LISTING 24.1 L24-1.ASM
; Program to illustrate operation of ALUs and latches of the VGA’s
; Graphics Controller. Draws a variety of patterns against
; a horizontally striped background, using each of the 4 available
; logical functions (data unmodified, AND, OR, XOR) in turn to combine
; the images with the background.
; By Michael Abrash.
;
stack segment para stack ‘STACK’
db 512 dup(?)
stack ends
;
VGA_VIDEO_SEGMENT equ 0a000h ;VGA display memory segment
SCREEN_HEIGHT equ 350
SCREEN_WIDTH_IN_BYTES equ 80
DEMO_AREA_HEIGHT equ 336 ;# of scan lines in area
; logical function operation
; is demonstrated in
DEMO_AREA_WIDTH_IN_BYTES equ 40 ;width in bytes of area
; logical function operation
; is demonstrated in
VERTICAL_BOX_WIDTH_IN_BYTES equ 10 ;width in bytes of the box used to
; demonstrate each logical function
;
; VGA register equates.
;
GC_INDEX equ 3ceh ;GC index register
GC_ROTATE equ 3 ;GC data rotate/logical function
; register index
GC_MODE equ 5 ;GC mode register index
;
dseg segment para common ‘DATA’
;
; String used to label logical functions.
;
LabelString label byte
db ‘UNMODIFIED AND OR XOR ’
LABEL_STRING_LENGTH equ $-LabelString
;
; Strings used to label fill patterns.
;
FillPatternFF db ‘Fill Pattern: 0FFh’
FILL_PATTERN_FF_LENGTH equ $ - FillPatternFF
FillPattern00 db ‘Fill Pattern: 000h’
FILL_PATTERN_00_LENGTH equ $ - FillPattern00
FillPatternVert db ‘Fill Pattern: Vertical Bar’
FILL_PATTERN_VERT_LENGTH equ $ - FillPatternVert
FillPatternHorz db ‘Fill Pattern: Horizontal Bar’
FILL_PATTERN_HORZ_LENGTH equ $ - FillPatternHorz
;
dseg ends
;
; Macro to set indexed register INDEX of GC chip to SETTING.
;
SETGC macro INDEX, SETTING
mov dx,GC_INDEX
mov ax,(SETTING SHL 8) OR INDEX
out dx,ax
endm
;
;
; Macro to call BIOS write string function to display text string
; TEXT_STRING, of length TEXT_LENGTH, at location ROW,COLUMN.
;
TEXT_UP macro TEXT_STRING, TEXT_LENGTH, ROW, COLUMN
mov ah,13h ;BIOS write string function
mov bp,offset TEXT_STRING ;ES:BP points to string
mov cx,TEXT_LENGTH
mov dx,(ROW SHL 8) OR COLUMN ;position
sub al,al ;string is chars only, cursor not moved
mov bl,7 ;text attribute is white (light gray)
int 10h
endm
;
cseg segment para public ‘CODE’
assume cs:cseg, ds:dseg
start proc near
mov ax,dseg
mov ds,ax
;
; Select 640x350 graphics mode.
;
mov ax,010h
int 10h
;
; ES points to VGA memory.
;
mov ax,VGA_VIDEO_SEGMENT
mov es,ax
;
; Draw background of horizontal bars.
;
mov dx,SCREEN_HEIGHT/4
;# of bars to draw (each 4 pixels high)
sub di,di ;start at offset 0 in display memory
mov ax,0ffffh ;fill pattern for light areas of bars
mov bx,DEMO_AREA_WIDTH_IN_BYTES / 2 ;length of each bar
mov si,SCREEN_WIDTH_IN_BYTES - DEMO_AREA_WIDTH_IN_BYTES
mov bp,(SCREEN_WIDTH_IN_BYTES * 3) - DEMO_AREA_WIDTH_IN_BYTES
BackgroundLoop:
mov cx,bx ;length of bar
rep stosw ;draw top half of bar
add di,si ;point to start of bottom half of bar
mov cx,bx ;length of bar
rep stosw ;draw bottom half of bar
add di,bp ;point to start of top of next bar
dec dx
jnz BackgroundLoop
;
; Draw vertical boxes filled with a variety of fill patterns
; using each of the 4 logical functions in turn.
;
SETGC GC_ROTATE, 0 ;select data unmodified
; logical function...
mov di,0
call DrawVerticalBox ;...and draw box
;
SETGC GC_ROTATE, 08h ;select AND logical function...
mov di,10
call DrawVerticalBox ;...and draw box
;
SETGC GC_ROTATE, 10h ;select OR logical function...
mov di,20
call DrawVerticalBox ;...and draw box
;
SETGC GC_ROTATE, 18h ;select XOR logical function...
mov di,30
call DrawVerticalBox ;...and draw box
;
; Reset the logical function to data unmodified, the default state.
;
SETGC GC_ROTATE, 0
;
; Label the screen.
;
push ds
pop es ;strings we’ll display are passed to BIOS
; by pointing ES:BP to them
;
; Label the logical functions, using the VGA BIOS’s
; write string function.
;
TEXT_UP LabelString, LABEL_STRING_LENGTH, 24, 0
;
; Label the fill patterns, using the VGA BIOS’s
; write string function.
;
TEXT_UP FillPatternFF, FILL_PATTERN_FF_LENGTH, 3, 42
TEXT_UP FillPattern00, FILL_PATTERN_00_LENGTH, 9, 42
TEXT_UP FillPatternVert, FILL_PATTERN_VERT_LENGTH, 15, 42
TEXT_UP FillPatternHorz, FILL_PATTERN_HORZ_LENGTH, 21, 42
;
; Wait until a key’s been hit to reset screen mode & exit.
;
WaitForKey:
mov ah,1
int 16h
jz WaitForKey
;
; Finished. Clear key, reset screen mode and exit.
;
Done:
mov ah,0 ;clear key that we just detected
int 16h
;
mov ax,3 ;reset to text mode
int 10h
;
mov ah,4ch ;exit to DOS
int 21h
;
start endp
;
; Subroutine to draw a box 80x336 in size, using currently selected
; logical function, with upper left corner at the display memory offset
; in DI. Box is filled with four patterns. Top quarter of area is
; filled with 0FFh (solid) pattern, next quarter is filled with 00h
; (empty) pattern, next quarter is filled with 33h (double pixel wide
; vertical bar) pattern, and bottom quarter is filled with double pixel
; high horizontal bar pattern.
;
; Macro to draw a column of the specified width in bytes, one-quarter
; of the height of the box, with the specified fill pattern.
;
DRAW_BOX_QUARTER macro FILL, WIDTH
local RowLoop, ColumnLoop
mov al,FILL ;fill pattern
mov dx,DEMO_AREA_HEIGHT / 4 ;1/4 of the full box height
RowLoop:
mov cx,WIDTH
ColumnLoop:
mov ah,es:[di] ;load display memory contents into
; GC latches (we don’t actually care
; about value read into AH)
stosb ;write pattern, which is logically
; combined with latch contents for each
; plane and then written to display
; memory
loop ColumnLoop
add di,SCREEN_WIDTH_IN_BYTES - WIDTH
;point to start of next line down in box
dec dx
jnz RowLoop
endm
;
DrawVerticalBox proc near
DRAW_BOX_QUARTER 0ffh, VERTICAL_BOX_WIDTH_IN_BYTES
;first fill pattern: solid fill
DRAW_BOX_QUARTER 0, VERTICAL_BOX_WIDTH_IN_BYTES
;second fill pattern: empty fill
DRAW_BOX_QUARTER 033h, VERTICAL_BOX_WIDTH_IN_BYTES
;third fill pattern: double-pixel
; wide vertical bars
mov dx,DEMO_AREA_HEIGHT / 4 / 4
;fourth fill pattern: horizontal bars in
; sets of 4 scan lines
sub ax,ax
mov si,VERTICAL_BOX_WIDTH_IN_BYTES ;width of fill area
HorzBarLoop:
dec ax ;0ffh fill (smaller to do word than byte DEC)
mov cx,si ;width to fill
HBLoop1:
mov bl,es:[di] ;load latches (don’t care about value)
stosb ;write solid pattern, through ALUs
loop HBLoop1
add di,SCREEN_WIDTH_IN_BYTES - VERTICAL_BOX_WIDTH_IN_BYTES
mov cx,si ;width to fill
HBLoop2:
mov bl,es:[di] ;load latches
stosb ;write solid pattern, through ALUs
loop HBLoop2
add di,SCREEN_WIDTH_IN_BYTES - VERTICAL_BOX_WIDTH_IN_BYTES
inc ax ;0 fill (smaller to do word than byte DEC)
mov cx,si ;width to fill
HBLoop3:
mov bl,es:[di] ;load latches
stosb ;write empty pattern, through ALUs
loop HBLoop3
add di,SCREEN_WIDTH_IN_BYTES - VERTICAL_BOX_WIDTH_IN_BYTES
mov cx,si ;width to fill
HBLoop4:
mov bl,es:[di] ;load latches
stosb ;write empty pattern, through ALUs
loop HBLoop4
add di,SCREEN_WIDTH_IN_BYTES - VERTICAL_BOX_WIDTH_IN_BYTES
dec dx
jnz HorzBarLoop
;
ret
DrawVerticalBox endp
cseg ends
end start
Graphics Programming Black Book © 2001 Michael Abrash