Date   : Sun, 25 Oct 2015 13:33:22 +0000 (WET)
From   : bbcmicro@... (Peter Coghlan)
Subject: Reading current colours

>
> I'd got as far as:
>   colour%   =FNosword(160,whatever)
>   maxColour%=FNosword(160,96)
>   IF maxColour%=3:colour%=colour% DIV 8
>   colour%=colour% AND maxColour%
>
> or:
>   LDX #whatever:JSR osword160:PHA         :\ read colour mask
>   LDX #96:JSR osword160:TAX:PLA           :\ read max. colour
>   CPX #3:BNE skip:LSR A:LSR A:LSR A:.skip :\ divide by 8 if max=3
>   STX temp:AND temp                       :\ AND with max. colour
>
> which works for 2-colour and 4-colour modes, but it looks like 16-colour 
> modes is going to end up being a chunk of special-case code. I was 
> hoping I could build something that was just arithmetic manipulation, 
> not a load of IF cases. But it looks like it will end up being a special 
> case for each colour depth.
>

I think this should work for all modes except the text only modes.  If you
AND the result with the maximum colour number, it should work for everything
(except mode 7 of course).


LDA #160    
LDX #87     \ Get foreground text colour byte
JSR osbyte
TXA
PHA

LDA #160
LDX #97     \ Get pixels per byte minus 1
JSR osbyte
INX         \ Get pixels per byte
STX counter

PLA

LDY #8      \ Go around once for each bit in byte

.loop1
LDX counter \ Get pixels per byte each time around

.loop2
LSR A       \ Discard one bit for each pixel per byte
DEX
BNE loop2

ROR result  \ Rotate in one discarded bit only
DEY
BNE loop1   \ Go around entire byte

LDA result
RTS

Regards,
Peter Coghlan.