Pixel Manipulation

z80 » Graphics

The first routine is heavily commented so that the others need not be. You should be able to adapt the first comments to the other routines. The chart at the end shows how to use them.

Make sure you have read the Find Pixel Section before reading on. The following code takes for granted that you have the FindPixel routine somewhere in your code so you can reference it.

pixel_on:
	call FindPixel	;hl=location in $fc00 (screen)
			;a=byte containing pixel at hl
			; with bit set for pixel
			; we want to change
	or (hl)		;logical op to make bit in
			; a that is set be included
			; in new a with original byte
	ld (hl),a       ;load modified byte onto screen
	ret             ;done

pixel_off:
	call FindPixel
	cpl
	and (hl)
	ld (hl),a
	ret

pixel_change:
	call FindPixel
	xor (hl)
	ld (hl),a
	ret

pixel_test:
	call FindPixel
	and (hl)        ;zero flag set if no pixel there
	ret

To use the above routines you need to put the coordinates, relative to the diagram below, into bc. You do this by ld bc,256*3+3 which puts coordinates (3,3) into bc which then can be used when you do call pixel_on to put the pixel at (3,3) on. That pixel would be in the bottom left hand of the screen. Consider the following representation of the TI86's screen.

PixelOn.asm gives an example of how to turn (blacken) a pixel at certain coordinates on the screen. It takes into account that you have the FindPixel routine in your include directory.

#include "ti86asm.inc"

.org _asm_exec_ram
	call _clrLCD	;clear screen

	ld bc,256*45+2	;coords (45,2)
	call pixel_on

	ld bc,256*3+62	;coords (3,62)
	call pixel_on

	ld bc,256*100+6	;coords (100,6)
	call pixel_on

	call _getkey	;wait for keypress
	jp _clrLCD	;clear screen and ret

pixel_on:
	call FindPixel	;hl=location in $fc00 (screen)
			;a=byte containing pixel at hl
			; with bit set for pixel
			; we want to change
	or (hl)		;logical op to make bit in
			; a that is set be included
			; in new a with original byte
	ld (hl),a       ;load modified byte onto screen
	ret             ;done
#include "findpixel.asm"
			;pastes the findpixel routine by clem
			; here
			;input:	(b,c)=(x,y)
			;output:hl=byte address in video memory
			;	bit set in a for offset
.end