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你看例子，GDT中有定义页目录表和页表：
INCLUDE 386SCD.INC
;============================================================================
PDT_AD = 200000h ;页目录表所在物理页的地址
PT0_AD = 202000h ;页表0所在物理页的地址
PT1_AD = 201000h ;页表1所在物理页的地址
PhVB_AD = 0b8000h ;物理视频缓冲区地址
LoVB_AD = 0f0000h ;程序使用的逻辑视频缓冲区地址
MPVB_AD = 301000h ;线性地址0B8000H所映射的物理地址
PhSC_AD = 303000h ;部分演示代码所在内存的物理地址
LoSC_AD = 402000h ;部分演示代码的逻辑地址
;============================================================================
GDTSeg SEGMENT PARA USE16 ;全局描述符表数据段(16位)
;----------------------------------------------------------------------------
;全局描述符表GDT
GDT LABEL BYTE
;空描述符
DUMMY Desc <>
;规范段描述符及选择子
Normal Desc <0ffffh,,,ATDW,,>
Normal_Sel = Normal-GDT
;页目录表所在段描述符(在保护方式下初始化时用)及选择子
PDT Desc <0fffh,PDT_AD AND 0ffffh,PDT_AD SHR 16,ATDW,,>
PDT_Sel = PDT-GDT
;页表0所在段描述符(在保护方式下初始化时用)及选择子
PT0 Desc <0fffh,PT0_AD AND 0ffffh,PT0_AD SHR 16,ATDW,,>
PT0_Sel = PT0-GDT
;页表1所在段描述符(在保护方式下初始化时用)及选择子
PT1 Desc <0fffh,PT1_AD AND 0ffffh,PT1_AD SHR 16,ATDW,,>
PT1_Sel = PT1-GDT
;逻辑视频缓冲区段描述符及选择子
LoVideo Desc <3999,LoVB_AD AND 0ffffh,LoVB_AD SHR 16,ATDW,,>
LoVideo_Sel = LoVideo-GDT
;逻辑上的部分演示代码段的描述符及选择子
LoCode Desc <SCodeLen-1,LoSC_AD AND 0ffffh,LoSC_AD SHR 16,ATCE,,>
LoCode_Sel = LoCode-GDT
;预定内存区域(用于部分演示代码)的段描述符及选择子
TPSCode Desc <SCodeLen-1,PhSC_AD AND 0ffffh,PhSC_AD SHR 16,ATDW,,>
TPSCode_Sel = TPSCode-GDT
;----------------------------------------------------------------------------
;以下是需额外初始化的描述符
EFFGDT LABEL BYTE
;临时代码段描述符及选择子
TempCode Desc <0ffffh,TempCodeSeg,,ATCE,,>
TempCode_Sel = TempCode-GDT
;演示代码段描述符及选择子
DemoCode Desc <DemoCodeLen-1,DemoCodeSeg,,ATCE,,>
DemoCode_Sel = DemoCode-GDT
;演示任务数据段描述符及选择子
DemoData Desc <DemoDataLen-1,DemoDataSeg,,ATDW,,>
DemoData_Sel = DemoData-GDT
;初始化时要移动的代码段描述符及选择子(移动时作为数据对待)
SCode Desc <SCodeLen-1,SCodeSeg,,ATDR,,>
SCode_Sel = SCode-GDT
;----------------------------------------------------------------------------
GDTLen = $-GDT ;全局描述符表长度
GDNum = ($-EFFGDT)/(SIZE Desc) ;需特殊处理的描述符数
;----------------------------------------------------------------------------
GDTSeg ENDS ;全局描述符表段定义结束
;============================================================================
;这部分代码在初始化时被复制到预定的内存区域,其功能是在屏幕上显示提示信息
;----------------------------------------------------------------------------
SCodeSeg SEGMENT PARA USE16
ASSUME CS:SCodeSeg,DS emoDataSeg
;----------------------------------------------------------------------------
SBegin PROC FAR
mov ax,LoVideo_Sel
mov es,ax
mov di,0
mov ah,17h
mov cx,MessLen
S1: lodsb
stosw
loop S1
JUMP16 DemoCode_Sel,Demo3
SBegin ENDP
;----------------------------------------------------------------------------
MLen = $-SBegin
SCodeLen = $
SCodeSeg ENDS
;============================================================================
DemoDataSeg SEGMENT PARA USE16 ;演示任务数据段
Mess DB 'Page is OK!'
MessLen = $-Mess
DemoDataLen = $
DemoDataSeg ENDS
;============================================================================
DemoCodeSeg SEGMENT PARA USE16 ;演示任务代码段
ASSUME CSemoCodeSeg
;----------------------------------------------------------------------------
DemoBegin PROC FAR
mov ax,PDT_Sel
mov es,ax
xor di,di
mov cx,1024
xor eax,eax ;先把全部表项置成无效
rep stosd ;再置表项0和表项1
mov DWORD PTR es:[0],PT0_AD OR (USU+RWW+PL)
mov DWORD PTR es:[4],PT1_AD OR (USU+RWW+PL)
mov ax,PT0_Sel ;初始化页表0
mov es,ax
xor di,di
mov cx,1024
xor eax,eax
or eax,USU+RWW+PL
Demo1: stosd
add eax,1000h ;先全部置成对应等地址的
loop Demo1 ;物理页,再特别设置两广表项
mov di,(PhVB_AD SHR 12)*4
mov DWORD PTR es:[di],MPVB_AD+USS+RWW+PL
mov di,(LoVB_AD SHR 12)*4
mov DWORD PTR es:[di],PhVB_AD+USU+RWR+PL
mov ax,PT1_Sel ;初始化页表1
mov es,ax
xor di,di
mov cx,1024
mov eax,400000h
Demo2: stosd ;先把全部表项设置为无效
add eax,1000h
loop Demo2 ;再特别设置1项
mov di,((LoSC_AD SHR 12)AND 3ffh)*4
mov DWORD PTR es:[di],PhSC_AD+USU+RWR+PL
mov eax,PDT_AD
mov cr3,eax
mov eax,cr0
or eax,80000000h
mov cr0,eax
jmp SHORT PageE
PageE: mov ax,DemoData_Sel
mov ds,ax
mov si,OFFSET Mess
JUMP16 LoCode_Sel,SBegin
Demo3: mov eax,cr0
and eax,7fffffffh ;关闭分页机制
mov cr0,eax
jmp SHORT PageD
PageD: mov ax,Normal_Sel
JUMP16 TempCode_Sel,ToDOS
DemoBegin ENDP
;----------------------------------------------------------------------------
DemoCodeLen = $
DemoCodeSeg ENDS
;============================================================================
TempCodeSeg SEGMENT PARA USE16 ;临时任务的代码段
ASSUME CS:TempCodeSeg
;----------------------------------------------------------------------------
Virtual PROC FAR
cld ;为演示在启用分页机制后执
mov ax,SCode_Sel ;行位于较高线性地址空间中
mov ds,ax ;的代码作准备
mov ax,TPSCode_Sel
mov es,ax
mov si,OFFSET SBegin
mov di,si
mov cx,MLen ;把分页演示代码复制到预定
rep movsb ;内存
JUMP16 DemoCode_Sel,DemoBegin
ToDOS: mov ds,ax
mov es,ax
mov eax,cr0 ;准备返回实模式
and al,11111110b
mov cr0,eax
JUMP16 <SEG Real>,<OFFSET Real>
Virtual ENDP
;----------------------------------------------------------------------------
TempCodeSeg ENDS
;============================================================================
RCodeSeg SEGMENT PARA USE16 ;实方式的初始化代码和数据
ASSUME CS:RCodeSeg,DS:RCodeSeg
;----------------------------------------------------------------------------
VGDTR PDesc <GDTLen-1,>
;----------------------------------------------------------------------------
Start PROC
push cs
pop ds
cld
call InitGDT ;初始化全局描述符表GDT
EnableA20
lgdt QWORD PTR VGDTR ;装载GDTR
cli ;关中断
mov eax,cr0
or al,1
mov cr0,eax
JUMP16 <TempCode_Sel>,<OFFSET Virtual>
Real: DisableA20
sti
mov ax,4c00h
int 21h
Start ENDP
;----------------------------------------------------------------------------
InitGDT PROC
push ds
mov ax,GDTSeg
mov ds,ax
mov cx,GDNum
mov si,OFFSET EFFGDT
InitG: mov ax,[si].BaseL
movzx eax,ax
shl eax,4
shld edx,eax,16
mov WORD PTR [si].BaseL,ax
mov BYTE PTR [si].BaseM,dl
mov BYTE PTR [si].BaseH,dh
add si,SIZE Desc
loop InitG
pop ds
mov bx,16
mov ax,GDTSeg
mul bx
mov WORD PTR VGDTR.Base,ax
mov WORD PTR VGDTR.Base+2,dx
ret
InitGDT ENDP
;----------------------------------------------------------------------------
RCodeSeg ENDS
END Start
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发表于 2014-01-16 09:39
