原文:https://blog.csdn.net/zhangjun1992/article/details/21962093
在Linux源代码中，以.S为扩展名的文件是“纯”汇编语言的文件。这里，我们结合具体的例子再介绍一些AT&T汇编语言的相关知识。
1．GNU汇编程序GAS（GNU Assembly和连接程序
当你编写了一个程序后，就需要对其进行汇编（assembly）和连接。在Linux下有两种方式，一种是使用汇编程序GAS和连接程序ld，一种是使用gcc。我们先来看一下GAS和ld:
GAS把汇编语言源文件（.o）转换为目标文件（.o），其基本语法如下：
as filename.s -o filename.o
一旦创建了一个目标文件，就需要把它连接并执行，连接一个目标文件的基本语法为：
ld filename.o -o filename
这里 filename.o是目标文件名，而filename 是输出(可执行) 文件。
GAS使用的是AT&T的语法而不是Intel的语法，这就再次说明了AT&T语法是Unix世界的标准，你必须熟悉它。
如果要使用GNC的C编译器gcc，就可以一步完成汇编和连接，例如：
gcc -o example example.S
这里，example.S是你的汇编程序，输出文件（可执行文件）名为example。其中，扩展名必须为大写的S，这是因为，大写的S可以使gcc自动识别汇编程序中的C预处理命令，像#include、#define、#ifdef、 #endif等，也就是说，使用gcc进行编译，你可以在汇编程序中使用C的预处理命令。
重点是：我们的汇编程序的后缀必须.S 注意是大写S，不是小写s
参考：http://oss.org.cn/kernel-book/ch02/2.6.2.htm
http://www.kerneltravel.net/chenlj/lecture6.pdf
http://blog.csdn.net/kevinshq/article/details/8228810 GNU汇编风格
---------------------------------
汇编语言中出现#include, #define的问题
U_boot源文件中，一些 .S文件中出现了#include, #define，如board\smdk2410目录下的lowlevel_init.S文件：
#include <config.h>
#include <version.h>
/* some parameters for the board */
/*
*
* Taken from linux/arch/arm/boot/compressed/head-s3c2410.S
*
* Copyright (C) 2002 Samsung Electronics SW.LEE <[email protected]>
*
*/
#define BWSCON 0x48000000
/* BWSCON */
#define DW8 (0x0)
#define DW16 (0x1)
#define DW32 (0x2)
#define WAIT (0x1<<2)
#define UBLB (0x1<<3)
#define B1_BWSCON (DW32)
#define B2_BWSCON (DW16)
#define B3_BWSCON (DW16 + WAIT + UBLB)
#define B4_BWSCON (DW16)
#define B5_BWSCON (DW16)
#define B6_BWSCON (DW32)
#define B7_BWSCON (DW32)
/* BANK0CON */
#define B0_Tacs 0x0 /* 0clk */
#define B0_Tcos 0x0 /* 0clk */
#define B0_Tacc 0x7 /* 14clk */
#define B0_Tcoh 0x0 /* 0clk */
#define B0_Tah 0x0 /* 0clk */
#define B0_Tacp 0x0
#define B0_PMC 0x0 /* normal */
/* BANK1CON */
#define B1_Tacs 0x0 /* 0clk */
#define B1_Tcos 0x0 /* 0clk */
#define B1_Tacc 0x7 /* 14clk */
#define B1_Tcoh 0x0 /* 0clk */
#define B1_Tah 0x0 /* 0clk */
#define B1_Tacp 0x0
#define B1_PMC 0x0
#define B2_Tacs 0x0
#define B2_Tcos 0x0
#define B2_Tacc 0x7
#define B2_Tcoh 0x0
#define B2_Tah 0x0
#define B2_Tacp 0x0
#define B2_PMC 0x0
#define B3_Tacs 0x0 /* 0clk */
#define B3_Tcos 0x3 /* 4clk */
#define B3_Tacc 0x7 /* 14clk */
#define B3_Tcoh 0x1 /* 1clk */
#define B3_Tah 0x0 /* 0clk */
#define B3_Tacp 0x3 /* 6clk */
#define B3_PMC 0x0 /* normal */
#define B4_Tacs 0x0 /* 0clk */
#define B4_Tcos 0x0 /* 0clk */
#define B4_Tacc 0x7 /* 14clk */
#define B4_Tcoh 0x0 /* 0clk */
#define B4_Tah 0x0 /* 0clk */
#define B4_Tacp 0x0
#define B4_PMC 0x0 /* normal */
#define B5_Tacs 0x0 /* 0clk */
#define B5_Tcos 0x0 /* 0clk */
#define B5_Tacc 0x7 /* 14clk */
#define B5_Tcoh 0x0 /* 0clk */
#define B5_Tah 0x0 /* 0clk */
#define B5_Tacp 0x0
#define B5_PMC 0x0 /* normal */
#define B6_MT 0x3 /* SDRAM */
#define B6_Trcd 0x1
#define B6_SCAN 0x1 /* 9bit */
#define B7_MT 0x3 /* SDRAM */
#define B7_Trcd 0x1 /* 3clk */
#define B7_SCAN 0x1 /* 9bit */
/* REFRESH parameter */
#define REFEN 0x1 /* Refresh enable */
#define TREFMD 0x0 /* CBR(CAS before RAS)/Auto refresh */
#define Trp 0x0 /* 2clk */
#define Trc 0x3 /* 7clk */
#define Tchr 0x2 /* 3clk */
#define REFCNT 1113 /* period=15.6us, HCLK=60Mhz, (2048+1-15.6*60) */
/**************************************/
_TEXT_BASE:
.word TEXT_BASE
.globl lowlevel_init
lowlevel_init:
/* memory control configuration */
/* make r0 relative the current location so that it */
/* reads SMRDATA out of FLASH rather than memory ! */
ldr r0, =SMRDATA
ldr r1, _TEXT_BASE
sub r0, r0, r1
ldr r1, =BWSCON /* Bus Width Status Controller */
add r2, r0, #13*4
0:
ldr r3, [r0], #4
str r3, [r1], #4
cmp r2, r0
bne 0b
/* everything is fine now */
mov pc, lr
.ltorg
/* the literal pools origin */
SMRDATA:
.word (0+(B1_BWSCON<<4)+(B2_BWSCON<<8)+(B3_BWSCON<<12)+(B4_BWSCON<<16)+(B5_BWSCON<<20)+(B6_BWSCON<<24)+(B7_BWSCON<<28))
.word ((B0_Tacs<<13)+(B0_Tcos<<11)+(B0_Tacc<<8)+(B0_Tcoh<<6)+(B0_Tah<<4)+(B0_Tacp<<2)+(B0_PMC))
.word ((B1_Tacs<<13)+(B1_Tcos<<11)+(B1_Tacc<<8)+(B1_Tcoh<<6)+(B1_Tah<<4)+(B1_Tacp<<2)+(B1_PMC))
.word ((B2_Tacs<<13)+(B2_Tcos<<11)+(B2_Tacc<<8)+(B2_Tcoh<<6)+(B2_Tah<<4)+(B2_Tacp<<2)+(B2_PMC))
.word ((B3_Tacs<<13)+(B3_Tcos<<11)+(B3_Tacc<<8)+(B3_Tcoh<<6)+(B3_Tah<<4)+(B3_Tacp<<2)+(B3_PMC))
.word ((B4_Tacs<<13)+(B4_Tcos<<11)+(B4_Tacc<<8)+(B4_Tcoh<<6)+(B4_Tah<<4)+(B4_Tacp<<2)+(B4_PMC))
.word ((B5_Tacs<<13)+(B5_Tcos<<11)+(B5_Tacc<<8)+(B5_Tcoh<<6)+(B5_Tah<<4)+(B5_Tacp<<2)+(B5_PMC))
.word ((B6_MT<<15)+(B6_Trcd<<2)+(B6_SCAN))
.word ((B7_MT<<15)+(B7_Trcd<<2)+(B7_SCAN))
.word ((REFEN<<23)+(TREFMD<<22)+(Trp<<20)+(Trc<<18)+(Tchr<<16)+REFCNT)
.word 0x32
.word 0x30
.word 0x30
在网上查找了一些资料，现将http://gcc.gnu.org/ml/gcc-help/2003-09/msg00111.html网站上的解释粘贴如下：
On Wed, Sep 10, 2003 at 01:07:25PM -0700, garret.spears wrote:
> Refernce:
> gcc-2.95.3 -m5200 -x assembler led.S
Before you compile an assembly source file, you should decide whether
you want to pass it through the C preprocessor (CPP) or not. If you
decide to use the preprocessor then you should name your source-file
"something.S" (capital "S"); if not, then you should name it
"something.s" (lowcase "s"). Provided that you follow this convention
you don't *have* to specify the source-language explicitly using the
"-x <lang>" option; gcc can figure-it out from the filename
extension. If you don't want to or can't use this naming convention,
then "-x <lang>" is required:
gcc-2.95.3 -m5200 asmcode.s -o asmcode.o
gcc-2.95.3 -m5200 -x assembler asmcode.asm -o asmcode.o
Compile without passing the source through CPP.
gcc-2.95.3 -m5200 asmcode1.S -o asmcode1.o
gcc-2.95.3 -m5200 -x assembler-with-cpp asmcode1.asm -o asmcode1.o
Pass the source through CPP and then compile.
If you decide to pass your assembly through CPP, then (and only then)
you *can* use all the C preprocessor goodies, like "#define",
"#include", "#ifdef", etc. But remember: these are handled by the
preprocessor *not* the assmebler!
>
> #DEFINE INIT_SECTION_ASM_OP // is this a requird line or should ther be
> another?
>
This is neither required nor allowed! First: you're not using
"assembler-with-cpp" so #DEFINE has no sense! Second: Even if you used
it, CPP is CaSe-SeNsItIvE, so it should be "#define" and not "#DEFINE"
> // Base addr of internal resources & SIM resources
> MBAR EQU 0x10000000 // alt I have seen ".set
What you should have said is (observe the dot!):
MBAR .EQU 0x10000000
Which is the same as saying
#define MBAR 0x10000000
>
> Should I be using a dot h file for some of this and a dot s file for my
> actual assembly coding?
>
You can use ".h" files (and the respective "#include" directives) only
if you pass your assembly through CPP. You don't *have* to use them,
though!
> Essentially when I did this years
> ago I dedicated a section to defines or equates, a section to data space,
> and a section to code - assembly language.
data section:
.data
... contents of the data-section ...
code section:
.text
... contents of the text-section ...
or if you use an object-format that can support arbitrarily-named
sections:
.section ".data"
... contents of the data-section ...
.section ".text"
... contents of the text-section ...
You don't need a section for the "equ"s since they produce no output!
Hope this helps
/npat
--
As for systems that are not like Unix, such as MSDOS, Windows, the
Macintosh, VMS, and MVS, supporting them is usually so much work that
it is better if you don't.
-- Richard Stallman "GNU Coding Standards"