从头写一个操作系统 11 (create an OS from scratch 11)

in esteem •  6 years ago  (edited)

你应该先google：C,ojbect code, linker, disassemble
目标：用C语言做底层汇编语言做的那些事

Compile

我们得研究研究C编译器如何编译代码，并且比较它与汇编器生成的机器码两者是否有所差别。

写一个只有一个简单函数的程序function.c。打开function.c看一眼。

int my_function() {
    return 0xbaba;
}

编译与系统无关的代码时需要加-ffreestanding:

i386-elf-gcc -ffreestanding -c function.c -o function.o

研究研究编译器生成的机器码:

i386-elf-objdump -d function.o

function.o:     file format elf32-i386

Disassembly of section .text:

00000000 <my_function>:
   0:   55                      push   %ebp
   1:   89 e5                   mov    %esp,%ebp
   3:   b8 ba ba 00 00          mov    $0xbaba,%eax
   8:   5d                      pop    %ebp
   9:   c3                      ret

是不是有点似曾相识？

Link

最后，如果要生成二进制文件，我们需要使用 linker，中文也叫链接器。在这个时候，我们一定要搞明白高级语言是如何调用函数的标签的。其实我们不知道，程序在内存中的偏移地址(offset)是多少。我们将offset定为0x0并且使用binary(二进制)格式生成二进制码，而不包含任何标签或链接器需要的数据。

i386-elf-ld -o function.bin -Ttext 0x0 --oformat binary function.o

注意：会有一个 warning(i386-elf-ld: warning: cannot find entry symbol _start; defaulting to 0000000000000000)，淡定的忽略掉它！

使用xxd命令查看两个文件，我们发现.bin文件是机器码，而.o文件包含很多调试信息。

geyu@geyu-All-Series:~/workdir/os-dev/os-tutorial/12-kernel-c$ xxd function.bin 
00000000: 5589 e5b8 baba 0000 5dc3 0000 1400 0000  U.......].......
00000010: 0000 0000 017a 5200 017c 0801 1b0c 0404  .....zR..|......
00000020: 8801 0000 1c00 0000 1c00 0000 d4ff ffff  ................
00000030: 0a00 0000 0041 0e08 8502 420d 0546 c50c  .....A....B..F..
00000040: 0404 0000            

geyu@geyu-All-Series:~/workdir/os-dev/os-tutorial/12-kernel-c$ xxd function.o
00000000: 7f45 4c46 0101 0100 0000 0000 0000 0000  .ELF............
00000010: 0100 0300 0100 0000 0000 0000 0000 0000  ................
00000020: cc00 0000 0000 0000 3400 0000 0000 2800  ........4.....(.
00000030: 0a00 0700 5589 e5b8 baba 0000 5dc3 0047  ....U.......]..G
00000040: 4343 3a20 2847 4e55 2920 342e 392e 3100  CC: (GNU) 4.9.1.
00000050: 1400 0000 0000 0000 017a 5200 017c 0801  .........zR..|..
00000060: 1b0c 0404 8801 0000 1c00 0000 1c00 0000  ................
00000070: 0000 0000 0a00 0000 0041 0e08 8502 420d  .........A....B.
00000080: 0546 c50c 0404 0000 002e 7379 6d74 6162  .F........symtab
00000090: 002e 7374 7274 6162 002e 7368 7374 7274  ..strtab..shstrt
000000a0: 6162 002e 7465 7874 002e 6461 7461 002e  ab..text..data..
000000b0: 6273 7300 2e63 6f6d 6d65 6e74 002e 7265  bss..comment..re
000000c0: 6c2e 6568 5f66 7261 6d65 0000 0000 0000  l.eh_frame......
000000d0: 0000 0000 0000 0000 0000 0000 0000 0000  ................
000000e0: 0000 0000 0000 0000 0000 0000 0000 0000  ................
000000f0: 0000 0000 1b00 0000 0100 0000 0600 0000  ................
00000100: 0000 0000 3400 0000 0a00 0000 0000 0000  ....4...........
00000110: 0000 0000 0100 0000 0000 0000 2100 0000  ............!...
00000120: 0100 0000 0300 0000 0000 0000 3e00 0000  ............>...
00000130: 0000 0000 0000 0000 0000 0000 0100 0000  ................
00000140: 0000 0000 2700 0000 0800 0000 0300 0000  ....'...........
00000150: 0000 0000 3e00 0000 0000 0000 0000 0000  ....>...........
00000160: 0000 0000 0100 0000 0000 0000 2c00 0000  ............,...
00000170: 0100 0000 3000 0000 0000 0000 3e00 0000  ....0.......>...
00000180: 1200 0000 0000 0000 0000 0000 0100 0000  ................
00000190: 0100 0000 3900 0000 0100 0000 0200 0000  ....9...........
000001a0: 0000 0000 5000 0000 3800 0000 0000 0000  ....P...8.......
000001b0: 0000 0000 0400 0000 0000 0000 3500 0000  ............5...
000001c0: 0900 0000 0000 0000 0000 0000 f402 0000  ................
000001d0: 0800 0000 0800 0000 0500 0000 0400 0000  ................
000001e0: 0800 0000 1100 0000 0300 0000 0000 0000  ................
000001f0: 0000 0000 8800 0000 4300 0000 0000 0000  ........C.......
00000200: 0000 0000 0100 0000 0000 0000 0100 0000  ................
00000210: 0200 0000 0000 0000 0000 0000 5c02 0000  ............\...
00000220: 8000 0000 0900 0000 0700 0000 0400 0000  ................
00000230: 1000 0000 0900 0000 0300 0000 0000 0000  ................
00000240: 0000 0000 dc02 0000 1800 0000 0000 0000  ................
00000250: 0000 0000 0100 0000 0000 0000 0000 0000  ................
00000260: 0000 0000 0000 0000 0000 0000 0100 0000  ................
00000270: 0000 0000 0000 0000 0400 f1ff 0000 0000  ................
00000280: 0000 0000 0000 0000 0300 0100 0000 0000  ................
00000290: 0000 0000 0000 0000 0300 0200 0000 0000  ................
000002a0: 0000 0000 0000 0000 0300 0300 0000 0000  ................
000002b0: 0000 0000 0000 0000 0300 0500 0000 0000  ................
000002c0: 0000 0000 0000 0000 0300 0400 0c00 0000  ................
000002d0: 0000 0000 0a00 0000 1200 0100 0066 756e  .............fun
000002e0: 6374 696f 6e2e 6300 6d79 5f66 756e 6374  ction.c.my_funct
000002f0: 696f 6e00 2000 0000 0202 0000            ion. .......

Decompile

好奇不好奇机器码对应着怎样的汇编代码？
ndisasm -b 32 function.bin

More

强烈建议多谢几个小程序，就像以下这样：

  • Local variables localvars.c
  • Function calls functioncalls.c
  • Pointers pointers.c

仔细看一看os-dev.pdf相关的章节，编译并反汇编上面的代码，对比研究它们的机器码。最后回答这个问题，pointers.c编译后为什么与我们预期不一致？"Hello"的ASCII码0x48656c6c6f在哪里？

THE ORIGIN ARTICALE IN GITHUB:[^1]

参考资料：
[1]:https://github.com/cfenollosa/os-tutorial/blob/master/12-kernel-c

版权注明：本文所有涉及到：https://github.com/cfenollosa/os-tutorial/ git仓库的内容，全部对应以下开源协议声明：
BSD 3-Clause License
Copyright (c) 2018,
Carlos Fenollosa

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