Ever since I began playing around with NES programming over a year ago, I’ve been constantly toying with the idea of building actual carts. Finding out how the code works in NES games was fascinating, but I also wanted to go a step further and learn more about the hardware side of NES games. The basic anatomy of an NES cart is straightforward enough. Well, I should say the basic anatomy of a basic NES cart is straightforward enough. In the most barebones, rudimentary of NES carts, like Super Mario Bros., two chips–one containing the graphics and one containing the programming–are connected to a printed circuit board along with a CIC chip. The CIC serves as the “key” to unlock the infamous lockout chip within your NES so you can actually play your game.
However, Super Mario Bros and many of the earliest NES games worked within extremely confined resources as far as memory and graphics. Hence, developers began placing additional components on the printed circuit boards within the cartridges to increase the possibilities available to them. Memory mapper chips were the major players here. Mappers greatly expanded the available memory that could be used for graphics and code. Other useful things included battery backups. These were RAM chips powered by an on-board battery that allowed players to save their games.
There’s a massive variety of boards between your different NES carts. When designing an NES game, a homebrewer must select a cart that matches the mapper they are using to manage the memory currently running through the NES’s CPU, among other things. For “Assimilate,” I’m using MMC1, the first memory mapper that Nintendo themselves developed. From there, we open the cart up, remove the original chips containing the graphics and programming (called CHR & PRG, respectively) and then solder new chips into those locations that have been programmed with the code in our games. Sometimes there is rewiring involved. Once it’s done, close the cart up and voila, we have our own official NES cart.
That’s obviously a VERY abridged version of what it takes to make a prototype, but I just wanted to give a broad idea of what goes into it. The pictures above are my first foray into building an actual NES cart for “Assimilate.” The top picture is the printed circuit board that I converted, and the bottom is the closed up, finished prototype itself.