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



**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);