从引导加载程序调用 C 代码

Question

我正在尝试编写引导加载程序。我想编译一些 C 代码，以便引导加载程序可以将其加载到内存中并跳转到那里。

我有两个问题:

• 调用约定是否与 x86 上的相同？即，堆栈上的参数，从左到右。
• 如何使用 gcc 生成原始二进制文件？

Answer 1

您可以使用链接器脚本使用 gcc 链接器创建纯二进制文件。关键是 OUTPUT_FORMAT(binary) 指令:

//========================================
FILE: linker.ld
//========================================
OUTPUT_FORMAT(binary)
SECTIONS {
    .text : { *(.text) }
    .data : { *(.data) }
    .bss  : { *(.bss)  }
}
//========================================

我在 makefile 中调用了链接器，如下所示(而 linker.ld 是链接器脚本文件):

//========================================
ld -T linker.ld loaderEntry.o loaderMain.o -o EOSLOAD.BIN -L$(lib) -lsys16
//========================================

我已经编译了目标代码

//========================================
gcc -nostdinc -nostdlib -ffreestanding -c <code files> -o theObjectCode.o
//========================================

为了摆脱不能在 16 位模式下工作的标准库。

对于握手 MBR 加载程序和引导加载程序，我使用了以下 loaderMain.S gcc 汇编代码(loaderMain.o 必须是传递给链接器的第一个文件，位于地址偏移 0x0000 处，如上所示)。我使用了 -code16gcc 指令来生成 16 位代码。但是，生成的代码可能无法在旧的 x86 机器上运行，因为我使用了 incompatible仅适用于较新型号的代码说明(%esp、$ebp、leave 等)。

//========================================
FILE: loaderEntry.S
//========================================
  .text
  .code16gcc

  // the entry point at 0x9000:0x0000 // this is where I did a far call to by the MBR
  .globl loaderMain   // loader C entry function name declaration
  push  %cs           // initialize data segments   with same value as code segment
  pop   %ax           // (I managed only tiny model so far ...)
  movw  %ax, %ds
  movw  %ax, %es
  movw  %ax, %fs
  movw  %ax, %gs
  movw  %ax, %ss      // initialize stack segment with same value as     code segment
  movl  $0xffff, %esp // initialize stack pointers with 0xffff (usage of extended (dword) offsets does not work, so we're stuck in tiny model)
  movl  %esp, %ebp
  call  loaderMain   // call C entry function

  cli // halt the machine for the case the main function dares to return
  hlt
//========================================

汇编代码调用了C语言文件loaderMain.c中定义的符号。为了生成 16 位模式兼容代码，您必须在您使用的每个 C 文件的第一行代码之前声明使用 16 位指令集。这只能通过内联汇编指令 AFAIK 完成:

  asm(".code16gcc\n"); // use 16bit real mode code set

  /*  ... some C code .. */


  // ... and here is the C entry code ... //
  void loaderMain() {
    uint cmdlen = 0;
    bool terminate = false;
    print(NL);
    print(NL);
    print("*** EOS LOADER has taken over control. ***\r\n\r\n");
    print("Enter commands on the command line below.\r\n");
    print("Command are executed by pressing the <ENTER> key.\r\n");
    print("The command \'help\' shows a list of all EOS LOADER commands.\r\n");
    print("HAVE FUN!\r\n");
    print(NL);
    while (!terminate) {
        print("EOS:>");
        cmdlen = readLine();
        buffer[cmdlen] = '\0';
        print(NL);
        terminate = command();
    }
    shutdown();
  }

到目前为止，我只能编写普通的 C 代码——到目前为止，我还没有成功编写 C++ 代码，而且我只设法生成了微型内存模型(意味着 CS、SS、DS 和 ES 都是一样的)。gcc 仅使用偏移量作为指针地址，因此如果没有额外的汇编代码，似乎很难克服内存模型问题。 (虽然我听说有人在gcc中解决了这个问题)

调用约定是最后一个参数首先压入堆栈，似乎所有值都是双字对齐的。下面贴出一个可以在.code16gcc C代码中调用的汇编代码示例:

//======================
  .text
  .code16gcc

  .globl kbdread             // declares a global symbol so that the function can be called from C
  .type  kbdread, @function  // declares the symbol as a function
kbdread:                     // the entry point label which has to the same as the symbol

  // this is the conventional stack frame for function entry
  pushl %ebp
  movl  %esp, %ebp

  // memory space for local variables would be allocated by decrementing the stack pointer accordingly
  // the parameter arguments are being addressed by the base pointer which points to the same address while bein within the function

  pushw %ds  // I'm paranoid, I know...
  pushw %es
  pushw %fs
  pushl %eax
  pushl %ebx
  pushl %ecx
  pushl %edx
  pushl %esi
  pushl %edi

  xorl %eax, %eax  // calls the keyboard interrupt in order to read char code and scan code
  int  $0x16

  xorl %edi, %edi
  movl 8(%ebp), %edi // moves the pointer to the memory location in which the char code will be stored into EDI             
  movb %al, (%edi)   // moves the char code from AL to the memory location to which EDI points

  xorl %edi, %edi // paranoid again (but who knows how well the bios handles extended registers??)..

  movl 12(%ebp), %edi // moves the pointer to the memory location in which the scan code will be stored into EDI
  movb %ah, (%edi)    // moves the scan code from AH to the memory location to which EDI points

  popl %edi // restoring the values from stack..
  popl %esi
  popl %edx
  popl %ecx
  popl %ebx
  popl %eax
  popw %fs
  popw %es
  popw %ds

  leave  // .. and the conventional end frame for functions.
  ret    // be aware that you are responsible to restore the stack when you have declared local variables on the stack ponter.
         // the leave instruction is a convenience method to do that. but it is part of not early X86 instruction set (as well as extended registers)
         // so be careful which instruftion you actually use if you have to stay compatible with older computer models.
//=====================

顺便说一句，该函数的 C 头文件声明如下所示:

//=====================
void kbdread(char* pc, (unsigned char)* psc);
//=====================

希望这在某种程度上有所帮助。干杯。