asm6.asm

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

EchoBegin       PROC    FAR
                push    ax                        ;保护现场
                push    ds
                push    es
                mov     ax,EchoData_Sel           ;置显示过程数据段
                mov     ds,ax
                mov     ax,Video_Sel              ;置视频缓冲区数据段
                mov     es,ax
                mov     ax,WORD PTR Mess
                mov     WORD PTR es:[0],ax
                pop     es
                pop     ds
                pop     ax
                iretd
EchoBegin       ENDP
;----------------------------------------------------------------------------
EchoCodeLen     =       $
EchoCodeSeg     ENDS
;----------------------------------------------------------------------------
;演示任务的堆栈段
;----------------------------------------------------------------------------
DemoStackSeg    SEGMENT PARA USE16
DemoStackLen    =       1024
                DB      DemoStackLen DUP(0)
DemoStackSeg    ENDS
;----------------------------------------------------------------------------
;演示任务的数据段
;----------------------------------------------------------------------------
DemoDataSeg     SEGMENT PARA USE16
Flag            DB      0
DemoDataLen     =       $
DemoDataSeg     ENDS
;----------------------------------------------------------------------------
;演示任务的代码段
;----------------------------------------------------------------------------
DemoCodeSeg     SEGMENT PARA USE16
                ASSUME  CS:DemoCodeSeg,DS:DemoDataSeg
;----------------------------------------------------------------------------
DemoBegin       PROC    FAR
                mov     ax,DemoStack_Sel          ;置堆栈
                mov     ss,ax
                mov     sp,DemoStackLen           ;置数据段
                mov     ax,DemoData_Sel
                mov     ds,ax
                mov     es,ax
                mov     fs,ax
                mov     gs,ax
                mov     al,11111110b              ;置中断屏蔽字
                out     IMREGP,al                 ;只开发时钟中断
                sti                               ;开中断
DemoConti:      cmp     BYTE PTR Flag,0           ;判标志
                jz      DemoConti                 ;直到不为0
                cli                               ;关中断
                ;转回临时代码段,准备回实方式
                JUMP16  TempCode_Sel,<OFFSET ToDos>
DemoBegin       ENDP
;----------------------------------------------------------------------------
DemoCodeLen     =       $
DemoCodeSeg     ENDS
;----------------------------------------------------------------------------
TempCodeSeg     SEGMENT PARA USE16                ;临时任务的代码段
                ASSUME  CS:TempCodeSeg
;----------------------------------------------------------------------------
Virtual         PROC    FAR
                JUMP16  DemoCode_Sel,DemoBegin    ;转演示任务
ToDos:          mov     ax,Normal_Sel             ;恢复实方式段描述符高速缓存
                mov     ds,ax
                mov     es,ax
                mov     fs,ax
                mov     gs,ax
                mov     ss,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
;============================================================================
RDataSeg        SEGMENT PARA USE16                ;实方式数据段
VGDTR           PDesc   <GDTLen-1,>               ;GDT伪描述符
VIDTR           PDesc   <IDTLen-1,>               ;IDT伪描述符
NORVIDTR        PDesc   <3ffh,>                   ;用于保存原IDTR值
SPVar           DW      ?                         ;用于保存实方式下的SP
SSVar           DW      ?                         ;用于保存实方式下的SS
IMaskRegV       DB      ?                         ;用于保存原中断屏蔽寄存器值
RDataSeg        ENDS
;----------------------------------------------------------------------------
RCodeSeg        SEGMENT PARA USE16                ;实方式代码段
                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
                call    InitIDT                   ;初始化中断描述符表IDT
                mov     SSVar,ss                  ;保存堆栈指针
                mov     SPVar,sp
                sidt    QWORD PTR NORVIDTR        ;保存IDTR
                in      al,IMREGP
                mov     BYTE PTR IMaskRegV,al
                lgdt    QWORD PTR VGDTR           ;装载GDTR
                cli                               ;关中断
                lidt    QWORD PTR VIDTR           ;装载IDTR
                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
                lidt    QWORD PTR NORVIDTR
                mov     al,IMaskRegV
                out     IMREGP,al
                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
;----------------------------------------------------------------------------
InitIDT         PROC
                mov     bx,16
                mov     ax,IDTSeg
                mul     bx
                mov     WORD PTR VIDTR.Base,ax
                mov     WORD PTR VIDTR.Base+2,dx
                ret
InitIDT         ENDP
;----------------------------------------------------------------------------
RCodeSeg        ENDS
                END     Start