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

.org 0x1000
　　 pg0:

.org 0x2000
　　 pg1:

.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
　　 after_page_tables:
　　 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
　　 L6:
　　 jmp L6 # main should never return here, but
　　 # just in case, we know what happens.

/* This is the default interrupt "handler" :-) */
　　 .align 2
　　 ignore_int:
　　 incb 0xb8000+160 # put something on the screen
　　 mo $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
　　 setup_paging:
　　 movl $1024*3,%ecx @@pg_dir pg_table 清零
　　 xorl %eax,%eax
　　 xorl %edi,%edi /* pg_dir is at 0x000 */
　　 cld;rep;stosl
　　 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) */
　　 std
　　 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),
　　 @@pg1,pg0页表逆向填充，pg0的首页仍为零
　　 @@这样映射，线性地址=物理地址

.align 2
　　 .word 0
　　 idt_descr:
　　 .word 256*8-1 # idt contains 256 entries
　　 .long _idt
　　 .align 2
　　 .word 0
　　 gdt_descr:
　　 .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