Inside a Gameboy cartridge.

Gameboy cart The most popular name for a Gameboy game-cartridge is 'cart'. I will use that name throughout this site since it saves on typing.

A cart contains the ROM that is neede to start the game. The ROM in fact IS the game. It is mapped in the lower 32 KB of the memorymap. The bottom 16 KB is fixed, the resst of the ROM is mapped in the second 16 KB of the memory map, using bank switching. For this purpose the Memory Bank Controller (MBC) is present. The cartridges I took apart all have the MBC-1 inside so I will only pay attention to this particular one. Please read the documentation section (see navigator) for all details. If you don't see a navigator, click here.

As I explained before, the most important parts of the game cart are:

For this particular cart, I removed the ROM and traced all signal lines, which I will tell about in the remainder of this page.

This cart is meant to become a kind of docking station for hardware extensions. I'm not sure how I will do it. There are roughly two methods right now:

  1. Connect a flatcable between the ROM connections to a ZIF socket for an external ROM and run all the I/O signals to a 34 pin header for external circuitry
  2. Only lead the I/O connector pins out
The first option is best since I will be able to use the original MBC for bankswitching.
On the other hand, a home made application that is not a game will rarely need more than 16 KB of object code, so the necessity of bankswitching will be very low. Time will tell what is the best method.

Signal pins of the I/O connector

I/O pin Signal name ROM MBC I/O pin Signal name ROM MBC
1 +5 Volts 32 24 2 CLK / 4    
3 /WR   22 4 /RD 24 11
5 /CS   23 6 A0 12  
7 A1 11   8 A2 10  
9 A3 9   10 A4 8  
11 A5 7   12 A6 6  
13 A7 5   14 A8 27  
15 A9 26   16 A10 23  
17 A11 25   18 A12 4  
19 A13 20 19 20 A14   20
21 A15 22 21 22 D0 13 1
23 D1 14 2 24 D2 15 3
25 D3 17 4 26 D4 18 5
27 D5 19   28 D6 20  
29 D7 21   30 /RESET   10
31 Audio in     32 Ground 16 12

The bankswitcher

Below, I show how the ROM and the MBC are interconnected. Of course this is just one example and a simple one too, but it shows how the hardware of the bankswitching took place.
Bankswitching was done by writing to some specific pages in the ROM area. Since the ROM doesn't react on Write pulses, the MBC can be programmed without any side effects. If someone is to install an EEPROM here, care must be taken however. 

          ROM pinout & connections                   MBC pinout & connections
     
              +----- v -----+                              +----- v -----+
     GND   1 -| NC      Vcc |- 32   +5V            D0   1 -|             |- 24  +5V
   MBC16   2 -| A16     A18 |- 31   MBC14          D1   2 -|             |o 23  CS
   MBC17   3 -| A15     A17 |- 30   MBC15          D2   3 -|             |o 22  WR
     A12   4 -| A12     A14 |- 29   MBC6           D3   4 -|             |- 21  A15
      A7   5 -| A7      A13 |- 28   A13            D4   5 -|      M      |- 20  A14
      A6   6 -| A6       A8 |- 26   A8           A14'   6 -|      B      |- 19  A13
      A5   7 -| A5   R   A9 |- 25   A9            GND   7 -|      C      |- 18  ??
      A4   8 -| A4   O  A11 |- 24   A11           +5V   8 -|             |- 17  A15'
      A3   9 -| A3   M   RD |o 24   /RD           GND   9 -|      1      |- 16  A16'
      A2  10 -| A2      A10 |- 23   A10         RESET  10 o|             |- 15  A17'
      A1  11 -| A1       CS |o 22   A15            RD  11 o|             |- 14  A18'
      A0  12 -| A0       D7 |- 21   D7            GND  12 -|             |- 13  GND
      D0  13 -| D0       D6 |- 20   D6                     +-------------+
      D1  14 -| D1       D5 |- 19   D5
      D2  15 -| D2       D4 |- 18   D4
     GND  16 -| GND      D3 |- 17   D3
              +-------------+
      |    |                   |    |
      |    |    |         |    |    |
      |    |    +---------o--- | -- | ------ Internal function
      |    |                   |    |
      |    +-------------------o -- | ------ Pin number
      |                             |
      +-----------------------------o------- Connection with Gameboy I/O connector
I'm not sure what pin 18 of the MBC is connected to. It is not connected on my cart, so I can only guess what it's for. The most probable guess is that it is A19', i.e. the bankswitchable A19. If you know what it's for, let me know. The E-mail address is in the topright section of the navigator frame.

According to Dr Pan's text, the MBC-1 should be able to address 2 megabyte ROM's so it needs 21 address line for this. A0 - A13 come from the CPU. So the MBC-1 will need to generate A14 upto and including A20. So most probaly the pins now assignd to GND (except pin 12) can be the extra address lines.

Page created December 2005,

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