asm4.asm

保护模式教程源码 The source code of protection mode in dos

                mov     ah,4eh                    ;置显示属性(红底黄字)
EchoSub1:       lodsb
                or      al,al
                jz      EchoSub2
                stosw
                jmp     EchoSub1
EchoSub2:       mov     eax,[ebp+8]               ;从堆栈中取调用程序的选择子
                and     al,3                      ;调用程序的CPL在CS的RPL字段
                add     al,'0'
                mov     ah,4eh                    ;置显示属性(红底黄字)
                stosw
                pop     ebp
                retf
EchoSub         ENDP
;----------------------------------------------------------------------------
EchoSubRLen     =       $
;----------------------------------------------------------------------------
EchoSubRSeg     ENDS                              ;显示子程序代码段结束
;----------------------------------------------------------------------------
DemoCodeSeg     SEGMENT PARA USE32                ;32位代码段(3级)
                ASSUME  CS:DemoCodeSeg
;----------------------------------------------------------------------------
DemoBegin       PROC    FAR
                CALL32  ToEcho_Sel,0              ;显示当前特权级(变换到1级)
                CALL32  ToT32B_Sel,0              ;转到过渡代码段(变换到0级)
DemoBegin       ENDP
DemoCodeLen     =       $
;----------------------------------------------------------------------------
DemoCodeSeg     ENDS                              ;32位代码段结束
;----------------------------------------------------------------------------
T32CodeSeg      SEGMENT PARA USE32                ;32位过渡代码段(0级)
                ASSUME  CS:T32CodeSeg
;----------------------------------------------------------------------------
T32Begin        PROC    FAR
                mov     ax,DemoStack0_Sel         ;建立0级堆栈
                mov     ss,ax
                mov     esp,DemoStack0Len
                push    DWORD PTR DemoStack3_Sel  ;压入3级堆栈指针
                push    DemoStack3Len
                push    DWORD PTR DemoCode_SEL    ;压入入口点
                push    OFFSET DemoBegin
                retf                              ;利用RET实现转3级的演示代码
T32Begin        ENDP
;----------------------------------------------------------------------------
T32End          PROC    FAR
                JUMP32  TempCode_Sel,<OFFSET ToReal>
T32End          ENDP
T32CodeLen      =       $
;----------------------------------------------------------------------------
T32CodeSeg      ENDS
;----------------------------------------------------------------------------
TempCodeSeg     SEGMENT PARA USE16                ;16位临时代码段(0级)
                ASSUME  CS:TempCodeSeg
;----------------------------------------------------------------------------
Virtual         PROC    FAR
                mov     ax,DemoTSS_Sel            ;装载TR
                ltr     ax
                mov     ax,DemoLDT_Sel            ;装载LDTR
                lldt    ax
                JUMP16  ToT32A_Sel,0              ;通过调用门转过渡段
ToReal:         mov     ax,Normal_Sel             ;准备切换回实模式
                mov     ds,ax
                mov     es,ax
                mov     fs,ax
                mov     gs,ax
                mov     ss,ax
                mov     eax,cr0
                and     al,11111110b
                mov     cr0,eax
                JUMP16  <SEG Real>,<OFFSET Real>
Virtual         ENDP
TempCodeLen     =       $
;----------------------------------------------------------------------------
TempCodeSeg     ENDS
;============================================================================
RDataSeg        SEGMENT PARA USE16                ;实方式数据段
VGDTR           PDesc   <GDTLen-1,>               ;GDT伪描述符
SPVar           DW      ?                         ;用于保存实方式下的SP
SSVar           DW      ?                         ;用于保存实方式下的SS
RDataSeg        ENDS
;----------------------------------------------------------------------------
RCodeSeg        SEGMENT PARA USE16
                ASSUME  CS:RCodeSeg,DS:RDataSeg
;----------------------------------------------------------------------------
Start           PROC
                mov     ax,RDataSeg
                mov     ds,ax
                cld
                CALL    InitGDT                   ;初始化全局描述符表GDT
                mov     ax,DemoLDTSeg
                mov     fs,ax
                mov     si,OFFSET DemoLDT
                mov     cx,DemoLDNum
                CALL    InitLDT                   ;初始化局部描述符表LDT
                mov     SSVar,ss
                mov     SPVar,sp
                lgdt    QWORD PTR VGDTR           ;装载GDTR并切换到保护方式
                cli
                mov     eax,cr0
                or      al,1
                mov     cr0,eax
                JUMP16  <TempCode_Sel>,<OFFSET Virtual>
Real:           mov     ax,RDataSeg
                mov     ds,ax
                lss     sp,DWORD PTR SPVar        ;又回到实方式
                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
;----------------------------------------------------------------------------
;入口参数:FS:SI=第一个要初始化的描述符,CX=要初始化的描述符数
;----------------------------------------------------------------------------
InitLDT         PROC
ILDT:           mov     ax,WORD PTR FS:[si].BaseL
                movzx   eax,ax
                shl     eax,4
                shld    edx,eax,16
                mov     WORD PTR fs:[si].BaseL,ax
                mov     BYTE PTR fs:[si].BaseM,dl
                mov     BYTE PTR fs:[si].BaseH,dh
                add     si,SIZE Desc
                loop    ILDT
                ret
InitLDT         ENDP
;----------------------------------------------------------------------------
RCodeSeg        ENDS
                END     Start