bootloader分析（BOIS）

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BOIS存储在系统的0x7c00位置，利用bochs的调试功能对其进行分析，设置0x7c00的断点，下面是运行的汇编命令

cli # Disable interrupts 关闭中断，设置IF寄存器为0
cld # String operations increment设置串处理的方向，置0，SI DI自增，串处理从低地址到高地址方向处理

        # Set up the important data segment registers (DS, ES, SS).对各个寄存器清零
        xorw    %ax,%ax            # Segment number zero
        movw    %ax,%ds            # -> Data Segment
        movw    %ax,%es            # -> Extra Segment
        movw    %ax,%ss            # -> Stack Segment

# Set up the stack pointer, growing downward from 0x7c00.
movw $start,%sp # Stack Pointer $start地址为0x7c00

#打开A20 Gate Option 详细为了读取1M以上的地址空间
#    * 读60h端口，读output buffer
#    * 写60h端口，写input buffer
#    * 读64h端口，读Status Register

seta20.1:    inb    $0x64,%al        # Get status 读状态寄存器 in al,0x64
        testb    $0x2,%al        # Busy? test al,0x02
        jnz    seta20.1        # Yes 占用则跳转
        movb    $0xd1,%al        # Command: Write
        outb    %al,$0x64        # output port
seta20.2:    inb    $0x64,%al        # Get status
        testb    $0x2,%al        # Busy?
        jnz    seta20.2        # Yes
        movb    $0xdf,%al        # Enable
        outb    %al,$0x60        # A20