Changes
Added MODE 2 version to complete it and fixed MODE 0 comment
RR E ;2 pixel per byte, Carry is X MOD 2
LD C, %10101010 ;Bitmask for MODE 10
JR NC, NSHIFT ;-> = 0, no shift
SHIFT LD C, %01010101 ;other bitmask for right pixel
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
</pre>
== 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).
<pre>
CMASK EQU &B338 ;Adress 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 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
