Hi again!
Imagine you have a ram buffer filled with 4bit color data (screen 1,2 or 3) and you want to replace white color for grey, light green for dark green... (for instance to simulate a darkening of the image).
xor a ; a =0
@@LOOP: rrd ; Rotate nibbles a -> [hl] -> a
ld e,a ; Lower byte of the color change table
ld a,[de] ; Paper color substitution
rrd ; Rotate nibbles a -> [hl] -> a
ld e,a ; Lower byte of the color change table
ld a,[de] ; Ink color substitution
rrd ; Rotate nibbles a -> [hl] -> a
cpi ; Inc HL, Dec BC
jp pe,@@LOOP ; If BC <> 0 then goto @@LOOP
ret ; Return
RRD rotates a 12 bits number (4 lower bits of A and 8 bits of [HL]) 4 bits to the right. RRL do the same to the left. Both are standard and documented Z80 instructions.
Usage of the routine:
HL -> Buffer address
DE -> Change color table address (lower byte MUST be zero! This speeds up a lot the routine)
BC -> Buffer length
The change color table is a 16 bytes area containing the destination color for each source color. For instance, the table:
.db 0,1,2,3,4,5,6,7,8,9,10,11,12,13,15,14
just exchange white and grey colors. Surely you can find more interesting uses for this little piece of code.
Regards
--
SapphiRe
