Bank Switching

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Bank switching is a technique to increase the amount of usable memory beyond the amount directly addressable by the processor. It can be used to configure a system differently at different times; for example, a ROM required to start a system from diskette could be switched out when no longer needed. In video game systems, bank switching allowed larger games to be developed for play on existing consoles.

Bank switching originated in minicomputer systems. Many modern micro controllers and microprocessors use bank switching to manage random-access memory, non-volatile memory, input-output devices and system management registers in small embedded systems. The technique was common in 8-bit microcomputer systems. Bank-switching may also be used to work around limitations in address bus width, where some hardware constraint prevents straightforward addition of more address lines. Some control-oriented microprocessors use a bank-switching technique to access internal I/O and control registers, which limits the number of register address bits that must be used in every instruction.

Unlike memory management by paging, data is not exchanged with a mass storage device like disk memory. Data remains in quiescent storage in a memory area that is not currently accessible to the processor, (although it may be accessible to the video display, DMA controller, or other subsystems of the computer).

1000px-Bankswitch memory map.svg.png A hypothetical memory map of bank-switched memory for a processor that can only address 64 KB. This scheme shows 200 KB of memory, of which only 64 Kb can be accessed at any time by the processor. The operating system must manage the bank-switching operation to ensure that program execution can continue when part of memory is not accessible to the processor.

Bank switching was also used in some video game consoles. The Atari 2600, for instance, could only address 4 KB of ROM, so later 2600 game cartridges contained their own bank switching hardware in order to permit the use of more ROM and thus allow for more sophisticated games (via more program code and, equally important, larger amounts of game data such as graphics and different game stages).

The Nintendo Entertainment System contained a modified 6502 but its cartridges sometimes contained a megabit or more of ROM, addressed via bank switching called a Multi-Memory Controller.

Game Boy cartridges used a chip called MBC (Memory Bank Controller), which not only offered ROM bank switching, but also cartridge SRAM bank switching, and even access to such peripherals as infrared links or rumble motors.

Bank switching was still being used on later game systems. Several Sega Mega Drive cartridges were over 4MB in size and required the use of this technique (4MB being the maximum address size). The GP2X handheld from Gamepark Holdings uses bank switching in order to control the start address (or memory offset) for the second processor.