mirror of
https://github.com/vxunderground/MalwareSourceCode.git
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99 lines
3.0 KiB
NASM
99 lines
3.0 KiB
NASM
; Resident program to provide flicker-free write_tty scroll for Color
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; Graphics Adapter clones with dual-ported memory. M. Abrash 5/3/86.
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; Make runnable with MASM-LINK-EXE2BIN.
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cseg segment
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assume cs:cseg
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org 100h ;necessary for COM file
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start proc near
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jmp makeres
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old_int10 dd ?
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; front end routine for BIOS video handler to scroll without flicker
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scroll_front_end:
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cmp ax,0e0ah ;only intercept write_tty function
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jnz pass_to_bios ; called with linefeed
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push ax
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push bx
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mov ah,0fh
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int 10h ;get current page & mode
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cmp al,2
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jz check_row ;BIOS only blanks in modes 2 & 3, so
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cmp al,3 ; only intercept linefeed scroll in
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jnz pass_to_bios2 ; modes 2 & 3
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check_row: ;see if cursor is on bottom row, in
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push cx ; which case linefeed causes scroll
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push dx
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mov ah,3
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int 10h ;get cursor location in current page
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cmp dh,24
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jnz pass_to_bios3 ;cursor not on bottom row, no scroll
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push ds ;meets all the criteria, so perform
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push es ; scroll in current page with special
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push si ; routine that doesn't disable video
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push di
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mov ah,0fh
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int 10h ;get # columns & page
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mov al,ah
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sub ah,ah ;convert to word
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push ax ;set aside # columns
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mov si,ax
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shl si,1 ;move from second row (each character=2 bytes)
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mov ah,24
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mul ah ;# words to move (24 rows)
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mov cx,ax
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sub ax,ax ;now adjust offsets for current page
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mov ds,ax ;buffer length is stored in BIOS segment
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mov al,bh ;get current page
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mul word ptr ds:[44ch] ;offset of start of current page
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add si,ax ;move data from second row of current page
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mov di,ax ; to top of current page
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mov ax,0b800h
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mov ds,ax
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mov es,ax ;will move data in display segment
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cld
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rep movsw ;scroll screen up
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mov ah,8 ;BH already has current page
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int 10h ;get attribute of character at cursor
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mov al,' ' ;fill with blanks & attribute just obtained
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pop cx ;# of words per row
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rep stosw ;blank bottom row-DI points to bottom row
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pop di ;done
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pop si
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pop es
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pop ds
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pop dx
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pop cx
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pop bx
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pop ax
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iret
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pass_to_bios3:
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pop dx
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pop cx
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pass_to_bios2:
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pop bx
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pop ax
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pass_to_bios: ;pass interrupt to normal BIOS handler
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jmp cs:[old_int10]
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endres:
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; make scroll front end handler resident & revector interrupt 10 to it
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makeres:
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push cs
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pop ds
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assume ds:cseg
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mov ax,3510h ;DOS get vector function, vector 10h
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int 21h ;get vector 10h
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mov word ptr [old_int10],bx ;set aside old vector to
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mov word ptr [old_int10+2],es ; allow pass to BIOS
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mov ax,2510h ;DOS set vector function, vector 10h
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mov dx,offset scroll_front_end ;revector interrupt
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int 21h ; 10h to front end routine
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mov dx,offset endres ;# of paragraphs to make
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mov cl,4 ; resident-can't do with an
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shr dx,cl ; expression because assembler can't
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inc dx ; calculate w/relocatable label
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mov ax,3100h ;DOS make resident fn, exit code=0
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int 21h ;terminate & stay resident
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start endp
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cseg ends
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end start
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