Screen Modes
There are 4 different screen modes available in the TMS9918A (as mentioned before, the TMS9918 lacks mode Graphic II):
Mode 0 (Text): 40×24 characters monochrome. As the display is 256 pixels width, the character set is only 6 pixels wide. This mode doesn't support sprites.
Mode 1 (Graphic 1): 32×24 characters. Each 8 characters in the character set has a foreground and background color. The chars "0"-"7" for example all have the same color attributes.
Mode 2 (Graphic 2): 32×24 characters or 256×192 bitmap, with a 2-color limitation for each 8 pixel wide line inside a character. Popular hybrids exists.
Mode 3 (Multicolor): 64×48 mode, very blocky and rarely used. Each 'pixel' can have its own color defined though, hence the name.
The TMS9918 has a fixed 16-color palette (actually 15 colors + transparent). The 4 bits (starting with the lowest valued bit) represent luminance, green, blue, and red. The color value of a 4-bit number can be computed based on the mixing of these primary colors, with the luminance (first) bit on meaning "light". Given this, both "0" and "1" should be black, so "0" is reserved as "transparent" and "1" is black. "14" ("E" in hex, "1110" in binary) is "gray" while "15" ("F" in hex, "1111" in binary) is white.
The "transparent" color is used by sprites to show the background. On the background, "transparent" will show the external video signal.
Read more about this topic: Texas Instruments TMS9918
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