Find a Pixel

z80 » Graphics

Find Pixel is a small routine that sets the basis for plotting pixels on the screen. It takes the x and y coordinates of where you want to put a pixel and transforms those coordinates into an address and a bit offset. The address it gives you is for the byte that that pixel coordinate is in the Video Memory.

Some routines give you this address relative to zero being the top left of the screen. You might have to add $fc00 to the address outputted in order to get the exact address in the Video Memory. The bit offset is stored as a bit set in the accumulator (a register). The bit cooresponds to the pixel you want. If you set the same bit in a as the address in the video memory given by hl, you will have a little black dot at the coordinates you want on the screen. Most routines accept the x coordinate in b and the y coordinate in c.

I've pasted Clem Vasseur's FindPixel.asm Routine here. You can download this along with many other Find Pixel routines that I've come across in the Download Section. Each line in the routine is represented by a number which corresponds to an explanation at the end of the routine. Follow each line and try to picture in your head what it is doing.

↓FindPixel:
↓	ld h,63
↓	ld a,c
↓	add a,a
↓	add a,a
↓	ld l,a
↓	ld a,b
↓	rra
↓	add hl,hl
↓	rra
↓	add hl,hl
↓	rra
↓	or l
↓	ld l,a
↓	ld a,b
↓	and %00000111
↓	ld bc,FP_Bits
↓	add a,c
↓	ld c,a
↓	adc a,b
↓	sub c
↓	ld b,a
↓	ld a,(bc)
↓	ret
↓FP_Bits:
↓	.db %10000000
↓	.db %01000000
↓	.db %00100000
↓	.db %00010000
↓	.db %00001000
↓	.db %00000100
↓	.db %00000010
↓	.db %00000001