linux内核原代码head.s部分的注释

　　/*
　　* head.s contains the 32-bit startup code.
　　*
　　* NOTE!!! Startup happens at absolute address 0x00000000, which is also where
　　* the page directory will exist. The startup code will be overwritten by
　　* the page directory.
　　*/
　　.text
　　.globl _idt,_gdt,_pg_dir
　　_pg_dir:
　　startup_32:
　　movl $0x10,%eax @@ds,es,fs,gs指向内核数据段
　　mov %ax,%ds
　　mov %ax,%es
　　mov %ax,%fs
　　mov %ax,%gs
　　lss _stack_start,%esp @@ds送ss esp 指向stack_start (在sched.c定义）
　　 @@进入保护模式的堆栈段的第一次变化，很奇怪
　　 @@为什么堆栈段也可正向增涨？
　　call setup_idt
　　call setup_gdt
　　movl $0x10,%eax # reload all the segment registers
　　mov %ax,%ds # after changing gdt. CS was already
　　mov %ax,%es # reloaded in 'setup_gdt' @@ 有reload??
　　mov %ax,%fs
　　mov %ax,%gs
　　lss _stack_start,%esp
　　xorl %eax,%eax
　　1: incl %eax # check that A20 really IS enabled
　　movl %eax,0x000000
　　cmpl %eax,0x100000 @@这是怎么测的 0x100000为什么值，
　　 @@明白，a20 not
　　enable,0x000000就是0x100000
　　je 1b
　　movl %cr0,%eax # check math chip
　　andl $0x80000011,%eax # Save PG,ET,PE
　　testl $0x10,%eax
　　jne 1f # ET is set - 387 is present
　　orl $4,%eax # else set emulate bit
　　1: movl %eax,%cr0
　　jmp after_page_tables @@注意，用jmp 不call，不返回
　　/*
　　* setup_idt
　　*
　　* sets up a idt with 256 entries pointing to
　　* ignore_int, interrupt gates. It then loads
　　* idt. Everything that wants to install itself
　　* in the idt-table may do so themselves. Interrupts
　　* are enabled elsewhere, when we can be relatively
　　* sure everything is ok. This routine will be over-
　　* written by the page tables.
　　*/
　　setup_idt:
　　lea ignore_int,%edx
　　movl $0x00080000,%eax
　　movw %dx,%ax /* selector = 0x0008 = cs */
　　movw $0x8E00,%dx /* interrupt gate - dpl=0, present */
　　 @@ ignore_int 低16-->ax 高16-->edx高字

@@ 8e00-->dx 8-->eax高字
　　lea _idt,%edi
　　mov $256,%ecx
　　rp_sidt:
　　movl %eax,(%edi)
　　movl %edx,4(%edi)
　　addl $8,%edi
　　dec %ecx
　　jne rp_sidt
　　lidt idt_descr
　　ret

/*
　　* setup_gdt
　　*
　　* This routines sets up a new gdt and loads it.
　　* Only two entries are currently built, the same
　　* ones that were built in init.s. The routine
　　* is VERY complicated at two whole lines, so this