DOS-debug

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;--- DPMIBK32.ASM: 32bit DPMI application written in MASM syntax.
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;--- this sample temporarily switches back to real-mode.
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;--- assemble: JWasm -mz dpmibk32.asm
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LF  equ 10
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CR  equ 13
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    .386
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    .model small
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    .dosseg     ;this ensures that stack segment is last
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    .stack 1024
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;--- DPMI real-mode call structure
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RMCS struct
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rEDI    dd ?
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rESI    dd ?
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rEBP    dd ?
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        dd ?
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rEBX    dd ?
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rEDX    dd ?
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rECX    dd ?
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rEAX    dd ?
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rFlags  dw ?
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rES     dw ?
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rDS     dw ?
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rFS     dw ?
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rGS     dw ?
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rIP     dw ?
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rCS     dw ?
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rSP     dw ?
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rSS     dw ?
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RMCS ends
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    .data
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szWelcome db "welcome in protected-mode",CR,LF,0
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szBackinPM db "back in protected-mode",CR,LF,0
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    .code
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start:
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    push eax
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    mov esi, offset szWelcome
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    call printstring
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    pop ebx
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;--- switch back to real-mode
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    sub esp, sizeof RMCS+2
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    mov ebp,esp
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    xor eax, eax
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    mov [ebp].RMCS.rIP, offset backtoreal
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    mov [ebp].RMCS.rCS, _TEXT16
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    mov [ebp].RMCS.rFlags, ax
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    mov [ebp].RMCS.rDS, ax
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    mov [ebp].RMCS.rES, ax
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    mov [ebp].RMCS.rFS, ax
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    mov [ebp].RMCS.rGS, ax
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    lea eax,[ebp-20h]
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    mov [ebp].RMCS.rSP, ax
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    mov [ebp].RMCS.rSS, bx
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    xor bx,bx
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    xor cx,cx
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    mov edi,ebp
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    push ss
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    pop es
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    mov ax,0301h	;temporarily switch to real-mode
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    int 31h
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    mov esi, offset szBackinPM
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    call printstring
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    mov ax, 4C00h   ;normal client exit
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    int 21h
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;--- print a string in protected-mode with simple
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;--- DOS commands not using pointers.
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printstring:
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    lodsb
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    and al,al
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    jz stringdone
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    mov dl,al
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    mov ah,2
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    int 21h
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    jmp printstring
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stringdone:
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    ret
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;--- now comes the 16bit initialization part
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_TEXT16 segment use16 word public 'CODE'
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backtoreal:
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    push cs
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    pop ds
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    mov dx,offset dBackinRM
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    mov ah,9
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    int 21h
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    retf            ;back to protected-mode
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start16:
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    mov ax,ss
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    mov cx,es
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    sub ax, cx
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    mov bx, sp
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    shr bx, 4
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    inc bx
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    add bx, ax
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    mov ah, 4Ah     ;free unused memory
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    int 21h
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    mov ax, 1687h   ;DPMI host installed?
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    int 2Fh
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    and ax, ax
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    jnz nohost
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    push es         ;save DPMI entry address
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    push di
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    and si, si      ;requires host client-specific DOS memory?
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    jz nomemneeded
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    mov bx, si
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    mov ah, 48h     ;alloc DOS memory
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    int 21h
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    jc nomem
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    mov es, ax
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nomemneeded:
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    mov ax, DGROUP
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    mov ds, ax
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    mov bx, ss
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    mov bp, sp
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    mov ax, 0001        ;start a 32-bit client
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    call far ptr [bp]   ;initial switch to protected-mode
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    jc initfailed
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;--- now in protected-mode
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;--- create a 32bit code selector and jump to 32bit code
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    push bx     ;save stack segment
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    mov cx,1
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    mov ax,0
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    int 31h
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    mov bx,ax
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    mov cx,_TEXT
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    mov dx,cx
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    shl dx,4
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    shr cx,12
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    mov ax,7
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    int 31h     ;set base address
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    mov dx,-1
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    mov cx,0
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    mov ax,8
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    int 31h     ;set descriptor limit to 64 kB
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    mov cx,cs
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    lar cx,cx
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    shr cx,8
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    or ch,40h
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    mov ax,9
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    int 31h     ;set code descriptors attributes to 32bit
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    pop ax      ;store stack segment in AX
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    push ebx
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    push offset start
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    retd        ;jump to 32-bit code
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nohost:
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    mov dx, offset dErr1
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    jmp error
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nomem:
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    mov dx, offset dErr2
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    jmp error
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initfailed:
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    mov dx, offset dErr3
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error:
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    push cs
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    pop ds
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    mov ah, 9
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    int 21h
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    mov ax, 4C00h
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    int 21h
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dErr1 db "no DPMI host installed",CR,LF,'$'
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dErr2 db "not enough DOS memory for initialisation",CR,LF,'$'
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dErr3 db "DPMI initialisation failed",CR,LF,'$'
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dBackinRM db "switched to real-mode",CR,LF,'$'
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_TEXT16 ends
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    end start16
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