X86内核启动分析三 内核的实模式阶段

header.S (c:\linux\linux-2.6.23\arch\i386\boot) //真正的入口是从第二个512字节开始的，bootloader(lilo/grub/uboot)会将控制权交给它
_start
=>.byte 0xeb        # short (2-byte) jump
.byte   start_of_setup-1f //跳转到start_of_setup
=>start_of_setup:
#ifdef SAFE_RESET_DISK_CONTROLLER
# Reset the disk controller.//复位硬盘控制器
movw    $0x0000, %ax       # Reset disk controller
movb    $0x80, %dl     # All disks
int $0x13
#endif
=># We will have entered with %cs = %ds+0x20, normalize %cs so
# it is on par with the other segments.
pushw   %ds
pushw   $setup2  //跳转到setup2
lretw
=>setup2:
# Force %es = %ds
movw    %ds, %ax
movw    %ax, %es
cld
=>andw  $~3, %sp  # dword align (might as well...)  //对齐堆栈到16B或者32B
jnz 1f
movw    $0xfffc, %sp   # Make sure we're not zero
1:  movzwl  %sp, %esp   # Clear upper half of %esp
sti
=># Check signature at end of setup  //检查55AA
cmpl    $0x5a5aaa55, setup_sig
jne setup_bad
=># Zero the bss
movw    $__bss_start, %di
movw    $_end+3, %cx
xorl    %eax, %eax
subw    %di, %cx
shrw    $2, %cx
rep; stosl

=># Jump to C code (should not return)
calll   main

main
=>copy_boot_params();//与bootloader交互的参数都在这里面进行二次加工，例如hdr和cmdline
=>validate_cpu()// x86指令检查一下CPU
=>detect_memory();
=>detect_memory_e820()//通过 int $0x15 中断获取内存分布
=>go_to_protected_mode();

machine_specific_memory_setup
=>who = "BIOS-e820";
=>sanitize_e820_map(E820_MAP, &E820_MAP_NR);//去掉重叠的entry
=>if (copy_e820_map(E820_MAP, E820_MAP_NR) < 0)// Copy the BIOS e820 map into a safe place.
copy_e820_map
=>do {//循环所有的entry，加入到map表里面
unsigned long long start = biosmap->addr;
unsigned long long size = biosmap->size;
unsigned long long end = start + size;
unsigned long type = biosmap->type;
add_memory_region(start, size, type);
} while (biosmap++,--nr_map);
if (ALT_MEM_K < EXT_MEM_K) {////如果e820不可获取，那么按照默认方式构建可用物理内存
mem_size = EXT_MEM_K;
who = "BIOS-88";
} else {
mem_size = ALT_MEM_K;
who = "BIOS-e801";
}
e820.nr_map = 0;
add_memory_region(0, LOWMEMSIZE(), E820_RAM);
add_memory_region(HIGH_MEMORY, mem_size << 10, E820_RAM);