mbc1b

Game Boy MBC1B Memory Bank Controller chip

KiCad Schematic

Many older Game Boy cartridges include an MBC1 chip, which is used to memory map larger ROM/RAM chips to the limited address space reserved to the cartridge on Game Boy.

The Game Boy address space has two reserved areas for cartridges:

• a 32 kB "cartridge ROM address range" (0x0000-0x7FFF): essentially a 15-bit address + A15 pin used as chip select
• a 8 kB "cartridge RAM address range" (0xA000-0xBFFF): essentially a 13-bit address + CS pin used as chip select

In order to use ROM chips larger than 32 kB and/or RAM chips larger than 8 kB, some kind of mapper chip is required.

For example, if a cartridge has a 128 kB ROM chip, the chip requires a 17-bit address and would not work alone since the console only provides a 15-bit address in the ROM address range. This kind of cartridge could wire the low 15 address bits directly to the ROM chip, and the high 2 address bits could be provided by an MBC1 chip. This concept is called "ROM banking", and the two extra bits provided by MBC1 are called the "bank number".

Note: There are multiple MBC1 versions and different implementations from several manufacturers. The chip reverse engineered here is MBC1B manufactured by NEC.

Repository contents

• schematic/: KiCad schematics
• MBC1B.svg: Inkscape SVG with all connections traced and all cells identified
• MBC1B.jpg: a stitched photo of a decapped MBC1B die, taken with a 20x microscope objective
• hdl/mbc1b_tb.vhd: a VHDL testbench with several tests, also verified on a real MBC1B chip using a simple GPIO bit-bang setup
• hdl/generated/: a structural VHDL model generated from the schematics using Kingfish HDL generator. The main purpose is automated testing via simulation, so the model is not meant to be readable by humans
• hdl/cells/: VHDL models for the standard cells on the chip

Running the HDL test suite

1. Install Poetry
2. Install GHDL
3. Run poetry install in the hdl directory
4. Run poetry run ./run.py in the hdl directory

Further reading

• Game Boy: Complete Technical Reference: contains a high-level description of MBC1
• Tauwasser's MBC1 wiki page: contains technical information and a readable VHDL model of MBC1