96 lines
2.1 KiB
NASM
96 lines
2.1 KiB
NASM
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; Functions:
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; al=0 then Disable communications interupt vector
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; al=1 then Enable communications interupt vector
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; Issue and int 44h
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;
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progseg segment para public 'CODE'
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public setcom
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assume cs:progseg, ds:progseg, es:progseg
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org 100h
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doscall equ 21h
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;
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startup proc far
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jmp setup
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;
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setcom proc far
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push ds
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push es
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push dx
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push ax
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mov ax,cs
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mov ds,ax
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mov es,ax
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pop ax
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cmp al,1 ;is function 1
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jz enable ;then enable
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;
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disable:
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mov dx,offset interupt ;get new vector address
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mov ax,cs
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mov ds,ax ;set segment
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mov ah,25h ;set interupt vector address function
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mov al,14h ;communications interupt vector
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int doscall ;set the interupt
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jmp exit ;exit
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enable:
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mov dx,word ptr vector ;set old segment
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mov ds,vector+2 ;set old communications vector
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mov ah,25h ;set interupt vector address function
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mov al,14h ;communications interupt vector
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int doscall
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exit:
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pop dx
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pop es
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pop ds
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;
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interupt proc far
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sub ax,ax ;zero return status
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iret
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interupt endp
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setcom endp
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;
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msg db 'Serial communications intercepter installed',0ah,0dh,'$'
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msg1 db 'Serial communications intercepter is already installed',0ah,0dh,'$'
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vector db 8 dup(0) ;only 4 needed 4 more for safety
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;
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setup:
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mov ah,35h ;get interupt vector address function
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mov al,44h ;communications interupt vector
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int doscall ;go get it
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cmp bx,0 ;check if vector used
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jnz lderr ;if used then exit
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mov ax,es ;check segment
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cmp ax,0
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jnz lderr
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;
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mov dx,offset msg
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mov ah,9
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int doscall
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;
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mov ah,35h ;get interupt vector address function
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mov al,14h ;communications interupt vector
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int doscall ;go get it
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;
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mov word ptr vector,bx ;save offset
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mov bx,es ;get segment address
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mov word ptr vector+2,bx ;save segment
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;
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mov dx,offset setcom ;get new vector address
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mov ax,cs
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mov ds,ax ;set segment
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mov ah,25h ;set interupt vector address function
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mov al,44h ;set to our new interupt vector
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int doscall ;set the interupt
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;
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mov dx,offset setup ;terminate and stay resident
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int 27h
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lderr:
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mov dx,offset msg1
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mov ah,9
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int doscall
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int 20h
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startup endp
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progseg ends
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;
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end startup
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