/*
*  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函数

.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
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
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
 
 

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