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Snes MaskROM Breakout
Snes MaskROM Breakout
Anders Lindqvist (breakin)

So a little detour. I de-soldered the so-called MaskROM from a cartridge. Read on to find out what game I destroyed! It was very easy and I'm beginning to think that this should have been how I extracted data from my first cartridge. Oh well sometimes a harder problem is more interesting to solve. If you always get the easiest problem, you never get to make something really hard (since you'll always be good enough not to be challenged).

Let's start the show and tell! MaskROM inside the cartridge:

MaskROM inside cartridge

... and outside:

MaskROM outside cartridge

So what is the MaskROM? Well wikipedia tells me that a MaskROM is a very cost-efficient way to store read-only memory. The name is related to the manufacturing procedure. This particular MaskROM has 32 legs (pins) and contains 1MB.

Let me insert a figure from the snes pinout page:

32PIN MaskROM                   36PIN MaskROM
=====================           =====================
A17  01       32  Vcc           A20  01       36  Vcc
A18  02       31  /OE           A21  02       35  A22
A15  03       30  A19           A17  03       34  Vcc
A12  04       29  A14           A18  04       33  /OE
 A7  05       28  A13           A15  05       32  A19
 A6  06       27  A18           A12  06       31  A14
 A5  07       26  A19            A7  07       30  A13
 A4  08       25  A11            A6  08       29  A8
 A3  09       24  A16            A5  09       28  A9
 A2  10       23  A10            A4  10       27  A11
 A1  11       22  /CS            A3  11       26  A16
 A0  12       21  D7             A2  12       25  A10
 D0  13       20  D6             A1  13       24  /CS
 D1  14       19  D5             A0  14       23  D7
 D2  15       18  D4             D0  15       22  D6
Vss  16       17  D3             D1  16       21  D5
                                 D2  17       20  D4
                                 Vss 18       19  D3

It shows the role of each pin. The D-pins are the 8-bit output data that is being read from the MaskROM. The address fed into the MaskROM does not have A15 going nowhere, all address pins are used. I inserted the MaskROM in my breadboard (yes, it is compatible!) and hooked it up to the RaspberryPI. My Raspberry does not have the 30 data-pins needed so I used my previous setup using counters to generate the adress. As usual I let it work at 3.3V instead of 5V to make sure the raspberry would survive. It looked something like this:

MaskROM in breadboard

and from another angle

MaskROM in breadboard #2

I've switched to dual 4-bit counters now which mean I can fit 24-bit adress generation using only 8*3=24 rows of the breadboard. However only 20-bits were needed today so 4 of the bits are fed straight to ground. In this cartridge each pin of the MaskROM is directly connected to a pin on the cartridge. For more advanced cartridges that supports save games (SRAM) or more advanced memory layouts that might not be true.

After much trial and error, as well as tearing it all apart and reverting to LED-debugging on each connection, it worked. The python program I ran was quite simple. It can be found here. The variables at the top specified to what data input/output I tied to pins on the MaskROM.

What did I do with the empty cartridge you might wonder? Let this image be a teaser!

SNES breakout

This image also says which cartridge I gutted. Hopefully it wasn't the last one!

That's all for this time!