.align 5之类的知识

经常会看到 arm-linux 汇编中有如下的指令:

.align n 它的含义就是使得下面的代码按一定规则对齐。

.align n 指令的对齐值有两种方案:n 或 2^n 。各种平台最初的汇编器一般都不是gas ,采取方案12 的都很多,gas 的目标是取代原来的汇编器,必然要保持和原来汇编器的兼容,因此在gas 中如何解释.align 指令会显得有些混乱,原因在于保持兼容。arm-linux 是按照 2^n 的方案对齐的,需要说明的是这个对齐和ld-script 里的对齐不同,不是一会事。下面的英文就不同平台的对齐进行了说明:版本2.11.92.0.12gasinfo(Mandrake 8.2 上的) 这样说:

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.

从这段文字来看,ARM.align 5就是 25次方对齐,也就是 4 字节对齐,通过反汇编也可以看出对齐方式:

反汇编:

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.

参考链接


.align 5之类的知识

王爽 汇编语言 课程设计2

附上BIOS内存分布图
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