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 I/O ports

ULAplus is controlled by two ports.

0xBF3B is the register port (write only)

The byte output will be interpreted as follows:

Bits 0-5: Select the register sub-group
Bits 6-7: Select the register group. Two groups are currently available:

     00 - palette group

          When this group is selected, the sub-group determines the
          entry in the palette table (0-63).

     01 - mode group

          The sub-group must be zero (colour) or one (greyscale). Sub-group entries 2-63 are reserved.

0xFF3B is the data port (read/write)

When the palette group is selected, the byte written will describe the color.

When the mode group is selected, the byte output will be interpreted as follows:

Bit 0: Palette mode select; on when set.

Implementations that support the Timex video modes use the #FF register to set the video mode, as per the Timex machines.
It is left to the individual implementations to determine if reading the port returns the previous write or the floating bus.


RGB palette entries
G3R3B2 encoding

For a device using the RGB colour space the palette entry is interpreted as follows

Bits 0-1: Blue intensity.
Bits 2-4: Red intensity.
Bits 5-7: Green intensity.

This colour space uses a sub-set of 9-bit RGB. The missing lowest blue bit is set to OR of the other two blue bits
(Bb becomes 000 for 00, and Bb1 for anything else). This gives access to a fixed half the potential 512 colour palette.
The reduces the jump in intensity in the lower range in the earlier version of the specification.
It also means the standard palette can now be represented by the ULAplus palette.


Greyscale palette entries

In greyscale mode, each palette entry describes an intensity from zero to 255. This can be achieved by simply removing the colour from the output signal.
Software

A rapidly growing collection of software is available including a palette editor, palette files to re-colour existing games,
a 6-bit RGB colour extension to CP/M plus, a slideshow of converted Commodore 64 hi-res pictures,
and brand new games, all of which can be found at [http://sites.google.com/site/ulaplus/home].

The palette editor enables you to create a palette for use with existing software without needing to rewrite the software.
You can save the palette and load it before loading the original software as normal.
In this way all existing software can be re-coloured without any programming knowledge required.

An introduction to using the ULAplus palette from BASIC is also available [1]
Limitations

Although in theory 64 colours can be displayed at once, in practice this is usually not possible except when displaying colour bars,
because the four CLUTs are mutually exclusive; it is not possible to mix colours from two CLUTs in the same cell.
Emulation

The 64 colour mode lookup table is organized as 4 palettes of 16 colours.

Bits 7 and 6 of each Spectrum attribute byte (normally used for FLASH and BRIGHT) will be used as an index value (0-3)
to select one of the four colour palettes.

Each colour palette has 16 entries (8 for INK, 8 for PAPER). Bits 0 to 2 (INK) and 3 to 5 (PAPER) of the attribute byte
will be used as indexes to retrieve colour data from the selected palette.

With the standard Spectrum display, the BORDER colour is the same as the PAPER colour in the first CLUT.
For example BORDER 0 would set the border to the same colour as PAPER 0 (with the BRIGHT and FLASH bits not set).

The complete index can be calculated as

ink_colour = (FLASH * 2 + BRIGHT) * 16 + INK
paper_colour = (FLASH * 2 + BRIGHT) * 16 + PAPER + 8

When scaling 3-bits of colour data to more bits for emulators that operate in high colour mode,
simply concatenate the bits repeatedly and then truncate to as many bits as needed. For example, for 8-bits the following conversion should be used:

    76543210
    hmlhmlhm

where h is the high bit, m is the middle bit, and l is the low bit of the original 3-bit value.