Programming:Fast plot

Fast plotting routine for MODE 1

This routine is approx. 6 times faster than the OS routine GRA PLOT, because it is dpendent to MODE 1 and doesn't perform range checks. It can draw 10500 pixels per second.

Input: DE = X (0..319), HL = Y (0..199)

CMASK	EQU &B338		;Adress for colormask
				;664/6128: &B6A3
						
FPLOT	LD A, L			;A = Lowbyte Y
	AND &X00000111		;isolate Bit 0..2
	LD H, A			;= y MOD 8 to H
	XOR L			;A = Bit 3..7 of Y
	LD L, A			;= (Y\8)*8 to L
	LD C, A			;store in C
	LD B, &60		;B = &C0\2 = Highbyte Screenstart\2
		
	ADD HL, HL		;HL * 2
	ADD HL, HL		;HL * 4
	ADD HL, BC		;+ BC = Startaddress
	ADD HL, HL		;of the raster line
		
	LD A, E			;Lowbyte X to A
	SRL D			;calculate X\4, because
	RR E			;4 pixel per byte
	SRL E
	ADD HL, DE		;+ HL = Screenaddress
		
	LD C, &X10001000	;Bitmask for MODE 1
	AND &X00000011		;A = X MOD 4
	JR Z, NSHIFT		;-> = 0, no shift
SHIFT 	SRL C			;move bitmask to pixel
        DEC A			;loop counter
	JR NZ,SHIFT		;-position
		
NSHIFT	LD A, (CMASK)		;get color mask
	XOR (HL)		;XOR screenbyte
	AND C			;AND bitmask
	XOR (HL)		;XOR screenbyte
	LD (HL), A		;new screenbyte
	RET			;done