DOS-debug

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;--- DPMICL16.ASM: 16bit DPMI application written in MASM syntax.
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;--- assemble: JWasm -bin -Fo dpmicl16.com dpmicl16.asm
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LF  equ 10
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CR  equ 13
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    .286
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    .model tiny
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    .code
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    org 100h
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;--- the 16bit initialization part
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start:
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    pop ax          ;get word saved on stack for COM files
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    mov bx, sp
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    shr bx, 4
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    jnz @F
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    mov bx,1000h    ;it was a full 64kB stack
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@@:
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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 bp, sp
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    mov ax, 0000    ;start a 16-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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    call printstring
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    db "welcome in protected-mode",CR,LF,0
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    mov ax, 4C00h   ;normal client exit
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    int 21h
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nohost:
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    call error
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    db "no DPMI host installed",CR,LF,'$'
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nomem:
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    call error
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    db "not enough DOS memory for initialisation",CR,LF,'$'
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initfailed:
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    call error
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    db "DPMI initialisation failed",CR,LF,'$'
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error:
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    push cs
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    pop ds
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    pop dx
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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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;--- 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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    pop si
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nextchar:
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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 nextchar
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stringdone:
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    push si
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    ret
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    end start
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