關於arm彙編中的align

經常會看到arm-linux彙編中有如下的指令：
.align n
它的含義就是使得下面的代碼按一定規則對齊，.align n 指令的對齊值有兩種方案，n 或 2^n ，各種平台最初的彙編器一般都不是gas，採取方案1或2的都很多，gas的目標是取代原來的彙編器，必然要保持和原來彙編器的兼容，因此在gas中如何解釋 .align指令會顯得有些混亂，原因在於保持兼容。
arm-linu是按照2^n的方案對齊的，需要說明的是這個對齊和ld-script里的對齊不同，不是一會事。下面的英文就不同平台的對齊進行了說明：
版本2.11.92.0.12的gas的info(Mandrake 8.2上的)這樣說：

The way the required alignment is specified varies from system to system. For the a29k, hppa, m68k, m88k, w65, sparc, and Hitachi SH, and i386 using ELF format, the first expression is the alignment request in bytes. For example `.align 8' advances the location counter until it is a multiple of 8. If the location counter is already a multiple of 8, no change is needed.

For other systems, including the i386 using a.out format, and the arm and strongarm, it is the number of low-order zero bits the location counter must have after advancement. For example `.align 3' advances the location counter until it a multiple of 8. If the location counter is already a multiple of 8, no change is needed.

從這段文字來看，ARM的.align 5就是2的5次方對齊，也就是4位元組對齊，通過反彙編也可以看出對齊方式：
.align 5
stmfd sp!, {r0 - r3, lr}
mov r0, ip
ldmfd sp!, {r0 - r3, pc}^

.align 5
stmfd sp!, {r0 - r3, lr}
mov r0, ip
mov ip, r0
ldmfd sp!, {r0 - r3, pc}^

反彙編：
00000000 <.text>:
0: e92d400f stmdb sp!, {r0, r1, r2, r3, lr}
4: e1a0000c mov r0, ip
8: e8fd800f ldmia sp!, {r0, r1, r2, r3, pc}^
...
20: e92d400f stmdb sp!, {r0, r1, r2, r3, lr}
24: e1a0000c mov r0, ip
28: e1a0c000 mov ip, r0
2c: e8fd800f ldmia sp!, {r0, r1, r2, r3, pc}^
30: e1a00000 nop (mov r0,r0)
34: e1a00000 nop (mov r0,r0)
38: e1a00000 nop (mov r0,r0)
3c: e1a00000 nop (mov r0,r0)

一些忠告：
In the future, everytime when you build an elf file, you need meantime created your map file. And then you will avoid mistakes like this align.

Also, please also pick up some linker script knowlege part. For embedded system, we frequently play the linker script to tune an image, for example, align some special section and so on for protection or/and cache purpose. wish helpful

以上就是我在網上找到並總結的有關.align的一些內容