X86内核启动分析四 打开保护模式之门

保护模式准备活动，上一章是假的准备，这次玩真的了

go_to_protected_mode
    =>realmode_switch_hook(); //关中断
    =>move_kernel_around(); //如果不是大内核，从0x10000挪到0x1000
    =>enable_a20()
    =>reset_coprocessor();
    =>mask_all_interrupts();
    =>setup_idt();
    setup_gdt();
    =>protected_mode_jump(boot_params.hdr.code32_start,
                (u32)&boot_params + (ds() << 4));

对于code32_start，是解压缩内核的起始地址
code32_start:               # here loaders can put a different
                    # start address for 32-bit code.
#ifndef __BIG_KERNEL__
        .long   0x1000      #   0x1000 = default for zImage
#else
        .long   0x100000    # 0x100000 = default for big kernel
#endif

我纵情一跳

    .code16

/*
 * void protected_mode_jump(u32 entrypoint, u32 bootparams);
*/
protected_mode_jump:
    =>movw  $__BOOT_DS, %cx

    movl    %cr0, %edx
    orb $1, %dl            # Protected mode (PE) bit
    movl    %edx, %cr0  //关键是设置cr0的 PE位
    movw    %cx, %ds    //各类段寄存器设置合适的置
    movw    %cx, %es
    movw    %cx, %fs
    movw    %cx, %gs
    movw    %cx, %ss

# Jump to the 32-bit entrypoint //我挥一挥手，不带走一片云彩
    .byte   0x66, 0xea      # ljmpl opcode
2:  .long   0           # offset
    .word   __BOOT_CS       # segment

参考
Linux kernel boot process——从实模式（real mode）到保护模式（protected mode），再到分页（paging）
http://blog.csdn.net/bokee/article/details/6900361