Technique · engine

PRG bank switching

Swapping which bank of program ROM the CPU sees in part of its address space, so a cartridge can hold more code and data than the CPU can address at once.

stubAlso: bank switching, program bank switching, PRG banking, mapper, trampolineUpdated 2026-10-23

Bank switching lets a cartridge hold more program than the CPU can address. A mapper chip on the cartridge decides which bank fills a window of the address space, and the game writes to the mapper to change it. CHR bank switching does the same for the tiles the video chip reads.

See also

Games

Every claim about this mechanic, by game (3)
Mega Man 2 1988Both games' bank switch routines map a PRG bank at $8000, but Mega Man 2's is new code for a different board: Mega Man writes the bank number once, to its own byte of a bank table (the disassembly's uxrom_prg_bank), and Mega Man 2 writes it to MMC1 one bit at a time, then runs any sound an NMI put off during the switch.Code
Mega Man 3 1990Both games flag a bank switch in progress and catch up on the frame's sound afterward if the NMI arrived during it, but Mega Man 3's switch is new code for a different board: Mega Man 2 maps one 16 KB bank at $8000 by writing its number to MMC1 one bit at a time, and Mega Man 3 writes two 8 KB banks, at $8000 and $A000, to the MMC3 as whole bytes.Code
Mega Man 5 1992Mega Man 5's routine that maps the two switchable PRG banks from their shadows, and then runs a sound request that arrived while it was busy, is Mega Man 4's, byte for byte (37 bytes). Mega Man 5 enters it through 6 new bytes that set both shadows from one bank number, the bank and the one after it.Code