At first, NES developers were getting along just fine with NROM. But players wanted more complexity in their games. The computer engineers at Nintendo figured out a solution: bankswitching the graphics ROM. This can be done by a CNROM board.
CNROM (iNES board class 3) is similar to NROM except that writes to the program area of the ROM go to a 74LS161 register that controls the most significant bits of the graphics ROM's address bus, allowing it to be bankswitched in 8 KB chunks. There are also some somewhat sneaky ways to stream map data out of the graphics ROM, making for a larger game. With a ROM size of 32 KB and a graphics ROM size of 16 KB or 32 KB (or higher on the Panesian CNROM clone), most CNROMs are 64 KB or smaller. Examples include Milon's Secret Castle, Chris Covell's Solar Wars, the independent Tengen version of Tetris, and several other games.
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