; Author unknown - 360x480x256 vid mode
;
; *** Listing 1 ***
;
; Program to demonstrate pixel drawing in 320x400 256-color
; mode on the VGA. Draws 8 lines to form an octagon, a pixel
; at a time. Draws 8 octagons in all, one on top of the other,
; each in a different color set. Although it's not used, a
; pixel read function is also provided.
;
VGA_SEGMENT equ 0a000h
SC_INDEX equ 3c4h ;Sequence Controller Index register
GC_INDEX equ 3ceh ;Graphics Controller Index register
CRTC_INDEX equ 3d4h ;CRT Controller Index register
MAP_MASK equ 2 ;Map Mask register index in SC
MEMORY_MODE equ 4 ;Memory Mode register index in SC
MAX_SCAN_LINE equ 9 ;Maximum Scan Line reg index in CRTC
START_ADDRESS_HIGH equ 0ch ;Start Address High reg index in CRTC
UNDERLINE equ 14h ;Underline Location reg index in CRTC
MODE_CONTROL equ 17h ;Mode Control register index in CRTC
READ_MAP equ 4 ;Read Map register index in GC
GRAPHICS_MODE equ 5 ;Graphics Mode register index in GC
MISCELLANEOUS equ 6 ;Miscellaneous register index in GC
SCREEN_WIDTH equ 320 ;# of pixels across screen
SCREEN_HEIGHT equ 400 ;# of scan lines on screen
WORD_OUTS_OK equ 1 ;set to 0 to assemble for
; computers that can't handle
; word outs to indexed VGA registers
;
_stack segment para stack 'STACK'
db 512 dup (?)
_stack ends
;
Data segment word 'DATA'
;
BaseColor db 0
;
; Structure used to control drawing of a line.
;
LineControl struc
StartX dw ?
StartY dw ?
LineXInc dw ?
LineYInc dw ?
BaseLength dw ?
LineColor db ?
LineControl ends
;
; List of descriptors for lines to draw.
;
LineList label LineControl
LineControl <130,110,1,0,60,0>
LineControl <190,110,1,1,60,1>
LineControl <250,170,0,1,60,2>
LineControl <250,230,-1,1,60,3>
LineControl <190,290,-1,0,60,4>
LineControl <130,290,-1,-1,60,5>
LineControl <70,230,0,-1,60,6>
LineControl <70,170,1,-1,60,7>
LineControl <-1,0,0,0,0,0>
Data ends
;
; Macro to output a word value to a port.
;
OUT_WORD macro
if WORD_OUTS_OK
out dx,ax
else
out dx,al
inc dx
xchg ah,al
out dx,al
dec dx
xchg ah,al
endif
endm
;
; Macro to output a constant value to an indexed VGA register.
;
CONSTANT_TO_INDEXED_REGISTER macro ADDRESS, INDEX, VALUE
mov dx,ADDRESS
mov ax,(VALUE shl 8) + INDEX
OUT_WORD
endm
;
Code segment
assume cs:Code, ds:Data
Start proc near
mov ax,Data
mov ds,ax
;
; Set 320x400 256-color mode.
;
call Set320By400Mode
;
; We're in 320x400 256-color mode. Draw each line in turn.
;
ColorLoop:
mov si,offset LineList ;point to the start of the
; line descriptor list
LineLoop:
mov cx,[si+StartX] ;set the initial X coordinate
cmp cx,-1
jz LinesDone ;a descriptor with a -1 X
; coordinate marks the end
; of the list
mov dx,[si+StartY] ;set the initial Y coordinate,
mov bl,[si+LineColor] ; line color,
mov bp,[si+BaseLength] ; and pixel count
add bl,[BaseColor] ;adjust the line color according
; to BaseColor
PixelLoop:
push cx ;save the coordinates
push dx
call WritePixel ;draw this pixel
pop dx ;retrieve the coordinates
pop cx
add cx,[si+LineXInc] ;set the coordinates of the
add dx,[si+LineYInc] ; next point of the line
dec bp ;any more points?
jnz PixelLoop ;yes, draw the next
add si,size LineControl ;point to the next line descriptor
jmp LineLoop ; and draw the next line
LinesDone:
call GetNextKey ;wait for a key, then
inc [BaseColor] ; bump the color selection and
cmp [BaseColor],8 ; see if we're done
jb ColorLoop ;not done yet
;
; Wait for a key and return to text mode and end when
; one is pressed.
;
call GetNextKey
mov ax,0003h
int 10h ;text mode
mov ah,4ch
int 21h ;done
;
Start endp
;
; Sets up 320x400 256-color modes.
;
; Input: none
;
; Output: none
;
Set320By400Mode proc near
;
; First, go to normal 320x200 256-color mode, which is really a
; 320x400 256-color mode with each line scanned twice.
;
mov ax,0013h ;AH = 0 means mode set, AL = 13h selects
; 256-color graphics mode
int 10h ;BIOS video interrupt
;
; Change CPU addressing of video memory to linear (not odd/even,
; chain, or chain 4), to allow us to access all 256K of display
; memory. When this is done, VGA memory will look just like memory
; in modes 10h and 12h, except that each byte of display memory will
; control one 256-color pixel, with 4 adjacent pixels at any given
; address, one pixel per plane.
;
mov dx,SC_INDEX
mov al,MEMORY_MODE
out dx,al
inc dx
in al,dx
and al,not 08h ;turn off chain 4
or al,04h ;turn off odd/even
out dx,al
mov dx,GC_INDEX
mov al,GRAPHICS_MODE
out dx,al
inc dx
in al,dx
and al,not 10h ;turn off odd/even
out dx,al
dec dx
mov al,MISCELLANEOUS
out dx,al
inc dx
in al,dx
and al,not 02h ;turn off chain
out dx,al
;
; Now clear the whole screen, since the mode 13h mode set only
; cleared 64K out of the 256K of display memory. Do this before
; we switch the CRTC out of mode 13h, so we don't see garbage
; on the screen when we make the switch.
;
CONSTANT_TO_INDEXED_REGISTER SC_INDEX,MAP_MASK,0fh
;enable writes to all planes, so
; we can clear 4 pixels at a time
mov ax,VGA_SEGMENT
mov es,ax
sub di,di
mov ax,di
mov cx,8000h ;# of words in 64K
cld
rep stosw ;clear all of display memory
;
; Tweak the mode to 320x400 256-color mode by not scanning each
; line twice.
;
mov dx,CRTC_INDEX
mov al,MAX_SCAN_LINE
out dx,al
inc dx
in al,dx
and al,not 1fh ;set maximum scan line = 0
out dx,al
dec dx
;
; Change CRTC scanning from doubleword mode to byte mode, allowing
; the CRTC to scan more than 64K of video data.
;
mov al,UNDERLINE
out dx,al
inc dx
in al,dx
and al,not 40h ;turn off doubleword
out dx,al
dec dx
mov al,MODE_CONTROL
out dx,al
inc dx
in al,dx
or al,40h ;turn on the byte mode bit, so memory is
; scanned for video data in a purely
; linear way, just as in modes 10h and 12h
out dx,al
ret
Set320By400Mode endp
;
; Draws a pixel in the specified color at the specified
; location in 320x400 256-color mode.
;
; Input:
; CX = X coordinate of pixel
; DX = Y coordinate of pixel
; BL = pixel color
;
; Output: none
;
; Registers altered: AX, CX, DX, DI, ES
;
WritePixel proc near
mov ax,VGA_SEGMENT
mov es,ax ;point to display memory
mov ax,SCREEN_WIDTH/4
;there are 4 pixels at each address, so
; each 320-pixel row is 80 bytes wide
; in each plane
mul dx ;point to start of desired row
push cx ;set aside the X coordinate
shr cx,1 ;there are 4 pixels at each address
shr cx,1 ; so divide the X coordinate by 4
add ax,cx ;point to the pixel's address
mov di,ax
pop cx ;get back the X coordinate
and cl,3 ;get the plane # of the pixel
mov ah,1
shl ah,cl ;set the bit corresponding to the plane
; the pixel is in
mov al,MAP_MASK
mov dx,SC_INDEX
OUT_WORD ;set to write to the proper plane for
; the pixel
mov es:[di],bl ;draw the pixel
ret
WritePixel endp
;
; Reads the color of the pixel at the specified location in 320x400
; 256-color mode.
;
; Input:
; CX = X coordinate of pixel to read
; DX = Y coordinate of pixel to read
;
; Output:
; AL = pixel color
;
; Registers altered: AX, CX, DX, SI, ES
;
ReadPixel proc near
mov ax,VGA_SEGMENT
mov es,ax ;point to display memory
mov ax,SCREEN_WIDTH/4
;there are 4 pixels at each address, so
; each 320-pixel row is 80 bytes wide
; in each plane
mul dx ;point to start of desired row
push cx ;set aside the X coordinate
shr cx,1 ;there are 4 pixels at each address
shr cx,1 ; so divide the X coordinate by 4
add ax,cx ;point to the pixel's address
mov si,ax
pop ax ;get back the X coordinate
and al,3 ;get the plane # of the pixel
mov ah,al
mov al,READ_MAP
mov dx,GC_INDEX
OUT_WORD ;set to read from the proper plane for
; the pixel
lods byte ptr es:[si] ;read the pixel
ret
ReadPixel endp
;
; Waits for the next key and returns it in AX.
;
; Input: none
;
; Output:
; AX = full 16-bit code for key pressed
;
GetNextKey proc near
WaitKey:
mov ah,1
int 16h
jz WaitKey ;wait for a key to become available
sub ah,ah
int 16h ;read the key
ret
GetNextKey endp
;
Code ends
;
end Start
