linux内核学习（12）启动全过程概述之三

最新推荐文章于 2021-12-19 13:35:36 发布

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最新推荐文章于 2021-12-19 13:35:36 发布 · 1.4k 阅读

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#linux内核 #alignment #input #output #image #struct

本文详细介绍了Linux内核启动过程，从arch/x86/compressed/head_32.S开始，讲解了解压缩、段寄存器加载、内存对齐、内核映像的临时定位、栈设置、BSS段清零、GOT调整、解压缩和最终跳转到解压缩后的内核等步骤。通过对关键代码的分析，揭示了内核启动的细节。

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进入了vmlinux内核映像部分，就进入了arch/x86/compressed/head_32.S。这个文件主要工作就是解压了，将真正的内核给释放出来，不过还得把这个文件分析完，内核才能出世。

来自arch/x86/compressed/head_32.S：

........省略了头文件......

__HEAD
ENTRY(startup_32)
    cld           /*di,si++*/
    /*
    * Test KEEP_SEGMENTS flag to see if the bootloader is asking
    * us to not reload segments
    */
    testb    $(1<<6), BP_loadflags(%esi)   /*esi = boot_params*/
    jnz    1f
# esi是之前就设置好了的，boot_params结构体的首地址，而loadflags的位6如果置位，则表示需要将段寄存器重新载入
    cli        /*close interrupt*/
    movl    $__BOOT_DS, %eax
    movl    %eax, %ds
    movl    %eax, %es
    movl    %eax, %fs
    movl    %eax, %gs
    movl    %eax, %ss
1:

/*
* Calculate the delta between where we were compiled to run
* at and where we were actually loaded at. This can only be done
* with a short local call on x86. Nothing else will tell us what
* address we are running at. The reserved chunk of the real-mode
* data at 0x1e4 (defined as a scratch field) are used as the stack
* for this calculation. Only 4 bytes are needed.
*/
    leal    (BP_scratch+4)(%esi), %esp /*boot_params->scratch*/
    call    1f
1:    popl    %ebp
    subl &nb