我是如何学习写一个操作系统（三）：操作系统的启动之保护模式

[SECTION .gdt]                              ; 利用宏定义定义gdt
                                            ; 段基址          段界限              属性
LABEL_GDT:          Descriptor        0,            0,                 0
LABEL_DESC_CODE32:  Descriptor        0,            0fffffh,           DA_C    | DA_32 | DA_LIMIT_4K
LABEL_DESC_VIDEO:   Descriptor        0B8000h,      0fffffh,           DA_DRW
LABEL_DESC_VRAM:    Descriptor        0,            0fffffh,           DA_DRW  | DA_LIMIT_4K

in    al,  92h                         ; 切换到保护模式
or    al,  00000010b
out   92h, al

mov   eax, cr0
or    eax , 1
mov   cr0, eax

mov	ax,#INITSEG	! this is done in bootsect already, but...
mov	ds,ax
mov	ah,#0x03	! read cursor pos
xor	bh,bh
int	0x10		! save it in known place, con_init fetches
mov	[0],dx		! it from 0x90000.

mov	ah,#0x88
int	0x15
mov	[2],ax

mov	ah,#0x0f
int	0x10
mov	[4],bx		! bh = display page
mov	[6],ax		! al = video mode, ah = window width

	mov	ax,#0x0000
	cld			! 'direction'=0, movs moves forward
do_move:
	mov	es,ax		! destination segment
	add	ax,#0x1000
	cmp	ax,#0x9000
	jz	end_move
	mov	ds,ax		! source segment
	sub	di,di
	sub	si,si
	mov 	cx,#0x8000
	rep
	movsw
	jmp	do_move

end_move:
	mov	ax,#SETUPSEG	! right, forgot this at first. didn't work :-)
	mov	ds,ax
	lidt	idt_48		! load idt with 0,0
	lgdt	gdt_48		! load gdt with whatever appropriate

call	empty_8042
mov	al,#0xD1		! command write
out	#0x64,al
call	empty_8042
mov	al,#0xDF		! A20 on
out	#0x60,al
call	empty_8042

mov	al,#0x11		! initialization sequence
out	#0x20,al		! send it to 8259A-1
.word	0x00eb,0x00eb		! jmp $+2, jmp $+2
out	#0xA0,al		! and to 8259A-2

mov	ax,#0x0001	! protected mode (PE) bit
lmsw	ax		! This is it!
jmpi	0,8		! jmp offset 0 of segment 8 (cs)

.word	0x07FF		! 8Mb - limit=2047 (2048*4096=8Mb)
.word	0x0000		! base address=0
.word	0x9A00		! code read/exec
.word	0x00C0		! granularity=4096, 386

setup_idt:
	lea ignore_int,%edx
	movl $0x00080000,%eax
	movw %dx,%ax		/* selector = 0x0008 = cs */
	movw $0x8E00,%dx	/* interrupt gate - dpl=0, present */

	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:
	lgdt gdt_descr
	ret

gdt_descr:
	.word 256*8-1		# so does gdt (not that that's any
	.long gdt		# magic number, but it works for me :^)

	.align 8

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

setup_paging:
	movl $1024*5,%ecx		/* 5 pages - pg_dir+4 page tables */
	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 $pg2+7,pg_dir+8		/*  --------- " " --------- */
	movl $pg3+7,pg_dir+12		/*  --------- " " --------- */
	movl $pg3+4092,%edi
	movl $0xfff007,%eax		/*  16Mb - 4096 + 7 (r/w user,p) */
	std
1:	stosl			/* fill pages backwards - more efficient :-) */
	subl $0x1000,%eax
	jge 1b
	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 */