Bank switching
Swapping which slice of an oversized ROM shows through the CPU's small address window — how 8-bit games broke their own memory ceiling.
A 6502 reached 64 KB of address space, but cartridges soon grew past it. A mapper chip on the cartridge — Nintendo's MMC1 and MMC3, Konami's VRC series — let code write a bank number to a register and so change which ROM bank sat in the fixed window. The trick was not new: home computers, MSX mega-ROMs and Atari cartridges all did it. The price was that the switched code must not be running precisely when it swapped itself out from under its own feet. Bank switching is why a several-hundred-KB NES cartridge was possible at all.