Bank Switching

Overview

Bank switching solves a fundamental problem: CPUs with 16-bit address buses can only address 64KB of memory, but games and programs often need more. The solution involves mapping different physical memory regions into the same address range, swapping “banks” as needed. This technique enabled NES cartridges to contain megabytes of data despite the console’s limited addressing.

Fast Facts

Aspect	Detail
Problem solved	16-bit address limit (64KB)
Method	Swap memory regions via hardware
Common platforms	NES, Game Boy, C64, ZX Spectrum 128K
Modern equivalent	Virtual memory, paging

The Address Space Problem

An 8-bit CPU with 16-bit addressing:

Addressing	Maximum
16-bit address bus	2^16 = 65,536 bytes
64KB ceiling	Hard limit for direct access
Game requirements	Often exceeded 64KB

How Bank Switching Works

The concept:

Step	Action
1	CPU sees fixed address range (e.g., $8000-$BFFF)
2	Hardware maps this to physical ROM/RAM
3	Writing to special register selects which bank
4	New bank appears at same addresses
5	Program code switches banks as needed

Platform Examples

NES Mappers

The NES used cartridge hardware called “mappers”:

Mapper	Features
NROM (0)	No switching, 32KB PRG
MMC1 (1)	256KB PRG, CHR switching
MMC3 (4)	512KB PRG, IRQ counter
MMC5 (5)	1MB PRG, extended features

Different games used different mappers depending on complexity.

Commodore 64

The C64 used ROM/RAM banking:

Region	Banks
$A000-$BFFF	BASIC ROM or RAM
$D000-$DFFF	I/O or character ROM or RAM
$E000-$FFFF	Kernal ROM or RAM

Switching via the processor port at $01.

ZX Spectrum 128K

The 128K Spectrum banked RAM at $C000-$FFFF:

Feature	Detail
Fixed bank	Bank 5 always at $4000-$7FFF
Switchable	Eight 16KB banks at $C000-$FFFF
Control	Port $7FFD

Technical Challenges

Challenge	Consequence
Code can’t span banks	Jump targets must be in same bank
Data access planning	Know which bank contains what
Interrupt handling	Bank state during interrupts
Performance overhead	Bank switching takes cycles

Programming Patterns

Pattern	Purpose
Fixed bank	Code that must always be accessible
Data banks	Level data, graphics, audio
Trampoline code	Jump between banks via fixed code
Bank tables	Track what’s where

NES Bank Switching Code

A simplified example:

; Switch to bank 3
LDA #$03
STA $8000    ; Bank select register (mapper-specific)

; Now $8000-$BFFF contains bank 3's data

The exact mechanism varies by mapper hardware.

Historical Context

Era	Approach
Arcade boards	Custom banking hardware
Console cartridges	Mapper chips in cart
Home computers	System-level banking
CD-ROM era	Streaming replaced banking

Legacy

Bank switching was the bridge between limited addressing and modern virtual memory. Developers learned to structure code and data around bank boundaries—skills that translated into understanding memory hierarchies, caching, and modern paging systems. Emulator developers must accurately emulate mapper behaviour to run games correctly.

Expanding memory through hardware tricks