关于bootasm.s理解

#include "asm.h"#include "memlayout.h"#include "mmu.h"#操作系统加载到内存中的第一个程序入口

#BIOS程序将该段程序加载到内存的0x7c00处## Start the first CPU: switch to 32-bit protected mode, jump into C.# The BIOS loads this code from the first sector of the hard disk into# memory at physical address 0x7c00 and starts executing in real mode# with %cs=0 %ip=7c00.由此处可以知道程序运行的CS段的值是0x0.code16                       # Assemble for 16-bit mode.globl startstart:  cli                         # BIOS enabled interrupts; disable  # Zero data segment registers DS, ES, and SS.  xorw    %ax,%ax             # Set %ax to zero  movw    %ax,%ds             # -> Data Segment  movw    %ax,%es             # -> Extra Segment  movw    %ax,%ss             # -> Stack Segment  # Physical address line A20 is tied to zero so that the first PCs   # with 2 MB would run software that assumed 1 MB.  Undo that.

  # 打开A20，准备进入保护模式seta20.1:  inb     $0x64,%al               # Wait for not busy  testb   $0x2,%al  jnz     seta20.1  movb    $0xd1,%al               # 0xd1 -> port 0x64  outb    %al,$0x64seta20.2:  inb     $0x64,%al               # Wait for not busy  testb   $0x2,%al  jnz     seta20.2  movb    $0xdf,%al               # 0xdf -> port 0x60  outb    %al,$0x60  # Switch from real to protected mode.  Use a bootstrap GDT that makes  # virtual addresses map directly to physical addresses so that the  # effective memory map doesn't change during the transition.  #进入保护模式以后不能按照cs:ip去算取指令的地址或者说是跳转的地址计算方法变了

  #例如下边的 ljmp    $(SEG_KCODE<<3), $start32调整指令

  #在实模式下：跳到$(SEG_KCODE<<3)×16+$start32的地址去运行

  #在保护模式下：要根据gdtdesc表去找到第$(SEG_KCODE<<3)个48位的段基址然后去该地址运行。  lgdt    gdtdesc

  movl    %cr0, %eax  orl     $CR0_PE, %eax     movl    %eax, %cr0  #PAGEBREAK!  # Complete transition to 32-bit protected mode by using long jmp  # to reload %cs and %eip.  The segment descriptors are set up with no  # translation, so that the mapping is still the identity mapping.<pre name="code" class="cpp"> 

  # 前面描述了一下这条指令的意思，同时这条指令也是真正的从实模式转变到保护模式。

<pre name="code" class="cpp">  ljmp    $(SEG_KCODE<<3), $start32.code32  # Tell assembler to generate 32-bit code now.start32:  # Set up the protected-mode data segment registers  movw    $(SEG_KDATA<<3), %ax    # Our data segment selector  movw    %ax, %ds                # -> DS: Data Segment  movw    %ax, %es                # -> ES: Extra Segment  movw    %ax, %ss                # -> SS: Stack Segment  movw    $0, %ax                 # Zero segments not ready for use  movw    %ax, %fs                # -> FS  movw    %ax, %gs                # -> GS  # Set up the stack pointer and call into C.  movl    $start, %esp  call    bootmain  # If bootmain returns (it shouldn't), trigger a Bochs  # breakpoint if running under Bochs, then loop.  movw    $0x8a00, %ax            # 0x8a00 -> port 0x8a00  movw    %ax, %dx  outw    %ax, %dx  movw    $0x8ae0, %ax            # 0x8ae0 -> port 0x8a00  outw    %ax, %dxspin:  jmp     spin

#这里就是前面的全局段描述表，这里只有3个，第0项都是0x000000000000,后边还有第1项，第2项。

<pre name="code" class="cpp"># Bootstrap GDT.p2align 2                                # force 4 byte alignmentgdt:  SEG_NULLASM                             # null seg  SEG_ASM(STA_X|STA_R, 0x0, 0xffffffff)   # code seg  SEG_ASM(STA_W, 0x0, 0xffffffff)         # data seg               #0x2gdtdesc:  .word   (gdtdesc - gdt - 1)             # sizeof(gdt) - 1  .long   gdt                             # address gdt

          似乎X86相关的BIOS都是把软盘里边的第0磁面0磁道1扇区的512B程序加载到0x7c00处，使用INT 0x19中断可以实现将第0磁面0磁道1扇区的512B程序加载到0x7c00处。