
7ed4c4c5 | 694 | |
4a6bc624 NS |
695 | @c NNNNNNNNNNNNNNNNNNNNNNNNNN |
696 | @c OOOOOOOOOOOOOOOOOOOOOOOOOO | |
7ed4c4c5 | 697 | |
4a6bc624 NS |
698 | @cindex @code{.object_arch} directive, ARM |
699 | @item .object_arch @var{name} | |
700 | Override the architecture recorded in the EABI object attribute section. | |
701 | Valid values for @var{name} are the same as for the @code{.arch} directive. | |
702 | Typically this is useful when code uses runtime detection of CPU features. | |
7ed4c4c5 | 703 | |
4a6bc624 NS |
704 | @c PPPPPPPPPPPPPPPPPPPPPPPPPP |
705 | ||
706 | @cindex @code{.packed} directive, ARM | |
707 | @item .packed @var{expression} [, @var{expression}]* | |
708 | This directive writes 12-byte packed floating-point values to the | |
709 | output section. These are not compatible with current ARM processors | |
710 | or ABIs. | |
711 | ||
712 | @cindex @code{.pad} directive, ARM | |
713 | @item .pad #@var{count} | |
714 | Generate unwinder annotations for a stack adjustment of @var{count} bytes. | |
715 | A positive value indicates the function prologue allocated stack space by | |
716 | decrementing the stack pointer. | |
7ed4c4c5 NC |
717 | |
718 | @cindex @code{.personality} directive, ARM | |
719 | @item .personality @var{name} | |
720 | Sets the personality routine for the current function to @var{name}. | |
721 | ||
722 | @cindex @code{.personalityindex} directive, ARM | |
723 | @item .personalityindex @var{index} | |
724 | Sets the personality routine for the current function to the EABI standard | |
725 | routine number @var{index} | |
726 | ||
4a6bc624 NS |
727 | @cindex @code{.pool} directive, ARM |
728 | @item .pool | |
729 | This is a synonym for .ltorg. | |
7ed4c4c5 | 730 | |
4a6bc624 NS |
731 | @c QQQQQQQQQQQQQQQQQQQQQQQQQQ |
732 | @c RRRRRRRRRRRRRRRRRRRRRRRRRR | |
733 | ||
734 | @cindex @code{.req} directive, ARM | |
735 | @item @var{name} .req @var{register name} | |
736 | This creates an alias for @var{register name} called @var{name}. For | |
737 | example: | |
738 | ||
739 | @smallexample | |
740 | foo .req r0 | |
741 | @end smallexample | |
742 | ||
743 | @c SSSSSSSSSSSSSSSSSSSSSSSSSS | |
7ed4c4c5 | 744 | |
7da4f750 | 745 | @anchor{arm_save} |
7ed4c4c5 NC |
746 | @cindex @code{.save} directive, ARM |
747 | @item .save @var{reglist} | |
748 | Generate unwinder annotations to restore the registers in @var{reglist}. | |
749 | The format of @var{reglist} is the same as the corresponding store-multiple | |
750 | instruction. | |
751 | ||
752 | @smallexample | |
753 | @exdent @emph{core registers} | |
754 | .save @{r4, r5, r6, lr@} | |
755 | stmfd sp!, @{r4, r5, r6, lr@} | |
756 | @exdent @emph{FPA registers} | |
757 | .save f4, 2 | |
758 | sfmfd f4, 2, [sp]! | |
759 | @exdent @emph{VFP registers} | |
760 | .save @{d8, d9, d10@} | |
fa073d69 | 761 | fstmdx sp!, @{d8, d9, d10@} |
7ed4c4c5 NC |
762 | @exdent @emph{iWMMXt registers} |
763 | .save @{wr10, wr11@} | |
764 | wstrd wr11, [sp, #-8]! | |
765 | wstrd wr10, [sp, #-8]! | |
766 | or | |
767 | .save wr11 | |
768 | wstrd wr11, [sp, #-8]! | |
769 | .save wr10 | |
770 | wstrd wr10, [sp, #-8]! | |
771 | @end smallexample | |
772 | ||
7da4f750 | 773 | @anchor{arm_setfp} |
7ed4c4c5 NC |
774 | @cindex @code{.setfp} directive, ARM |
775 | @item .setfp @var{fpreg}, @var{spreg} [, #@var{offset}] | |
4a6bc624 | 776 | Make all unwinder annotations relative to a frame pointer. Without this |
7ed4c4c5 NC |
777 | the unwinder will use offsets from the stack pointer. |
778 | ||
a5b82cbe | 779 | The syntax of this directive is the same as the @code{add} or @code{mov} |
7ed4c4c5 NC |
780 | instruction used to set the frame pointer. @var{spreg} must be either |
781 | @code{sp} or mentioned in a previous @code{.movsp} directive. | |
782 | ||
783 | @smallexample | |
784 | .movsp ip | |
785 | mov ip, sp | |
786 | @dots{} | |
787 | .setfp fp, ip, #4 | |
a5b82cbe | 788 | add fp, ip, #4 |
7ed4c4c5 NC |
789 | @end smallexample |
790 | ||
4a6bc624 NS |
791 | @cindex @code{.secrel32} directive, ARM |
792 | @item .secrel32 @var{expression} [, @var{expression}]* | |
793 | This directive emits relocations that evaluate to the section-relative | |
794 | offset of each expression's symbol. This directive is only supported | |
795 | for PE targets. | |
796 | ||
cab7e4d9 NC |
797 | @cindex @code{.syntax} directive, ARM |
798 | @item .syntax [@code{unified} | @code{divided}] | |
799 | This directive sets the Instruction Set Syntax as described in the | |
800 | @ref{ARM-Instruction-Set} section. | |
801 | ||
4a6bc624 NS |
802 | @c TTTTTTTTTTTTTTTTTTTTTTTTTT |
803 | ||
804 | @cindex @code{.thumb} directive, ARM | |
805 | @item .thumb | |
806 | This performs the same action as @var{.code 16}. | |
807 | ||
808 | @cindex @code{.thumb_func} directive, ARM | |
809 | @item .thumb_func | |
810 | This directive specifies that the following symbol is the name of a | |
811 | Thumb encoded function. This information is necessary in order to allow | |
812 | the assembler and linker to generate correct code for interworking | |
813 | between Arm and Thumb instructions and should be used even if | |
814 | interworking is not going to be performed. The presence of this | |
815 | directive also implies @code{.thumb} | |
816 | ||
817 | This directive is not neccessary when generating EABI objects. On these | |
818 | targets the encoding is implicit when generating Thumb code. | |
819 | ||
820 | @cindex @code{.thumb_set} directive, ARM | |
821 | @item .thumb_set | |
822 | This performs the equivalent of a @code{.set} directive in that it | |
823 | creates a symbol which is an alias for another symbol (possibly not yet | |
824 | defined). This directive also has the added property in that it marks | |
825 | the aliased symbol as being a thumb function entry point, in the same | |
826 | way that the @code{.thumb_func} directive does. | |
827 | ||
828 | @c UUUUUUUUUUUUUUUUUUUUUUUUUU | |
829 | ||
830 | @cindex @code{.unreq} directive, ARM | |
831 | @item .unreq @var{alias-name} | |
832 | This undefines a register alias which was previously defined using the | |
833 | @code{req}, @code{dn} or @code{qn} directives. For example: | |
834 | ||
835 | @smallexample | |
836 | foo .req r0 | |
837 | .unreq foo | |
838 | @end smallexample | |
839 | ||
840 | An error occurs if the name is undefined. Note - this pseudo op can | |
841 | be used to delete builtin in register name aliases (eg 'r0'). This | |
842 | should only be done if it is really necessary. | |
843 | ||
7ed4c4c5 | 844 | @cindex @code{.unwind_raw} directive, ARM |
4a6bc624 | 845 | @item .unwind_raw @var{offset}, @var{byte1}, @dots{} |
7ed4c4c5 NC |
846 | Insert one of more arbitary unwind opcode bytes, which are known to adjust |
847 | the stack pointer by @var{offset} bytes. | |
848 | ||
849 | For example @code{.unwind_raw 4, 0xb1, 0x01} is equivalent to | |
850 | @code{.save @{r0@}} | |
851 | ||
4a6bc624 | 852 | @c VVVVVVVVVVVVVVVVVVVVVVVVVV |
ee065d83 | 853 | |
4a6bc624 NS |
854 | @cindex @code{.vsave} directive, ARM |
855 | @item .vsave @var{vfp-reglist} | |
856 | Generate unwinder annotations to restore the VFP registers in @var{vfp-reglist} | |
857 | using FLDMD. Also works for VFPv3 registers | |
858 | that are to be restored using VLDM. | |
859 | The format of @var{vfp-reglist} is the same as the corresponding store-multiple | |
860 | instruction. | |
ee065d83 | 861 | |
4a6bc624 NS |
862 | @smallexample |
863 | @exdent @emph{VFP registers} | |
864 | .vsave @{d8, d9, d10@} | |
865 | fstmdd sp!, @{d8, d9, d10@} | |
866 | @exdent @emph{VFPv3 registers} | |
867 | .vsave @{d15, d16, d17@} | |
868 | vstm sp!, @{d15, d16, d17@} | |
869 | @end smallexample | |
e04befd0 | 870 | |
4a6bc624 NS |
871 | Since FLDMX and FSTMX are now deprecated, this directive should be |
872 | used in favour of @code{.save} for saving VFP registers for ARMv6 and above. | |
e04befd0 | 873 | |
4a6bc624 NS |
874 | @c WWWWWWWWWWWWWWWWWWWWWWWWWW |
875 | @c XXXXXXXXXXXXXXXXXXXXXXXXXX | |
876 | @c YYYYYYYYYYYYYYYYYYYYYYYYYY | |
877 | @c ZZZZZZZZZZZZZZZZZZZZZZZZZZ | |
ee065d83 | 878 | |
252b5132 RH |
879 | @end table |
880 | ||
881 | @node ARM Opcodes | |
882 | @section Opcodes | |
883 | ||
884 | @cindex ARM opcodes | |
885 | @cindex opcodes for ARM | |
49a5575c NC |
886 | @code{@value{AS}} implements all the standard ARM opcodes. It also |
887 | implements several pseudo opcodes, including several synthetic load | |
888 | instructions. | |
252b5132 | 889 | |
49a5575c NC |
890 | @table @code |
891 | ||
892 | @cindex @code{NOP} pseudo op, ARM | |
893 | @item NOP | |
894 | @smallexample | |
895 | nop | |
896 | @end smallexample | |
252b5132 | 897 | |
49a5575c NC |
898 | This pseudo op will always evaluate to a legal ARM instruction that does |
899 | nothing. Currently it will evaluate to MOV r0, r0. | |
252b5132 | 900 | |
49a5575c NC |
901 | @cindex @code{LDR reg,=<label>} pseudo op, ARM |
902 | @item LDR | |
252b5132 RH |
903 | @smallexample |
904 | ldr <register> , = <expression> | |
905 | @end smallexample | |
906 | ||
907 | If expression evaluates to a numeric constant then a MOV or MVN | |
908 | instruction will be used in place of the LDR instruction, if the | |
909 | constant can be generated by either of these instructions. Otherwise | |
910 | the constant will be placed into the nearest literal pool (if it not | |
911 | already there) and a PC relative LDR instruction will be generated. | |
912 | ||
49a5575c NC |
913 | @cindex @code{ADR reg,<label>} pseudo op, ARM |
914 | @item ADR | |
915 | @smallexample | |
916 | adr <register> <label> | |
917 | @end smallexample | |
918 | ||
919 | This instruction will load the address of @var{label} into the indicated | |
920 | register. The instruction will evaluate to a PC relative ADD or SUB | |
921 | instruction depending upon where the label is located. If the label is | |
922 | out of range, or if it is not defined in the same file (and section) as | |
923 | the ADR instruction, then an error will be generated. This instruction | |
924 | will not make use of the literal pool. | |
925 | ||
926 | @cindex @code{ADRL reg,<label>} pseudo op, ARM | |
927 | @item ADRL | |
928 | @smallexample | |
929 | adrl <register> <label> | |
930 | @end smallexample | |
931 | ||
932 | This instruction will load the address of @var{label} into the indicated | |
a349d9dd | 933 | register. The instruction will evaluate to one or two PC relative ADD |
49a5575c NC |
934 | or SUB instructions depending upon where the label is located. If a |
935 | second instruction is not needed a NOP instruction will be generated in | |
936 | its place, so that this instruction is always 8 bytes long. | |
937 | ||
938 | If the label is out of range, or if it is not defined in the same file | |
939 | (and section) as the ADRL instruction, then an error will be generated. | |
940 | This instruction will not make use of the literal pool. | |
941 | ||
942 | @end table | |
943 | ||
252b5132 RH |
944 | For information on the ARM or Thumb instruction sets, see @cite{ARM |
945 | Software Development Toolkit Reference Manual}, Advanced RISC Machines | |
946 | Ltd. | |
947 | ||
6057a28f NC |
948 | @node ARM Mapping Symbols |
949 | @section Mapping Symbols | |
950 | ||
951 | The ARM ELF specification requires that special symbols be inserted | |
952 | into object files to mark certain features: | |
953 | ||
954 | @table @code | |
955 | ||
956 | @cindex @code{$a} | |
957 | @item $a | |
958 | At the start of a region of code containing ARM instructions. | |
959 | ||
960 | @cindex @code{$t} | |
961 | @item $t | |
962 | At the start of a region of code containing THUMB instructions. | |
963 | ||
964 | @cindex @code{$d} | |
965 | @item $d | |
966 | At the start of a region of data. | |
967 | ||
968 | @end table | |
969 | ||
970 | The assembler will automatically insert these symbols for you - there | |
971 | is no need to code them yourself. Support for tagging symbols ($b, | |
972 | $f, $p and $m) which is also mentioned in the current ARM ELF | |
973 | specification is not implemented. This is because they have been | |
974 | dropped from the new EABI and so tools cannot rely upon their | |
975 | presence. | |
976 | ||
7da4f750 MM |
977 | @node ARM Unwinding Tutorial |
978 | @section Unwinding | |
979 | ||
980 | The ABI for the ARM Architecture specifies a standard format for | |
981 | exception unwind information. This information is used when an | |
982 | exception is thrown to determine where control should be transferred. | |
983 | In particular, the unwind information is used to determine which | |
984 | function called the function that threw the exception, and which | |
985 | function called that one, and so forth. This information is also used | |
986 | to restore the values of callee-saved registers in the function | |
987 | catching the exception. | |
988 | ||
989 | If you are writing functions in assembly code, and those functions | |
990 | call other functions that throw exceptions, you must use assembly | |
991 | pseudo ops to ensure that appropriate exception unwind information is | |
992 | generated. Otherwise, if one of the functions called by your assembly | |
993 | code throws an exception, the run-time library will be unable to | |
994 | unwind the stack through your assembly code and your program will not | |
995 | behave correctly. | |
996 | ||
997 | To illustrate the use of these pseudo ops, we will examine the code | |
998 | that G++ generates for the following C++ input: | |
999 | ||
1000 | @verbatim | |
1001 | void callee (int *); | |
1002 | ||
1003 | int | |
1004 | caller () | |
1005 | { | |
1006 | int i; | |
1007 | callee (&i); | |
1008 | return i; | |
1009 | } | |
1010 | @end verbatim | |
1011 | ||
1012 | This example does not show how to throw or catch an exception from | |
1013 | assembly code. That is a much more complex operation and should | |
1014 | always be done in a high-level language, such as C++, that directly | |
1015 | supports exceptions. | |
1016 | ||
1017 | The code generated by one particular version of G++ when compiling the | |
1018 | example above is: | |
1019 | ||
1020 | @verbatim | |
1021 | _Z6callerv: | |
1022 | .fnstart | |
1023 | .LFB2: | |
1024 | @ Function supports interworking. | |
1025 | @ args = 0, pretend = 0, frame = 8 | |
1026 | @ frame_needed = 1, uses_anonymous_args = 0 | |
1027 | stmfd sp!, {fp, lr} | |
1028 | .save {fp, lr} | |
1029 | .LCFI0: | |
1030 | .setfp fp, sp, #4 | |
1031 | add fp, sp, #4 | |
1032 | .LCFI1: | |
1033 | .pad #8 | |
1034 | sub sp, sp, #8 | |
1035 | .LCFI2: | |
1036 | sub r3, fp, #8 | |
1037 | mov r0, r3 | |
1038 | bl _Z6calleePi | |
1039 | ldr r3, [fp, #-8] | |
1040 | mov r0, r3 | |
1041 | sub sp, fp, #4 | |
1042 | ldmfd sp!, {fp, lr} | |
1043 | bx lr | |
1044 | .LFE2: | |
1045 | .fnend | |
1046 | @end verbatim | |
1047 | ||
1048 | Of course, the sequence of instructions varies based on the options | |
1049 | you pass to GCC and on the version of GCC in use. The exact | |
1050 | instructions are not important since we are focusing on the pseudo ops | |
1051 | that are used to generate unwind information. | |
1052 | ||
1053 | An important assumption made by the unwinder is that the stack frame | |
1054 | does not change during the body of the function. In particular, since | |
1055 | we assume that the assembly code does not itself throw an exception, | |
1056 | the only point where an exception can be thrown is from a call, such | |
1057 | as the @code{bl} instruction above. At each call site, the same saved | |
1058 | registers (including @code{lr}, which indicates the return address) | |
1059 | must be located in the same locations relative to the frame pointer. | |
1060 | ||
1061 | The @code{.fnstart} (@pxref{arm_fnstart,,.fnstart pseudo op}) pseudo | |
1062 | op appears immediately before the first instruction of the function | |
1063 | while the @code{.fnend} (@pxref{arm_fnend,,.fnend pseudo op}) pseudo | |
1064 | op appears immediately after the last instruction of the function. | |
1065 | These pseudo ops specify the range of the function. | |
1066 | ||
1067 | Only the order of the other pseudos ops (e.g., @code{.setfp} or | |
1068 | @code{.pad}) matters; their exact locations are irrelevant. In the | |
1069 | example above, the compiler emits the pseudo ops with particular | |
1070 | instructions. That makes it easier to understand the code, but it is | |
1071 | not required for correctness. It would work just as well to emit all | |
1072 | of the pseudo ops other than @code{.fnend} in the same order, but | |
1073 | immediately after @code{.fnstart}. | |
1074 | ||
1075 | The @code{.save} (@pxref{arm_save,,.save pseudo op}) pseudo op | |
1076 | indicates registers that have been saved to the stack so that they can | |
1077 | be restored before the function returns. The argument to the | |
1078 | @code{.save} pseudo op is a list of registers to save. If a register | |
1079 | is ``callee-saved'' (as specified by the ABI) and is modified by the | |
1080 | function you are writing, then your code must save the value before it | |
1081 | is modified and restore the original value before the function | |
1082 | returns. If an exception is thrown, the run-time library restores the | |
1083 | values of these registers from their locations on the stack before | |
1084 | returning control to the exception handler. (Of course, if an | |
1085 | exception is not thrown, the function that contains the @code{.save} | |
1086 | pseudo op restores these registers in the function epilogue, as is | |
1087 | done with the @code{ldmfd} instruction above.) | |
1088 | ||
1089 | You do not have to save callee-saved registers at the very beginning | |
1090 | of the function and you do not need to use the @code{.save} pseudo op | |
1091 | immediately following the point at which the registers are saved. | |
1092 | However, if you modify a callee-saved register, you must save it on | |
1093 | the stack before modifying it and before calling any functions which | |
1094 | might throw an exception. And, you must use the @code{.save} pseudo | |
1095 | op to indicate that you have done so. | |
1096 | ||
1097 | The @code{.pad} (@pxref{arm_pad,,.pad}) pseudo op indicates a | |
1098 | modification of the stack pointer that does not save any registers. | |
1099 | The argument is the number of bytes (in decimal) that are subtracted | |
1100 | from the stack pointer. (On ARM CPUs, the stack grows downwards, so | |
1101 | subtracting from the stack pointer increases the size of the stack.) | |
1102 | ||
1103 | The @code{.setfp} (@pxref{arm_setfp,,.setfp pseudo op}) pseudo op | |
1104 | indicates the register that contains the frame pointer. The first | |
1105 | argument is the register that is set, which is typically @code{fp}. | |
1106 | The second argument indicates the register from which the frame | |
1107 | pointer takes its value. The third argument, if present, is the value | |
1108 | (in decimal) added to the register specified by the second argument to | |
1109 | compute the value of the frame pointer. You should not modify the | |
1110 | frame pointer in the body of the function. | |
1111 | ||
1112 | If you do not use a frame pointer, then you should not use the | |
1113 | @code{.setfp} pseudo op. If you do not use a frame pointer, then you | |
1114 | should avoid modifying the stack pointer outside of the function | |
1115 | prologue. Otherwise, the run-time library will be unable to find | |
1116 | saved registers when it is unwinding the stack. | |
1117 | ||
1118 | The pseudo ops described above are sufficient for writing assembly | |
1119 | code that calls functions which may throw exceptions. If you need to | |
1120 | know more about the object-file format used to represent unwind | |
1121 | information, you may consult the @cite{Exception Handling ABI for the | |
1122 | ARM Architecture} available from @uref{http://infocenter.arm.com}. |