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#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 start
start:
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,$0x64
seta20.2:
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, %dx
spin:
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 alignment
gdt:
SEG_NULLASM # null seg
SEG_ASM(STA_X|STA_R, 0x0, 0xffffffff) # code seg
SEG_ASM(STA_W, 0x0, 0xffffffff) # data seg
#0x2
gdtdesc:
.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。