As the Game Boy’s 16-bit address bus offers only limited space for ROM and RAM addressing, many games are using Memory Bank Controllers (MBCs) to expand the available address space by bank switching. These MBC chips are located in the game cartridge (that is, not in the Game Boy itself).
In each cartridge, the required (or preferred) MBC type should be specified in the byte at $0147 of the ROM, as described in the cartridge header. Several MBC types are available:
Among Nintendo MBCs, only the MBC5 is guaranteed by Nintendo to support the tighter timing of CGB Double Speed Mode. There have been rumours that older MBCs (like MBC1-3) wouldn’t be fast enough in that mode. If so, it might be nevertheless possible to use Double Speed during periods which use only code and data which is located in internal RAM. Despite the above, a self-made MBC1-EPROM card appears to work stable and fine even in Double Speed Mode.