BBC Palette Extender Hardware
=============================
There are various add-ons to give the BBC series a full 12-bit palette. Most
of them are controlled at the hardware level in different ways, but usually
provide code to interface to them transparently without having to know the
hardware details.


Clwyd Technics Palette Enhancer, 1982
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Plugs into the RGB output socket to change the physical displayed colours.
Controlled via the User Port.

Set user port to Output with ?&FE62=&FF

Write colours to PortB at &FE60 with:
  %cccc1000 - colour number
  %rrrr1001 - RED
  %gggg1010 - GREEN
  %bbbb1011 - BLUE
 Data written with b3=0, b3=1, b3=0, b3=1
 ie, write xxxx0xxx
     write xxxx1xxx
     write xxxx0xxx
     write xxxx1xxx

Using the top four bits for the colour control was probably chosen to make
interfacing to VDU 19 easier, as the significant colour data is in the top
four bits.


CTS Recognition Colour Card 500, 1986
-------------------------------------
Plugs into the RGB output socket to change the physical displayed colours.
Controlled via the 1MHz bus.

Write colour with:
  ?(&FCA0+colour)=%xxxxrrrr - RED
  ?(&FCA8+colour)=%xxxxgggg - GREEN
  ?(&FCB0+colour)=%xxxxbbbb - BLUE


WildVision PaletteMate, 1986
----------------------------
Plugs into the RGB output socket to change the physical displayed colours.
Unknown control, probably similar to the Chameleon, though one reference
refers to controlling 16 colours, which would suggest it interfaces via
the video ULA.


Chameleon, Mike Cook/Micro User, 1990
-------------------------------------
Plugs into the RGB output socket to change the physical displayed colours.
Controlled via the User Port.

Set user port to Output with CB2 strobe with ?&FE62=&FF, &FE6C=&AE.

Write colours to PortB at &FE60 with:
  %111xcccc - colour number
  %000xrrrr - inverted RED
  %010xgggg - inverted GREEN
  %100xbbbb - inverted BLUE


VideoNuLA, 2017
---------------
Plugs into the video ULA socket to change the logical displayed colours.
Controlled by writing to extra VULA registers.

Reset with ?&FE23=&40, this resets to 3-bit colour mapping.

Write colours with:
 ?&FE23=%ccccrrrr, then
 ?&FE23=%ggggbbbb


Software control
----------------
A relocatable interface module PALSUB can be built for different palette
hardware, giving a uniform interface that can be called from BASIC to set
palette entries.

It is loaded with:
  DIM PALETTE 200
  OSCLI "LOAD PALSUB "+STR$~PALETTE
  CALL PALETTE+3

Palette entries can be set with:
  CALL PALETTE,colour,red,green,blue

The code is built with the PALSUB/S program, and will only work in the I/O
processor.