; Program to demonstrate use of the DAC registers by selecting a
; smoothly contiguous set of 256 colors, then filling the screen
; with concentric diamonds in all 256 colors so that they blend
; into one another to form a continuum of color.
;
	.model small
	.stack	200h
	.data

; Table used to set all 256 DAC entries.
;
; Table format:
;	Byte 0: DAC register 0 red value
;	Byte 1: DAC register 0 green value
;	Byte 2: DAC register 0 blue value
;	Byte 3: DAC register 1 red value
;	Byte 4: DAC register 1 green value
;	Byte 5: DAC register 1 blue value
;			:
;	Byte 765: DAC register 255 red value
;	Byte 766: DAC register 255 green value
;	Byte 767: DAC register 255 blue value

ColorTable	label	byte

; The first 64 entries are increasingly dim pure green.
X=0
	REPT	64
	db	0,63-X,0
X=X+1
	ENDM

; The next 64 entries are increasingly strong pure blue.
X=0
	REPT	64
	db	0,0,X
X=X+1
	ENDM

; The next 64 entries fade through violet to red.
X=0
	REPT	64
	db	X,0,63-X
X=X+1
	ENDM

; The last 64 entries are increasingly dim pure red.
X=0
	REPT	64
	db	63-X,0,0
X=X+1
	ENDM

	.code
Start:
	mov	ax,0013h	;AH=0 selects set mode function,
				; AL=13h selects 320x200 256-color
	int	10h		; mode

				;load the DAC registers with the
				; color settings
	mov	ax,@data	;point ES to the default
	mov	es,ax		; data segment
	mov	dx,offset ColorTable
				;point ES:DX to the start of the
				; block of RGB three-byte values
				; to load into the DAC registers
	mov	ax,1012h	;AH=10h selects set color function,
				; AL=12h selects set block of DAC
				; registers subfunction
	sub	bx,bx		;load the block of registers
				; starting at DAC register #0
	mov	cx,100h		;set all 256 registers
	int	10h		;load the DAC registers

				;now fill the screen with
				; concentric diamonds in all 256
				; color attributes
	mov	ax,0a000h	;point DS to the display memory
	mov	ds,ax		; segment
				;
				;draw diagonal lines in the upper
				; left quarter of the screen
	mov	al,2		;start with color attribute #2
	mov	ah,-1		;cycle down through the colors
	mov	bx,320		;draw top to bottom (distance from
				; one line to the next)
	mov	dx,160		;width of rectangle
	mov	si,100		;height of rectangle
	sub	di,di		;start at (0,0)
	mov	bp,1		;draw left to right (distance from
				; one column to the next)
	call	FillBlock	;draw it
				;
				;draw diagonal lines in the upper
				; right quarter of the screen
	mov	al,2		;start with color attribute #2
	mov	ah,-1		;cycle down through the colors
	mov	bx,320		;draw top to bottom (distance from
				; one line to the next)
	mov	dx,160		;width of rectangle
	mov	si,100		;height of rectangle
	mov	di,319		;start at (319,0)
	mov	bp,-1		;draw right to left (distance from
				; one column to the next)
	call	FillBlock	;draw it

				;draw diagonal lines in the lower
				; left quarter of the screen
	mov	al,2		;start with color attribute #2
	mov	ah,-1		;cycle down through the colors
	mov	bx,-320		;draw bottom to top (distance from
				; one line to the next)
	mov	dx,160		;width of rectangle
	mov	si,100		;height of rectangle
	mov	di,199*320	;start at (0,199)
	mov	bp,1		;draw left to right (distance from
				; one column to the next)
	call	FillBlock	;draw it
				;
				;draw diagonal lines in the lower
				; right quarter of the screen
	mov	al,2		;start with color attribute #2
	mov	ah,-1		;cycle down through the colors
	mov	bx,-320		;draw bottom to top (distance from
				; one line to the next)
	mov	dx,160		;width of rectangle
	mov	si,100		;height of rectangle
	mov	di,199*320+319	;start at (319,199)
	mov	bp,-1		;draw right to left (distance from
				; one column to the next)
	call	FillBlock	;draw it

	mov	ah,1		;wait for a key
	int	21h		;

	mov	ax,0003h	;return to text mode
	int	10h		;

	mov	ah,4ch		;done--return to DOS
	int	21h

; Fills the specified rectangular area of the screen with diagonal
; lines.
;
; Input:
;	AL = initial attribute with which to draw
;	AH = amount by which to advance the attribute from
;		one pixel to the next
;	BX = distance to advance from one pixel to the next
;	DX = width of rectangle to fill
;	SI = height of rectangle to fill
;	DS:DN = screen address of first pixel to draw
;	BP = offset from the start of one column to the start of
;		the next

FillBlock:
FillHorzLoop:
	push	di		;preserve pointer to top of column
	push	ax		;preserve initial attribute
	mov	cx,si		;column height
FillVertLoop:
	mov	[di],al		;set the pixel
	add	di,bx		;point to the next row in the column
	add	al,ah		;advance the attribute
	loop	FillVertLoop	;
	pop	ax		;restore initial attribute
	add	al,ah		;advance to the next attribute to
				; start the next column
	pop	di		;retrieve pointer to top of column
	add	di,bp		;point to next column
	dec	dx		;have we done all columns?
	jnz	FillHorzLoop	;no, do the next column
	ret			;

	end	Start