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	Typical games
NROM (0)	No switching, 16KB or 32KB PRG	Donkey Kong, Mario Bros
UxROM (2)	256KB PRG bank-switching, fixed last bank	Mega Man, Castlevania
CNROM (3)	32KB PRG fixed, 32KB CHR bank-switching	Paperboy
MMC1 (1)	Up to 512KB PRG + 128KB CHR, mirroring control, 5-bit serial register	Zelda, Metroid
MMC3 (4)	512KB PRG + 256KB CHR, scanline IRQ via PPU A12 edge, paired even/odd writes	Super Mario Bros 3
MMC5 (5)	1MB PRG + 1MB CHR, extended attributes, expansion audio	Castlevania III

Different games used different mappers depending on complexity. The NES 2.0 spec catalogues several hundred distinct mappers across licensed and unlicensed cartridges.

Commodore 64

The C64 banks system ROMs and I/O in and out via the 6510's on-chip processor port at $01:

Region	Banks (controlled by `$01` bits 0-2)
$8000-$9FFF	RAM, or cartridge LO ROM (when `/EXROM` is asserted)
$A000-$BFFF	RAM, BASIC ROM, or cartridge HI ROM (LORAM bit)
$D000-$DFFF	RAM, character ROM, or I/O area (CHAREN bit)
$E000-$FFFF	RAM, KERNAL ROM, or cartridge HI ROM (HIRAM bit)

Bit 0 = LORAM, bit 1 = HIRAM, bit 2 = CHAREN. The Ultimax cartridge mode (/EXROM=0, /GAME=0) replaces large parts of the map with cartridge ROM regardless of $01.

ZX Spectrum 128K

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

Feature	Detail
Fixed Bank 5	Always at $4000-$7FFF (the regular screen)
Fixed Bank 2	Always at $8000-$BFFF
Switchable	Eight 16KB banks at $C000-$FFFF (banks 0-7, selected by `$7FFD` bits 0-2)
Control	Port `$7FFD` (write-only); bit 5 locks paging until reset

The +2A/+3 add a second paging port at $1FFD for "all-RAM" configurations and ROM disk routing.

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

Trampoline pattern

The trampoline lives in a fixed bank (e.g. MMC3's last 8 KB at $E000-$FFFF, which never switches). It saves the current bank, switches to the target bank, calls into it, and restores the original bank on return. Code in switchable banks calls foreign functions only via the trampoline:

; Lives at $E000 in the fixed bank
far_call:
    PHA                  ; save bank arg & target addr (pushed by caller)
    ; ... swap to target bank using mapper-specific writes ...
    JSR (target_addr)    ; run the routine in the now-paged bank
    ; ... swap back to caller's bank ...
    RTS

Without a trampoline, an RTS from a foreign bank would land back in the wrong bank's code.

NES bank switching by mapper

The bank-select interface varies enormously between mappers. Three patterns cover most of them.

UxROM, CNROM, AxROM — direct write

A single write to any address in the cartridge range latches the bank number:

    LDA #$03
    STA $8000        ; UxROM: select PRG bank 3 at $8000-$BFFF
                     ; (last bank stays fixed at $C000-$FFFF)

MMC1 — 5-bit serial shift register

MMC1 collects five sequential writes into an internal shift register. Each write latches one bit (bit 0 of the value); on the fifth write the assembled 5-bit value is committed to one of four registers, picked by the address of the fifth write:

    LDA #%10000000   ; bit 7 set = reset shift register
    STA $8000        ; (use this to abort a partial write sequence)

    ; Now write the 5 bits of the bank number, LSB first
    LDA #$1          ; bit 0
    STA $E000
    LDA #$1
    STA $E000
    LDA #$0
    STA $E000
    LDA #$0
    STA $E000
    LDA #$0
    STA $E000        ; fifth write to $E000-$FFFF latches PRG bank %00011 = 3

MMC3 — paired bank-select / bank-data

MMC3 splits the operation across even and odd addresses. Write the register index to an even address in $8000-$9FFF, then the bank value to the odd address one byte later:

    LDA #$06         ; index 6 = PRG bank at $8000 (in default mode)
    STA $8000        ; bank select
    LDA #$0A
    STA $8001        ; bank data — PRG bank $0A now at $8000

MMC3 also holds an 8-bit IRQ counter that decrements on each rising edge of PPU A12, used for split-screen and status-bar effects.

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