In this post, I’ll give you an overview of the NES hardware. I’ll discuss both the console and the cartridge, what chips are in them and what they do.
The schematics
This image shows a schematic overview of the most important parts from the inside of a NES console and cartridge.
Let’s dissect what we’ve got here.
The CPU
The Central Processing Unit (CPU) of the NES is basically what runs all code and logic. It gets instructions and data from the cartridge’s program memory (MASK ROM) and tells the other processing units what to do. The NES has 2kB of work RAM (WRAM) space in addition to the CPU. This is only used if the cartridge also has and uses its own WRAM, often battery powered. This is what allows a game status to be saved to the cartridge, like The Legend of Zelda does. Controller input is directly handled by the CPU as well.
The PPU
The Picture Processing Unit (PPU) is what renders the graphics on screen. Graphics are split into sprites (which can move freely across the screen) and backgrounds (which are in a fixed grid). It gets its graphics from the cartridges character memory chip (CHR ROM/RAM). The PPU can hold 8 kB of character memory, stored inyp two 4kB groups of character data. In addition, the PPU stores palette and attribute data as well. The NES also has 2kB of dedicated video RAM space (VRAM) which tells the PPU what to draw, and how to draw it. Data is stored in VRAM using nametables.
The APU
The Audio Processing Unit (APU) is what generates sound for the NES. It is contolled by the CPU, which tells the chip what sounds to make. The APU and CPU are bundled together inside a single chip, called the RP2A03G.
The LC
Finally, both the NES and the cartridge have a Lockout Chip (LC). This chip is to prevent people from pirating or publishing unauthorized games. The NES’ lockout chip runs a program which checks if the corresponding chip is available inside the cartridge. If not, the game will not load. Or at least that was the idea; in practice, disabling the lockout chip was as easy as cutting one of the pins of the console’s LC.
…that was brief
I know, there’s a lot more to be told about the NES hardware. For now, this at least gives you an idea what the NES looks like on the inside, and what it does, broadly speaking.