asm5.asm

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

                loop    SubR3
                popad
                pop     ebp
                ret     8
SubRB           ENDP
;----------------------------------------------------------------------------
HToASCII        PROC
                and     al,0fh
                add     al,90h
                daa
                adc     al,40h
                daa
                ret
HToASCII        ENDP
;----------------------------------------------------------------------------
SubRLen         =       $
SubRSeg         ENDS                              ;子程序代码段结束
;----------------------------------------------------------------------------
DemoCodeSeg     SEGMENT PARA USE32                ;演示任务的32位代码段
                ASSUME  CS:DemoCodeSeg,DS:DemoDataSeg
;----------------------------------------------------------------------------
DemoBegin       PROC    FAR
                ;把要复制的参数个数置入调用门
                mov     BYTE PTR fs:ToSubR.DCount,2
                ;向2级堆栈中压入参数
                push    DWORD PTR gs:TempTask.TREIP
                push    OFFSET Message
                ;通过调用门调用SubRB
                CALL32  ToSubR_Sel,0
                ;把指向规范数据段描述符的选择子填入临时任务TSS
                ASSUME  DS:TempTSSSeg
                push    gs
                pop     ds
                mov     ax,Normal_Sel
                mov     WORD PTR TempTask.TRDS,ax
                mov     WORD PTR TempTask.TRES,ax
                mov     WORD PTR TempTask.TRFS,ax
                mov     WORD PTR TempTask.TRGS,ax
                mov     WORD PTR TempTask.TRSS,ax
                ;通过任务门切换到临时任务
                JUMP32  ToTempT_Sel,0
                jmp     DemoBegin
DemoBegin       ENDP
DemoCodeLen     =       $
;----------------------------------------------------------------------------
DemoCodeSeg     ENDS                              ;演示任务的32位代码段结束
;----------------------------------------------------------------------------
TempTSSSeg      SEGMENT PARA USE16                ;临时任务的任务状态段TSS
TempTask        TSS     <>
                DB      0ffh                      ;I/O许可位图结束标志
TempTSSLen      =       $
TempTSSSeg      ENDS
;----------------------------------------------------------------------------
TempCodeSeg     SEGMENT PARA USE16                ;临时任务的代码段
                ASSUME  CS:TempCodeSeg
;----------------------------------------------------------------------------
Virtual         PROC    FAR
                mov     ax,TempTSS_Sel            ;装载TR
                ltr     ax
                JUMP16  DemoTSS_Sel,0             ;直接切换到演示任务
                clts                              ;清任务切换标志
                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
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,ES: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
                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    InitLDT
                ret
InitLDT         ENDP
;----------------------------------------------------------------------------
RCodeSeg        ENDS
                END     Start