Date   : Sun, 28 Feb 2016 12:38:55 +0000
From   : jgh@... (J.G.Harston)
Subject: Reading current colours

Several long journeys with my mind in neutral resulted in trimming the 
code down to this:

   LDA #0:STA col              :\ Clear result
   LDX #87:LDA #160:JSR osbyte :\ Get foreground text colour
   STX mask                    :\ Save colour bitmap
   LDX #96:LDA #160:JSR osbyte :\ Get maximum colour
   STX max                     :\ Save maximum colour
   INX:TXA:ORA #8              :\ Use bitmap from max as counter
   .lp
   ROL mask:ROL mask:ROL col   :\ Rotate every second bit into result
   ROR A:BCC lp                :\ Loop until set bit rotated out
   LDA col:AND max             :\ Get result, AND with max colour

The epiphany was realising I could use the maximum number of colours 
value as a counter by moving the bits out and waiting for the bit to 
change. With a bit of a fudge to make '16 colours' rotate 3 times 
instead of 4 times.

The simplest BASIC equivalent I could work out was this:

   col%=FNosbyte(160,whatever,0)
   FOR loop%=1 TO 3-INTSQR((FNosbyte(160,96,0)+1)/4)
   col%=4*INTSQR((col%DIV16)+1)+SQR((col%AND15)+1)-5
   NEXT loop%

I've written it up and added it to the Wiki.

-- 
J.G.Harston - jgh@...      - mdfs.net/jgh