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%SUBTTL "Scroll Window Left routine with sample driver"
; Scroll2.Asm
; see ScrollLeft description for complete explanation of this program
MODEL small
STACK 200h ; this is more than enough stack
; this test stub scrolls the window at (4,3)-(22,68) left 9 columns,
; filling in with spaces of attribute white on black (07)
mov al,9 ; number of columns to scroll
mov bh,07 ; attribute to use for blanked chars
mov ch,4 ; row of upper left hand corner of window
mov cl,3 ; col of upper left hand corner of window
mov dh,22 ; row of lower right hand corner of window
mov dl,68 ; col of lower right hand corner of window
call ScrollLeft
mov ah,4ch ; exit with errorlevel from scroll routine
int 21h
; ScrollLeft
; PURPOSE: scrolls a rectangular region of a text screen (window) to the
; left by a variable number of columns
; INPUTS: AL = number of columns to scroll (if zero, no effect)
; BH = attribute to use for blanked characters
; CH,CL = row, col for upper left hand corner of window
; DH,DL = row, col for lower right hand corner of window
; NOTES: upper left hand corner of screen (home) is (0,0)
; video adapter is checked to see if it's in text mode
; screen dimensions are taken from BIOS data
; calling sequence is nearly identical to BIOS scroll routine
; display pages supported
; dual monitors supported
; MDA/CGA/EGA/VGA supported
; RETURNS: On error, Carry flag set, AX contains error number
; otherwise carry is clear, AX contains 0
; Error codes: 0 - no error
; -1 - some error occurred
PROC ScrollLeft
ScrRows equ 0484h ; location of BIOS rows data
ScrCols equ 044ah ; location of BIOS cols data
ScrMode equ 0449h ; location of BIOS mode data
ScrOffs equ 044eh ; location of BIOS scrn offset data
; first, save all of the registers we're going to trash
push bx
push cx
push dx
push si
push di
push bp
push es
push ds
mov bl,al ; stow cols in BL
xor ax,ax
mov ds,ax ; point data seg to 0000 to read BIOS data
cmp ch,dh ; srow > erow ?
ja @@badcoords
cmp cl,dl ; scol > ecol ?
ja @@badcoords
; now we need to load ScrRows, if we haven't got EGA or better
mov al,[ScrRows] ; is MAXROWS = 0 ?
cmp al,0 ; if so, then assume we've got MDA or CGA
jne @@ega_plus ; if not, we've got correct value already
mov al,24 ; otherwise, assume 25 rows (and load 25-1)
cmp dh,al ; erow >= MAXROWS?
; note that BIOS actually stores MAXROWS-1 in this location, so the
; actual jump instruction is (correctly) written as ja
ja @@badcoords
cmp dl,[ScrCols] ; ecol >= MAXCOLS?
jae @@badcoords
mov ah,bl ; remember cols
add ah,cl ; cols + scol
cmp ah,dl ; (cols + scol) > ecol ?
ja @@badcoords
; figure out where the video buffer starts
mov bp,0b800h ; first guess
mov al,[ScrMode] ; get mode from BIOS' RAM
cmp al,4 ; if mode is 0 through 3, we're all set
jb @@modeOK
mov bp,0b000h ; second guess
cmp al,7 ; is it mode 7 (monochrome 80 col) ?
je @@modeOK ; if so, we're still OK
mov ax,-1 ; set error code
stc ; and set error (carry) flag
jmp @@exit
mov es,bp ; set up our video pointer segment
; ES:DI = endloc = Screen + 2 * (MAXCOLS * srow + scol)
mov di,[ScrOffs] ; offset of screen buffer
xor ah,ah ; clear out high half of AX
mov al,[ScrCols] ; get the width of the screen
mul ch ; multiply width by the row number
add al,cl ; now add the column number
adc ah,0 ; propagate carry bit to other half of AX
shl ax,1 ; now multiply by 2 bytes/char
add di,ax ; add the offset to the screen start address
; DS:DI = startloc = endloc + 2 * cols
xor ah,ah ; clear top half of ax
mov al,bl ; recall cols
mov si,di ; start address = end address
add si,ax ; add cols
add si,ax ; and again
; start on count calculation (figure ecol-scol+1)
sub dl,cl ; ecol - scol
inc dl ; now dl = ecol - scol + 1
; calculate increment and stow in BP
; increment = (MAXCOLS - (ecol - scol + 1)) * 2
xor ah,ah ; clear top half of ax
mov al,dl ; use partial count calculation
neg ax
add al,[ScrCols] ; now add in screen width
adc ah,0 ; propagate carry bit to hi half of AX
shl ax,1 ; now double it (2 bytes/char)
mov bp,ax
; finish count calculations and put in DL
; count = (ecol - scol + 1) - cols
sub dl,bl ; figure in cols
mov ax,es ; recall our video pointer
mov ds,ax ; now duplicate it
; load up AX with char and attribute for blank space
mov al,32 ; ASCII space character
mov ah,bh ; passed attribute byte
sub dh,ch ; get row count
mov dh,bh ; save loop count (rows to move) in bh
xor ch,ch ; zero out hi half of CX
cld ; assure that we move the right direction (up)
mov cl,dl ; load in count (words to move)
rep movsw ; move the line over
mov cl,bl ; recall cols (blanks to insert)
rep stosw ; fill in the rest of the line
add di,bp ; advance to next line by adding increment
mov si,di ; now set up the other (source) pointer
mov cl,bl ; recall cols
add si,cx ; add in cols
add si,cx ; and again (because 2 bytes/char)
dec bh ; decrement loop counter
ja @@looptop ; skip back if we're not done yet
mov ax,0 ; set error code to zero (all OK)
clc ; and clear error (carry) flag
pop ds ; restore all of our registers
pop es
pop bp
pop di
pop si
pop dx
pop cx
pop bx
ret ; mosey on home again
ENDP ScrollLeft
END start
-+------- cut here -----------