ZX Prism: Screen decoding

As you may have noted from previous posts, ZX Prism goes through a number of different stages to convert values in memory into what's displayed on the screen.

Firstly, the addresses in VRAM to be read are calculated; this takes into consideration a number of things: the type of mode being displayed - (standard (pixel+attribute), 16bit attribute (pixel+2 attribute), planar, text, tiled), the size of attribute (8x8, 8x1) etc. The appropriate parts of memory are then read.

If a planar screen mode is selected, the pixel data from each plane is combined to produce a "logical colour" number. If a pixel+attribute screen mode is selected data is then decoded using a selectable attribute decode method, producing a "logical colour" number.

Finally, the logical colour number is decoded into a physical colour - There are three selectable palettes to do this.

The Default palette's first 16 colours are mapped to be the same as the colours on the ZX Spectrum (including colour 8 - "bright black" being identical to colour 0). The next 16 colours are the same again except duller.

There is also a fully redefinable 256 colour palette. Finally, there's a palette which is directly mapped to the 256 colours available to choose from in ULAplus (this palette is automatically selected when ULAplus is used)

These different parts of the screen data decoding process are all controlled using ZX Prism's 4-bit control registers. The registers are manipulated with an OUT to port 0x8E3B (36411).

The value to be sent to port 0x8E3B is constructed as follows: The most significant 4 bits of the value choose the register to be written to (in the case of the attribute decode method, this is register 3 - "0011" in binary). The least significant 4 bits of the value is the data to be sent to the register. Again, using attribute decode method as an example:

0000 selects Spectrum attribute decode (3 bits ink colour, 3 bits paper colour, 1 bit bright, 1 bit flash)
0001 selects 16+16 attribute decode (3 bits ink, 3 bits paper, 1 bit bright for ink, 1 bit bright for paper)
0010 selectes 32 colour mode (3 bits ink, 3 bits paper, 2 bits bright)
0100 selectes 256 colour mode 1 (8 bits ink colour, paper colour is same as the border colour)
0110 selects 16 colour planar mode (which doesn't use an attribute byte at all!)

So:
OUT 36411,48 selects spectrum attribute decode
OUT 36411,49 selects 16+16 mode
OUT 36411,50 selects 32 colour mode
OUT 36411,52 selects 256 colour mode 1