MMC3
Nintendo's third-generation memory controller for NES cartridges, which besides bank switching counted scanlines and so gave the console the raster interrupt it lacked.
Memory Management Controller 3 arrived in 1988 with Super Mario Bros. 3 and ended up in hundreds of games, among them Kirby's Adventure and Mega Man 3–6. Like its predecessors it handles bank switching, in 8 KB chunks for code and down to 1 KB for graphics, so tile sets can be swapped quickly.
The new part was the counter. The chip watches an address line that the PPU toggles roughly once per scanline, counts down and fires an IRQ at zero. A game can then change the scroll position on exactly the right line — the fixed status bar under the scrolling world in Mario 3 is the textbook example. Without it, developers leaned on the sprite 0 hit, which only works once per frame. Emulator authors know the chip as mapper 4.