Graphical programming in Assembly (DOS)

Contents

1. Change to a VGA resolution.

With BIOS interrupt 13 AH = 00 we're ably to change the current screen resolution (See below).

        Al (hex)        video mode

0
1
2
3
4
5
6
7
...
F
10
11
12
text 40 x 25 16 grey
text 40 x 25 16 color
text 80 x 25 16 grey
text 80 x 25 16 color
graph (CGA) 320 x 200 color
graph (CGA) 320 x 200 black / white
graph (CGA) 640 x 200 black / white
text 80 x 25 black / white (MDA, Hercules)
...
graph (EGA,VGA) 640x350 grey
graph (EGA,VGA) 640x350 16 colors
graph (VGA) 2 colors
graph (VGA) 16 colors

In the next paragraphs we'll discuss meanly the standard VGA resolution (12h) These days most VGA-boards has "Super VGA" resolutions, in the next table you'll find the information to change to a "VESA Super VGA"

INT 10hAX = 4F02h
BX = mode
        BX        video mode

100h
101h
102h
103h
104h
105h
106h
107h
108h
109h
10Ah
10Bh
10Ch
graph 640x400   256 colors
graph 640x480   256 colors
graph 800x600   16   colors
graph 800x600   256 colors
graph 1024x768   16   colors
graph 1024x768  256 colors
graph 1280x1024 16   colors
graph 1280x1024 256 colors
text 80x60
text 132x25
text 132x43
text 132x50
text 132x60

When your VGA BIOS support this function AH will become 00h, otherwise AH will return 01h. The issues in the next paragraph can also be use in the VESA resolution 800 x 600 16 colors.

2. Write to the video memory

The standard IBM VGA card has 16 color therefor every pixel needs 4 bits (2^4 = 16) The VGA graphical card uses bitplanes, every main color has his own memory area in the video memory. Because the memory address is equal for every main color (A000:0000h memory mapped) we have to program the sequencer to select the right memory bank.

register selection with port 03C4h.

   register   name

00
01
02
03
04
reset
Clocking mode
Map Mask
Character Map Select   
Memory mode

With the Map Mask register we can select to which color plane we write.

[gfx vga_fig1.jpg]

When we set bit 0 we'll write to the blue bitplane, if we set bit 0 and 1 (3) we'll write to the to blue bitplane. The next example will elucidate the above theory.


 .MODEL SMALL
 .CODE
start:
 mov ax,012h ;VGA mode
 int 10h ;640 x 480 16 colors.
 mov ax,0A000h
 mov es,ax ;ES points to the video memory.
 mov dx,03C4h ;dx = indexregister
 mov ax,0F02h ;INDEX = MASK MAP, 
 out dx,ax ;write all the bitplanes.
 mov di,0 ;DI pointer in the video memory.
 mov cx,38400 ;(640 * 480)/8 = 38400
 mov ax,0FFh ;write to every pixel.
 rep stosb ;fill the screen
 mov ah,4ch ;go back 
 int 21h ; to DOS.
 END start

 

With mov ax,0F02h, out dx,ax we select all the bitplanes this results in the color white. If we change mov ax,0F02h to mov ax,0102h we'll get a blue screen ;-).

3. Video Palette

Another advantange of VGA is the ability to modify the colors, so it's possible to give pictures a more "real" look.
There're two way the modify the video palette program direct to the video hardware or use the video software interrupt (INT 10h). Below you'll find howto change the palette by INT 10h.

With INT 10h AX = 1010h we can change the palette of one color, BX contains the color register, CH the green value, CL the blue value and DH the red value. There is a difference between "color register" and "color number" we can solve this by first request the "color register" of the "color number". We can do this with INT 10 AX = 1007h, BL contains the color number the color register will be returned in BH. The next example will proof that practice is less complicated that the theory.

 
;------------------------------------------
;Modify the palette of one color.
;BX = color number
;ch = green 
;cl = blue 
;dh = red 
;------------------------------------------
pal PROC
 push ax
 mov bh,0
 mov ax,1007h
 int 10h
 mov bl,bh
 mov bh,0
 mov ax,1010h
 int 10h
 pop ax
 ret
pal ENDP

 

4. Reading the Video memory

Reading the Video memory is more or less the same as writing to it. We have to change the graphical control to read mode. This is possible with port address 03CEh and 03CFh.
The next table gives you a overview of the possible functions.

 

Mode adjustment by port 03CEh.

 
   register   Name

1
2
3
4
5
6
7
8
Set/Reset
Enable Set/Reset
Data Rotate/Function Select    
Read Map Select
Graphics mode
Miscellaneous
Color don't care
Bit Mask

 

When we set port 03CEh to 3 we set the VGA card in read mode with port 03CFh we can specify the color bitplane to read. In the next table contains the colors with equivalent values.

bitplane selection by port 03CFh

      3      
2
1
0		             blue            
green
red
intensity

 

The next procedure will demonstrate the above theory. It read one byte in the 4 color bitplanes (BGRI), with SI we specify the byte address.

 
 PUBLIC readbyte
;--------------------------------------------
;Read one byte in the video memory
;call : SI byte address
;answer : al,ah,bl,bh -> bl,gr,rood,inten
;--------------------------------------------
readbyte PROC NEAR
 push cx
 push dx
 push si
 push es
 mov ax,0A000h
 mov es,ax
 mov dx,03ceh 
 mov ax,0005h
 out dx,ax
 mov ax,0304h ;AH = bitplane, AL = read modus
rb01: out dx,ax ;go to read mode 
 mov bh,es:[si] ;read the blue byte
 dec ah
 out dx,ax ;AH = green
 mov bl,es:[si] ;read the green byte
 dec ah
 out dx,ax ;AH = red
 mov ch,es:[si] ;read the red byte
 dec ah 
 out dx,ax ;AH = intensity
 mov cl,es:[si] ;read the intensity byte
 mov ax,cx
 pop es
 pop si
 pop dx
 pop cx
 ret
readbyte ENDP

 

5. Different write modes

There're 4 different write modes; replace, AND, OR and XOR, we change the write mode by the "Data Rotate/Function Select" register (see table in paragraph 2).By putting 3 in the selection register 03CEh we select the "Data Rotate/Function Select".

Like the name suggest we also have the ability the rotate the date, this isn't often used since it's faster to rotate the date with the normal processor.

With the dataregister at port address 03CFh we can specify how to the data is written to the screen.

write mode selection by port 03CFh

      bit 4             bit3                  function            

         0                   0
         0                   1
         1                   0
         1                   1
replace
AND
OR
XOR

 

sample:

mov ax,1803h
mov dx,03CEh
out dx,ax

 

Will set the VGA card write mode to XOR.

6. Drawing a line

Drawing a line in assembly isn't that easy... and it is very important not to use to complicated functions (cos, sin ...) since this is too difficult to program and (more important) will slow the function.
The most widely used function to draw a line is the so called Bresenham algoritme which is published in IBM System Journal in 1965.

The equation of a line : y = (dy/dx).x

[gfx vga_fig2.jpg]

When the line is closer to point A, we have to draw point A. When the line is closer to point B we need to draw point B.


a = (yi+1) - (dy/dx).xi

b = (dy/dx).xi - yi

(b-a) = 2.(dy/dx).xi - 2.yi - 1

(b-a).dx = 2.(dy.xi - yi.dx) - dx

suppose di = (b-a).dx

di = 2.(dy.xi - yi.dx) - dx

 

We need to calculate di with the information of the last drew point:


(di - di-1) = 2.(dy.xi - yi.dx) - dx - 2.(dy.xi-1 - yi-1.dx) + dx

(di - di-1) = 2.(dy.(xi - xi-1) - dx(yi - yi-1))

 

For the step in the x direction (xi - xi-1) we choose 1.


(di - di-1) = 2.(dy - dx(yi - yi-1))

 

The value of (yi - yi-1) depends on where we have to draw the point, (yi - yi-1) is 1 when we need to draw point A (yi-1 +1) and will be 0 if we need to draw point B (yi = yi-1).

When we have to draw point A : (di - di-1) = 2.(dy - dx)
When we have to draw point B : (di - di-1) = 2.dy

Below you'll find a Turbo Pascal program that uses the Bresenham algoritme.


program tst_lijn;
uses crt,graph,dos;
var regs: registers;
 d,dx,dy: integer;
 Aincr,Bincr,Yincr: integer;
 x,y: integer;
procedure lijn (x1,y1,x2,y2 : integer);
var j :integer;
begin
if (x1 > x2) then begin j:=x1;x1:=x2;x2:=j;j:=y1;y1:=y2;y2:=j;end;
if (y2 > y1) then yincr:=1 else yincr :=-1;
dx := x2 - x1;dy:= abs(y2-y1);
d := 2*dy-dx;
aincr := 2*(dy-dx); {stap indien punt A geplaatst wordt}
bincr := 2*dy; {stap indien punt B geplaatst wordt}
x := x1;y:=y1; 
putpixel(x,y,15); {tekenen van het eerste punt}
for x := (x1+1) to x2 do
begin
if d>=0 then begin y := y+yincr;d := d +aincr;end
 else d := d+bincr;
putpixel (x,y,15);
end;
end;
procedure open_graph;
var
 grdriver,grmode,errcode : integer;
begin
 grdriver:=detect;
 initgraph(grdriver,grmode,'c:\tp\bgi');
 errcode:=graphresult;
 if errcode<>grok then
 begin
 closegraph;
 writeln('Graphics driver niet OK!');
 writeln('Programma gestopt.');
 halt;
 end;
end;
begin

open_graph;
lijn(0,0,640,480);
repeat until keypressed;
end.

7. Resources: