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103 lines
4.5 KiB

input equ 080h ;command line tail buffer
cr equ 0dh ;ASCII carriage return
cseg segment byte
assume cs:cseg,ds:cseg
org 0100h ;since this will be
; a COM file
clear: ;initialize display...
;call BIOS video driver to
mov ah,15 ;get current display mode:
int 10h ;returns AL = mode, and
;AH = no. of columns.
cmp al,7 ;if we are in graphics modes
je clear0 ;(modes 4,5,6) then exit
cmp al,3 ;but if we are in mode 0-3
ja clear9 ;or 7 then continue.
clear0: ;set up size of window to
;be initialized...
xor cx,cx ;set upper left corner of
;window to (X,Y)=(0,0)
mov dh,24 ;set Y to 24 for lower right
mov dl,ah ;corner, and X to the number
dec dl ;of columns returned by BIOS
;minus 1
mov bh,7 ;initialize attribute byte
;to "normal" video display,
;i.e. white on black.
;set SI=address of command
;tail's length byte
mov si,offset input
cld ;clear the Direction Flag
;for "LODS" string instruction.
lodsb ;check length byte to see if
or al,al ;there's any command tail.
jz clear8 ;no,go clear the screen
;with normal video attribute
clear1: lodsb ;check the next byte of
;the command tail,
cmp al,cr ;if carriage return
je clear8 ;we are done.
or al,20h ;fold the character to
;lower case.
cmp al,'a' ;make sure it's in range a-z
jb clear1 ;no, skip it
cmp al,'z'
ja clear1 ;no, skip it
cmp al,'i' ;I=Set intensity
jne clear2 ;jump if not I
or bh,08 ;set intensity bit
jmp short clear1
clear2: cmp al,'r' ;R=Reverse
jne clear3 ;jump if not R
and bh,088h ;mask off old foreground/
;background bits and
or bh,070h ;change to reverse video
jmp short clear1
clear3: cmp al,'u' ;U=Underline
jne clear4 ;jump if not U
and bh,088h ;mask off old foreground/
;background bits and
or bh,01h ;change to underline
jmp short clear1
clear4: cmp al,'b' ;B=Blink
jne clear5 ;jump if not B
or bh,080h ;set blink bit
jmp short clear1
clear5: cmp al,'s' ;S=Silent
jne clear1 ;if not S try next char.
mov bh,0 ;if S command, rig for
;silent running. Clear
;the foreground/background
;display control fields, and
;don't bother to look for
;any more command characters.
clear8: ;now we have decoded all
;the characters in the
;command tail, and are ready
;to initialize the display.
;BH= desired attribute
;CL,CH=(X,Y),upper left
; corner of window
;DL,DH=(X,Y),lower right
; corner of window
mov ax,0600h ;AH = function type 6,
;AL = lines to scroll (zero)
int 10h ;request initialization
;of window by BIOS
mov ah,2 ;now set the cursor to
mov bh,0 ;(X,Y)=(0,0), Page=0
xor dx,dx
int 10h
clear9: int 20h ;exit to PC-DOS
cseg ends
end clear