调用函数时，寄存器到底发生了那些变化？

linux x86 gcc3.2.3 AT&T格式的汇编

代码如下：

void
fun()
{
int a = 'A';
}

void main()
{
int b;
fun();

return;
}

编译
gcc -g -o t t.c

开始调试
[sanool@sanool ex2]$ gdb t
GNU gdb Red Hat Linux (6.0post-0.20031117.6rh)
Copyright 2003 Free Software Foundation, Inc.
GDB is free software, covered by the GNU General Public License, and you are
welcome to change it and/or distribute copies of it under certain conditions.
Type "show copying" to see the conditions.
There is absolutely no warranty for GDB. Type "show warranty" for details.
This GDB was configured as "i386-redhat-linux-gnu"...(no debugging symbols found)...Using host libthread_db library "/lib/tls/libthread_db.so.1".

(gdb) disas main
Dump of assembler code for function main:
0x08048323 : push %ebp
0x08048324 : mov %esp,%ebp
0x08048326 : sub $0x8,%esp
0x08048329 : and $0xfffffff0,%esp
0x0804832c : mov $0x0,%eax
0x08048331 : sub %eax,%esp
0x08048333 : call 0x8048314
0x08048338 : leave
0x08048339 : ret
0x0804833a : nop
0x0804833b : nop
End of assembler dump.
(gdb) disas fun
Dump of assembler code for function fun:
0x08048314 : push %ebp
0x08048315 : mov %esp,%ebp
0x08048317 : sub $0x4,%esp
0x0804831a : movl $0x41,0xfffffffc(%ebp)
0x08048321 : leave
0x08048322 : ret
End of assembler dump.

解释如下：
**当程序下一步执行 0x08048333 : call 0x8048314 时
esp = 0xbfffe660 （运行时）
ebp = 0xbfffe668 （运行时）
eip = 0x08048333

**然后执行 call 0x8048314 也就是
push %eip ( 相当于 sub $4 %esp 再 mov %eip %esp )
movl $0x8048314, %eip

则0xbfffe65c 处为 eip = 0x08048338
且esp = 0xbfffe65c
eip = 0x8048314
ebp = 0xbfffe668

**执行0x08048314 : push %ebp后
esp = 0xbfffe658
ebp = 0xbfffe668
0xbfffe658处的值为 ebp = 0xbfffe668

**继续0x08048315 : mov %esp,%ebp
将esp的值赋值给ebp
即 ebp = esp = 0xbfffe658

**开始执行 0x08048321 : leave 前
eip = 0x08048321
ebp = 0xbfffe658
esp = 0xbfffe654

**开始执行 0x08048321 : leave 时
即进行
movl %ebp, %esp ( 即 esp = ebp = 0xbfffe658)
pop %ebp ( 也就是 mov %esp,%ebp 再 add $4,%esp )

此时 ebp = 0xbfffe668 回到了原函数的ebp值，

**再执行 0x08048322 : ret
即 pop %eip
( 也就是 mov %esp,%eip 再 add $4,%esp )
此时 eip = 0x08048338
程序继续执行 main 中的 leave

调用fun函数结束

补充：
i reg //观察寄存器的值 (注意ebp esp eip)

x/20x $esp //查看内存中$esp(如0xbfffef50)后面的数据内容