MBC1

(max 2MByte ROM and/or 32 KiB RAM)

This is the first MBC chip for the Game Boy. Any newer MBC chips work similarly, so it is relatively easy to upgrade a program from one MBC chip to another — or to make it compatible with several types of MBCs.

In its default configuration, MBC1 supports up to 512 KiB ROM with up to 32 KiB of banked RAM. Some cartridges wire the MBC differently, where the 2-bit RAM banking register is wired as an extension of the ROM banking register (instead of to RAM) in order to support up to 2 MiB ROM, at the cost of only supporting a fixed 8 KiB of cartridge RAM. All MBC1 cartridges with 1 MiB of ROM or more use this alternate wiring. Also see the note on MBC1M multi-game compilation carts below.

Note that the memory in range 0000–7FFF is used both for reading from ROM and writing to the MBCs Control Registers.

Memory

0000–3FFF — ROM Bank X0 [read-only]

This area normally contains the first 16 KiB (bank 00) of the cartridge ROM.

In 1 MiB and larger cartridges (that use the 2-bit second banking register for extended ROM banking), entering mode 1 (see below) will allow that second banking register to apply to reads from this region in addition to the regular 4000–7FFF banked region, resulting in accessing banks $20/$40/$60 for regular large ROM carts, or banks $10/$20/$30 for an MBC1M multi-cart (see note below).

4000–7FFF — ROM Bank 01-7F [read-only]

This area may contain any of the further 16 KiB banks of the ROM. If the main 5-bit ROM banking register is 0, it reads the bank as if it was set to 1.

For 1 MiB+ ROM, this means any bank that is possible to accessible via the 0000–3FFF region is not accessible in this region. i.e. banks $00/$20/$40/$60 in regular large ROM carts, or banks $00/$10/$20/$30 in MBC1M multi-game compilation carts. Instead, it automatically maps to 1 bank higher ($01/$21/$41/$61 or $01/$11/$21/$31 respectively).

A000–BFFF — RAM Bank 00–03, if any

This area is used to address external RAM in the cartridge (if any). The RAM is only accessible if RAM is enabled, otherwise reads return open bus values (often $FF, but not guaranteed) and writes are ignored.

Available RAM sizes are 8 KiB (at $A000–BFFF) and 32 KiB (in form of four 8K banks at $A000–BFFF). 32 KiB is only available in cartridges with ROM <= 512 KiB.

External RAM is often battery-backed, allowing for the storage of game data while the Game Boy is turned off, or if the cartridge is removed from the Game Boy. External RAM is no slower than the Game Boy’s internal RAM, so many games use part of the external RAM as extra working RAM, even if they use another part of it for battery-backed saves.

Registers

All of the MBC1 registers default to $00 on power-up, which for the “ROM Bank Number” register is treated as $01.

0000–1FFF — RAM Enable (Write Only)

Before external RAM can be read or written, it must be enabled by writing $A to anywhere in this address space. Any value with $A in the lower 4 bits enables the RAM attached to the MBC, and any other value disables the RAM. It is unknown why $A is the value used to enable RAM.

It is recommended to disable external RAM after accessing it, in order to protect its contents from corruption during power down of the Game Boy or removal of the cartridge. Once the cartridge has completely lost power from the Game Boy, the RAM is automatically disabled to protect it.

2000–3FFF — ROM Bank Number (Write Only)

This 5-bit register (range $01-$1F) selects the ROM bank number for the 4000–7FFF region. Higher bits are discarded — writing $E1 (binary 11100001) to this register would select bank $01.

If this register is set to $00, it behaves as if it is set to $01. This means you cannot duplicate bank $00 into both the 0000–3FFF and 4000–7FFF ranges by setting this register to $00.

If the ROM Bank Number is set to a higher value than the number of banks in the cart, the bank number is masked to the required number of bits. e.g. a 256 KiB cart only needs a 4-bit bank number to address all of its 16 banks, so this register is masked to 4 bits. The upper bit would be ignored for bank selection.

Even with smaller ROMs that use less than 5 bits for bank selection, the full 5-bit register is still compared for the bank 00→01 translation logic. As a result if the ROM is 256 KiB or smaller, it is possible to map bank 0 to the 4000–7FFF region — by setting the 5th bit to 1 it will prevent the 00→01 translation (which looks at the full 5-bit register, and sees the value $10, not $00), while the bits actually used for bank selection (4, in this example) are all 0, so bank $00 is selected.

On larger carts which need a >5 bit bank number, the secondary banking register at 4000–5FFF is used to supply an additional 2 bits for the effective bank number: Selected ROM Bank = (Secondary Bank << 5) + ROM Bank.¹

These additional two bits are ignored for the bank 00→01 translation. This causes a problem — attempting to access banks $20, $40, and $60 only set bits in the upper 2-bit register, with the lower 5-bit register set to 00. As a result, any attempt to address these ROM Banks will select Bank $21, $41 and $61 instead. The only way to access banks $20, $40 or $60 at all is to enter mode 1, which remaps the 0000–3FFF range. This has its own problems for game developers as that range contains interrupt handlers, so it’s usually only used in multi-game compilation carts (see below).

4000–5FFF — RAM Bank Number — or — Upper Bits of ROM Bank Number (Write Only)

This second 2-bit register can be used to select a RAM Bank in range from $00–$03 (32 KiB ram carts only), or to specify the upper two bits (bits 5-6) of the ROM Bank number (1 MiB ROM or larger carts only). If neither ROM nor RAM is large enough, setting this register does nothing.

In 1MB MBC1 multi-carts (see below), this 2-bit register is instead applied to bits 4-5 of the ROM bank number and the top bit of the main 5-bit main ROM banking register is ignored.

6000–7FFF — Banking Mode Select (Write Only)

This 1-bit register selects between the two MBC1 banking modes, controlling the behaviour of the secondary 2-bit banking register (above). If the cart is not large enough to use the 2-bit register (≤ 8 KiB RAM and ≤ 512 KiB ROM) this mode select has no observable effect. The program may freely switch between the two modes at any time.

The banking mode can be:

• 0 = simple (default): 0000–3FFF and A000–BFFF are locked to bank 0 of ROM and SRAM respectively.
• 1 = advanced: 0000–3FFF and A000-BFFF can be bank-switched via the 4000–5FFF register.

Addressing diagrams

The following diagrams show how the address within the ROM/RAM chips are calculated from the accessed address and banking registers

0000–3FFF

4000–7FFF

A000–BFFF

“MBC1M”: 1 MiB Multi-Game Compilation Carts

Known as MBC1M, these carts have an alternative wiring, that ignores the top bit of the main ROM banking register (making it effectively a 4-bit register for banking, though the full 5 bit register is still used for 00→01 translation) and applies the 2-bit register to bits 4-5 of the bank number (instead of the usual bits 5-6). This means that in mode 1 the 2-bit register selects banks $00, $10, $20, or $30, rather than the usual $00, $20, $40 or $60.

These carts make use of the fact that mode 1 remaps the 0000–3FFF area to switch games. The 2-bit register is used to select the game — switching the zero bank and the region of banks that the 4000–7FFF ROM area can access to those for the selected game and then the game only changes the main ROM banking register. As far as the selected game knows, it’s running from a 256 KiB cart!

These carts can normally be identified by having a Nintendo copyright header in bank $10. A badly dumped multi-cart ROM can be identified by having duplicate content in banks $10-$1F (dupe of $00–$0F) and banks $30-$3F (dupe of $20-$2F). There is a known bad dump of the Mortal Kombat I & II collection around.

An “MBC1M” compilation cart ROM can be converted into a regular MBC1 ROM by increasing the ROM size to 2 MiB and duplicating each sub-ROM — $00–$0F duplicated into $10-$1F, the original $10-$1F placed in $20-$2F and duplicated into $30-$3F and so on.

MBC1M addressing diagrams

0000–3FFF

4000–7FFF