Reading ROMs and Saving Pokemon

Early Game Boy cartridges that save games use battery-backed SRAM cartridges to hold data between plays. It’s recently arisen that the batteries in them are expected to start dying soon. Critically: a lot of Pokemon from a lot of childhoods are facing extinction.

Luckily, thanks to the Pandocs [1] and a lot of patience from other developers, people have started building their own dumpers. This post details another implementation, built for PIC16 devices instead of the existing Arduino and AVR implementations. It is heavily based on the InsideGadgets Arduino GBCartRead project [2].

There's a ROM dumper under this rat's nest, honest.

The Hardware

I built my dumper around a PIC16F690. It’s got just enough pins to drive a couple of shift registers, a UART and to leave enough pins left over for I/O. The most difficult part to get is the cartridge connector – a donor Game Boy is required unfortunately.

Schematic for the dumper

The schematic is fairly straightforward. The PIC communicates with the PC via a MAX232. The address lines are driven via two 74HC594 shift registers. Data and control pins take up the remaining pins. Power is supplied via a LM7805 linear regulator (not shown).

The cartridge requires a connection to GND and +5V, as well as a pull-up resistor from +5V to the /RST pin (30)

The Software

Game Boy cartridges are fairly simple devices. They use a 32-pin connector which consists of power, three control lines, a 16-bit address bus and an 8-bit data bus. The cartridge shares the address bus with other devices and is mapped to the region 0x0000 to 0x7FFFF [3].

Talking to a cartridge is fairly simple:

  1. Write the address you want to read or write to the address bus
  2. (optional) Write the data you want to write to the data bus
  3. (optional) Raise MREQ if you’re reading SRAM (Pokemon live there)
  4. Raise RD or WR to read or write respectively
  5. Drop RD, WR and MREQ
  6. (optional) Read the data from the address bus

The WR, RD and MREQ pins are all active high.

Cartridges larger 16KB require bank switching which is handled by  a device called the memory bank controller (MBC). The MBC also controls access to the SRAM, as well as any other peripherals on the cartridge.

The MBC is controlled by writing commands to specific addresses. These addresses vary between MBCs, although most seem to enable RAM when 0xA0 is written to 0x0000. The MBC type is readable from the cartridge descriptor which is at 0x0134-0x0148 [1].

The firmware for the PIC is available at GitHub [4].

Rescuing the Pokemon

To use the dumper just connect to it via the serial port. The interface is fairly straight forward. It waits for confirmation before reading the descriptor, it’s pretty straight forward from there. The most important mode to use first is the diagnostics mode, which needs to be run without a cartridge connected.

On the Importance of Testing

Continuity testing is important!

The diagnostics mode is ideal for catching faults before they have real implications.

Faults like this one. In this case the WR pin hadn’t been properly soldered. Since the pins are active low and they’re connected via pull-down resistors. That means that the WR signal was 1, so the attempt to read RAM ended up writing garbage to it. I euthanised my Pokemon.

Dumping Data

Retrieving data (hopefully having not destroyed it) can be done using RealTerm [5]. It features true raw logging which is necessary, as, for example, PuTTY interprets some control characters, even in raw logging mode.

Future Work

At the moment the interface is a bit clunky, as is having to use RealTerm to dump things. I’ll be changing it to speak the same protocol as the original Arduino GBCartRead [2] which will make dumping things easier.


[1] : The Pandocs – Everything you wanted to know about Game Boy but were afraid to ask

[2] : GBCartRead

[3] : Game Boy memory map

[4] : GBCartRead-PIC16

[5] : RealTerm


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