An NES emulator written in pure C

Here lies an NES emulator I wrote to learn about emulation and generally sharpen my understanding of the C language. Don't let this fool you though. It is a pretty decent emulator with full 6502 instruction set (official, unofficial and most illegal opcodes) to say the least.

What is implemented

Below shows the features that have been implemented. This is obviously subject to change as I will continue to implement features where I can over time. Any help is welcome.

• CPU (6502)
  • Official opcodes
  • Unofficial opcodes
  • Cycle accuracy (official & unofficial)
  • BCD arithmetic (Not needed by NES anyway)
• Memory
  • Extended RAM
  • Expansion ROM
  • Battery backed (persistent) save RAM
  • Open-bus behaviour
• PPU (Picture Processing Unit)
  • NTSC
  • PAL
  • Dendy
• APU (Audio Processing Unit)
  • Pulse channel
  • Triangle channel
  • Noise channel
  • Delta Modulation Channel
• Gaming input
  • Keyboard input
  • Keyboard (multiplayer)
  • Original NES controller
  • Gamepad controller (Multiplayer)
  • Turbo keys
  • Custom Touch controller (Android)
• Formats
  • iNES
  • NES2.0
  • NSF
  • NSF2
  • UNIF
  • FDS
  • IPS
• Mappers
  • #0 NROM
  • #1 MMC1
  • #2 UxROM
  • #3 CNROM
  • #4 MMC3
  • #5 MMC5
  • #7 AxROM
  • #66 GNROM
• Game genie (for cheat codes)

What can be played

I haven't really taken time to do a survey but based on mappers implemented we can put the estimate at 51.7% of NES games. This is assuming none of them are using a quirk that I haven't gotten round to implementing. Games using mid-scanline trickery may not work as expected unless I find time to iron out timing issues in the PPU. Below are a few demos:

Contra

Metal gear

Legend of Zelda

1943: Battle of Midway

Controller setup

Xbox and Playstation controllers have not been tested on the emulator and are not guaranteed to work as shown here.

Key	Keyboard	Playstation	Xbox
Start	Enter	Start	Menu
Select	Shift	Select	View
A	J	▢	X
B	K	◯	B
Turbo A	H	△	Y
Turbo B	B	X	A
up	Up	D-pad/stick Up	D-pad/stick Up
down	Down	D-pad/stick Down	D-pad/stick Down
left	Left	D-pad/stick Left	D-pad/stick Left
right	Right	D-pad/stick Right	D-pad/stick Right

Compiling

Compiling the emulator requires:

• a C11 (or higher) compiler e.g. gcc
• SDL2 library for graphics
• cmake
• make (linux)

Assuming have the above requirements installed on your system, Acquire and build the source as follows:

$ git clone https://github.com/obaraemmanuel/NES
$ cd NES
$ mkdir build && cd build
$ cmake ..
$ make

You can now run the built emulator

$ ./nes ~/nes/roms/Contra.nes

To run with Game Genie enabled provide path to the original game genie ROM as an extra argument

$ ./nes ~/nes/roms/Contra.nes ~/nes/roms/game_genie.nes