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Happy coding

linux汇编常见问题

Ted posted @ 2008年9月02日 23:34 in 未分类 with tags ASM , 4937 阅读

1.gcc嵌入汇编
(1). 在gcc嵌入汇编中输入输出使用相同的寄存器？

static void * __memcpy(void * to, const void * from, size_t n)
{
 int d0,d1,d2;
 __asm__ __volatile__(
  "rep;movsl\n\t"
  "testb $2,%b4\n\t"
  "je 1f\n\t"
  "movsw\n"
  "1:\ttestb $1,%b4\n\t"
  "je 2f\n\t"
  "movsb\n"
  "2:"
  :"=&c" (d0), "=&D" (d1), "=&S" (d2)
  :"0" (n/4), "q" (n), "1" ((long) to), "2" ((long) from)
  :"memory");
 return (to);
}

操作数0，1，2和3，5，6使用相同的寄存器的理解：
a. 3，5，6在输入过程中将值n/4，to和from的地址读入ecx，edi，esi寄存器中；
b. 0，1，2在输出过程中将ecx，edi，esi寄存器中的值存入d0，d1，d2内存变量中。
c. 注意在上面的语句中也有"&"限定符，但输入和输出仍使用相同的寄存器，如操作数0和3都使用寄存器ecx，这是因为"0"限定的结果，如果

把"0" (n/4)换成"c" (n/4)，则因为"=&c"的限定而使得编译时报错。

(2). 关于gcc嵌入式汇编中"&"限定符的作用？
"&"限定符用于输出操作数，使其唯一的使用某寄存器

int bar,foo;
__asm__ __volatile__(
 "call func \n\t"
 "mov ebx,%1"
 :"=a" (foo)
 :"r" (bar));

在gcc编译时默认会让bar也使用eax寄存器，如果把"=a"改为"=&a",那么foo将唯一使用eax，而让bar使用其它的寄存器。

(3). _start和main的关系？
main是gcc看到的程序入口点，而ld和as所看到的程序入口点其实是_start。libc库中的_start会调用main函数。
a. 编译带_start的汇编程序时的步骤：as -o a.o a.s，ld -o a a.o; gcc -g -c a.s,ld -o a a.o。(使用gcc编译可以增加调试选项-g，这

时不能直接用gcc -o a a.s编译的原因是gcc会默认在程序中查找main函数，并且会将libc中的_start加入进来。如果直接用gcc -o a a.s编译

，则会报两个错误"重复的_start"和"没找到main")
b. 编译带main的汇编程序或C程序时的步骤：gcc -o a a.s；gcc -o a a.c。

(4).section和.previous
将这两个.section和.previous中间的代码汇编到各自定义的段中，然后跳回去，将这之后的的代码汇编到上一个section中(一般是.text段)，

也就是自定义段之前的段。.section和.previous必须配套使用。

2. AT&T汇编
(1).data，.section等都是伪操作，不能直接翻译成机器码，只有相应的assembler才能识别
(2).section将程序分成几个片断，如.data,.text,.bss
(3).globl 函数名表示该函数被export，并可以被其它文件中的函数调用
(4).bss可以用来申请一块空间，但不需要对其进行初始化
(5)使用内核定义函数如open，read等可以通过int $80中断来完成
(6)MOVL $FOO,%EAX是把FOO在内存中的地址放到EAX中，而MOVL FOO,%EAX是把FOO这个变量的内容放入EAX
(7).include "文件名"，将其它文件包含进来
(8)如何在汇编中表示结构？如c语言中的如下结构:
struct para
{
 char Firstname[40];
 char Lastname[40];
 char Address[240];
 long Age;//4 bytes
}
在汇编中可以表示成:
.section data
record1:
.ascii "Fredrick\0"
.rept 31 #Padding to 40 bytes
.byte 0
.endr
.ascii "Bartlett\0"
.rept 31 #Padding to 40 bytes
.byte 0
.endr
.ascii "4242 S Prairie\nTulsa, OK 55555\0"
.rept 209 #Padding to 240 bytes
.byte 0
.endr
.long 45
其中.rept n和.endr表示重复两者之间的序列n次，可用于填充数据
(9)当汇编程序由多个文件构成时，可以采用以下方式编译与连接：
as write-records.s -o write-records.o (gcc -g -c write-records.s)
as write-record.s -o write-record.o (gcc -g -c write-record.s)
ld write-record.o write-records.o -o write-records
(10)如何在汇编语言中使用动态库中的函数？
#helloworld-lib.s
.section .data
helloworld:
.ascii "hello world\n\0"
.section .text
.globl _start
_start:
pushl $helloworld
call printf
pushl $0
call exit

as helloworld-lib.s -o helloworld-lib.o
ld -dynamic-linker /lib/ld-linux.so.2 -o helloworld-lib helloworld-lib.o -lc
产生动态库：ld -shared write-record.o read-record.o -o librecord.so
(11)使用汇编文件生成动态库
as write-record.s -o write-record.o
as read-record.s -o read-record.o
ld -shared write-record.o read-record.o -o librecord.so
as write-records.s -o write-records.o
ld -L . -dynamic-linker /lib/ld-linux.so.2 -o write-records -lrecord write-records.o
记得运行write-records时还需要将动态库路径加到/etc/ld.so.conf中，并运行ldconfig
(12)编译汇编文件时如何产生调试符号
as --gstabs a.s -0 a.o 或者gcc -g -c a.s

附录：
(1)函数调用时栈的情况
#Parameter #N <--- N*4+4(%ebp)
#...
#Parameter 2 <--- 12(%ebp)
#Parameter 1 <--- 8(%ebp)
#Return Address <--- 4(%ebp)
#Old %ebp <--- (%ebp)
#Local Variable 1 <--- -4(%ebp)
#Local Variable 2 <--- -8(%ebp) and (%esp)

(2)例子
 .include "external_func.s"

 .section .data
data_array:     #定义long型数组
 .long 3,67,34,0        
data_strings:    #定义字符串
 .ascii "Hello there\0"
data_long: #定义long型变量
 .long 5

 .section .bss
 .lcomm my_buffer, 500   #申请一块500字节的内存

 .section .text
 .equ LINUX_SYSCALL, 0x80 #定义符号LINUX_SYSCALL的值为0x80
 .globl _start
_start:
 pushl %edx
 movl data_long,%edx     #将data_long变量的值放入edx寄存器
 movl $data_long,%edx    #将data_long的地址放入edx寄存器
 popl %edx

 pushl $3      #push second argument
 pushl $2      #push first argument
 call power    #call the function
 addl $8, %esp #move the stack pointer back
 pushl %eax    #save the first answer before,calling the next function

 movl $1, %eax #exit (%ebx is returned)
 int $LINUX_SYSCALL    

 .type power, @function #定义函数power
power:
 pushl %ebp         #save old base pointer
 movl %esp, %ebp    #make stack pointer the base pointer
 subl $4, %esp      #get room for our local storage
 movl 8(%ebp), %eax #put first argument in %eax
 movl 12(%ebp), %ebx #put second argument in %ebx
 imull %ebx,%eax
 movl %ebp, %esp    #restore the stack pointer
 popl %ebp          #restore the base pointer
 ret

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