CharPad 2.7 User Manual - Subchrist Software, 2020.
Display Modes - Extended Colour Character Mode (ECM).
Extended Colour character mode allows up to 64 different character images to be displayed on a 40x25 byte screen matrix using one of 4 different background colours per screen cell.
Each character image is 8x8 pixels in size and uses a bit depth of 1 bit-per-pixel (two colours).
The foreground colour (0-15) of each displayed character image can be set using the 40x25 byte colour matrix at $d800 (55296).
The lower 6 bits of each screen matrix cell are used to select the character image (0-63) and the upper two bits are used to select a background colour...