Screen Mode 2 Detail
Technically, mode 2 is a character mode with a colorful character set. The screen is vertically divided into three 256×64 pixel areas, each of which gets its own character set. By sequentially printing the characters 0 through 255 in all three areas, the program can simulate a graphics mode where each pixel can be set individually. However, the resulting framebuffer is non-linear.
The program can also use three identical character sets, and then deal with the screen like a text mode with a colorful character set. Background patterns and sprites then consist of colorful characters. This was commonly used in games, because to fill/scroll the entire screen, only 32×24 bytes had to be moved. Games on other home computers such as the Commodore 64 also worked on a character basis. The graphics can be drawn such that the 8×8 pixel borders are not too obvious, an art where Konami was particularly well known for their excellence.
This is the TMS9918 screen mode 2 challenge: every 8×1 pixel area has two colors, foreground and background. They may be freely picked out of the 16 color palette. But within each 8×1 pixel area, only two different colors can exist. When manipulating the screen in BASIC with the LINE command, one easily could exceed the limit of max 2 colors per 8×1 area and end up with "color spill".
In comparison, the MOS Technology VIC-II used in the Commodore 64 limited programmers to 4 colors per 4×8 fat-pixel area. This meant there was less local color pressure, but more global color pressure: while three of the 4 colors could be freely picked out of palette of 16, the remaining fourth was a universal background color.
The TMS9918 does not have any scroll registers. Scrolling must be done in software.
Read more about this topic: Texas Instruments TMS9918
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