# Programming:Fast plot

From CPCWiki - THE Amstrad CPC encyclopedia!

## Fast plotting routine for MODE 0

Converted from the Mode 1 routine by The Executioner

**Input:** DE = *X (0..159)*, HL = *Y (0..199)*

CMASK EQU &B338 ;Address for colormask ;664/6128: &B6A3 FPLOT LD A, L ;A = Lowbyte Y AND %00000111 ;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 SRL E ;calculate X\2, because 2 pixel per byte, Carry is X MOD 2 LD C, %10101010 ;Bitmask for MODE 0 JR NC, NSHIFT ;-> = 0, no shift SHIFT LD C, %01010101 ;other bitmask for right pixel NSHIFT ADD HL, DE ;+ HL = Screenaddress 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

## Fast plotting routine for MODE 1

This routine is approx. 6 times faster than the OS routine GRA PLOT, because it is dependent 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 ;Address for colormask ;664/6128: &B6A3 FPLOT LD A, L ;A = Lowbyte Y AND %00000111 ;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, %10001000 ;Bitmask for MODE 1 AND %00000011 ;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

## Fast plotting routine for MODE 2

And the MODE 2 routine. Perhaps this could be improved since there are only two colours, and the pixel shift may need to operate up to 7 times (so perhaps a table would average out better).

CMASK EQU &B338 ;Address for colormask ;664/6128 - &B6A3 FPLOT LD A, L ;A = Lowbyte Y AND %00000111 ;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\8, because RR E ;8 pixel per byte SRL D RR E SRL E ADD HL,DE ;+ HL = Screenaddress LD C,%10000000 ;Bitmask for MODE 2 AND %00000111 ;A = X MOD 8 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