where bits can be 8 or 9.

The default colour mode of Layer 1 modes (except LoRes and HiRes) is the standard colour attribute display one. In order to enable the extended colour attribute display mode we need to enable the EnhancedULA functionality. For this you must use the PALETTE FORMAT which takes the form:

PALETTE FORMAT ink_count

where ink_count is a numerical expression specifying the number of inks to be in the palette (0,1,3,7,15,31,63,127 or 255). When the EnhancedULA is enabled, BRIGHT and FLASH are ignored, and INK and PAPER accept the appropriate new range of values. Note here that although you can specify INK and PAPER values up to 255 when writing a program, attempting to execute the program in Layer 0 will result into a K Invalid Colour error when the EnhancedULA is not enabled. To disable the EnhancedULA functionality you will need to specify an ink count of 0. The standard attributes with 8 inks, 8 papers, bright and flash are then once again supported.

As we saw in Fig. 13 there is an order of display of different layers on screen. Although it is not immediately apparent this means that it's also possible to mix display output from more than one graphical layers. That is achieved by assigning a global transparency mask for the regular layers or, in the case of the Sprites layer, a transparency index, and then colouring the areas or sprites we want to be transparent with the specific colour.

You can set the transparency colour mask or transparency colour index using the following statement:

PALETTE OVER value

where value is an 8-bit numeric expression which identifies a colour either in R3G3B2 8-bit format (in the case of regular graphics layers) or the index to the 9bit colour value we want to be transparent (in the case of the Sprites layer). The default global transparency mask and transparency colour index is light magenta / 227 (11100011 in binary).

To reset all palette data and settings to default, use the PALETTE CLEAR statement.

In the Palette-based hybrid linear bitmapped colour display section, we first discussed the existence of two palettes per display layer (note here that in this case layer is meant in the memory usage paradigm displayed in Fig. 13 so ULA layers get grouped together).

We can switch between palettes using the compound keyword:

LAYER PALETTE n

where n is the palette to use (0 or 1) for the current memory usage layer (ie. if you're in any ULA layer all of it gets affected but not Layer 2 etc).

You can point a palette for the current layer to palette data you have previously stored in memory using the following compound command:

LAYER PALETTE number BANK bank, offset

where number is the palette to update (0 or 1) for the current memory usage layer, bank is the memory bank to point to, and offset is the offset within that memory bank (For more information about BANK see Chapter 23 – The Memory).

Palette data should be either 256 double byte colour entries (for 9-bit), or 256 single byte entries (for 8-bit). As per what we discussed earlier in the chapter we need to encode the colour information in an R3G3B2 (for 8-bit) or RGB3 (for 9-bit) with every colour component value describing 8 intensities per colour.

In the double-byte entry method, the second byte in each sequence only has one bit defined for colour: the 3rd blue bit as well as one bit for priority (which only applies to palettes