* 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.
  .globl _idt,_gdt,_pg_dir
  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)
  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
  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.
  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
  movl %eax,(%edi)
  movl %edx,4(%edi)
  addl $8,%edi
  dec %ecx
  jne rp_sidt
  lidt idt_descr

  * 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
  * rather long comment is certainly needed :-).
  * This routine will beoverwritten by the page tables.
  lgdt gdt_descr
  ret @@跳到main函数

.org 0x1000

.org 0x2000

.org 0x3000
  pg2: # This is not used yet, but if you
   # want to expand past 8 Mb, you'll have
   # to use it.

.org 0x4000
  pushl $0 # These are the parameters to main :-)
  pushl $0
  pushl $0
  pushl $L6 # return address for main, if it decides to.
  pushl $_main
  jmp setup_paging @@再jmp
  jmp L6 # main should never return here, but
   # just in case, we know what happens.

/* This is the default interrupt "handler" :-) */
  .align 2
  incb 0xb8000+160 # put something on the screen
  movb $2,0xb8000+161 # so that we know something
  iret # happened

  * Setup_paging
  * This routine sets up paging by setting the page bit
  * in cr0. The page tables are set up, identity-mapping
  * the first 8MB. The pager assumes that no illegal
  * addresses are produced (ie >4Mb on a 4Mb machine).
  * NOTE! Although all physical memory should be identity
  * mapped by this routine, only the kernel page functions
  * use the >1Mb addresses directly. All "normal" functions
  * use just the lower 1Mb, or the local data space, which
  * will be mapped to some other place - mm keeps track of
  * that.
  * For those with more memory than 8 Mb - tough luck. I've
  * not got it, why should you :-) The source is here. Change
  * it. (Seriously - it shouldn't be too difficult. Mostly
  * change some constants etc. I left it at 8Mb, as my machine
  * even cannot be extended past that (ok, but it was cheap :-)
  * I've tried to show which constants to change by having
  * some kind of marker at them (search for "8Mb"), but I
  * won't guarantee that's all :-( )
  .align 2
  movl $1024*3,%ecx @@pg_dir pg_table 清零
  xorl %eax,%eax
  xorl %edi,%edi /* pg_dir is at 0x000 */
  movl $pg0+7,_pg_dir /* set present bit/user r/w */
  movl $pg1+7,_pg_dir+4 /* --------- " " --------- */
  movl $pg1+4092,%edi
  movl $0x7ff007,%eax /* 8Mb - 4096 + 7 (r/w user,p) */
  1: stosl /* fill pages backwards - more efficient :-) */
  subl $0x1000,%eax
  jge 1b @@ greater or equal
  xorl %eax,%eax /* pg_dir is at 0x0000 */
  movl %eax,%cr3 /* cr3 - page directory start */
  movl %cr0,%eax
  orl $0x80000000,%eax
  movl %eax,%cr0 /* set paging (PG) bit */
  ret /* this also flushes prefetch-queue */

@@页目录pg_dir=0 仅两项(8M),

.align 2
  .word 0
  .word 256*8-1 # idt contains 256 entries
  .long _idt
  .align 2
  .word 0
  .word 256*8-1 # so does gdt (not that that's any @@和前面head.S差一??
  .long _gdt # magic number, but it works for me :^)

.align 3
  _idt: .fill 256,8,0 # idt is uninitialized

_gdt: .quad 0x0000000000000000 /* NULL descriptor */
  .quad 0x00c09a00000007ff /* 8Mb */
  .quad 0x00c09200000007ff /* 8Mb */
  .quad 0x0000000000000000 /* TEMPORARY - don't use */
  .fill 252,8,0 /* space for LDT's and TSS's etc */
  @@内核 逻辑地址(不含段寄存器)=线性地址