 The CPU interface combined the 4 planes in exactly the same way (a feature called "chain-4") so that the pixel appeared to the CPU as a packed 8-bit value.This color number is exactly which color register is used.Since the colors 0 ... 15 are still supposed to result in the CGA colors, the color registers are not preset to contain the EGA palette; instead it contains the 16 CGA colors in the first entries.