LaiNES

Compact, cycle-accurate NES emulator in ~1000 lines of C++.

How are lines counted?

There have been some discussions about how the number of lines has been calculated. The claim of ~1k lines doesn't include the libraries (boost and nes_apu). Future plans include eliminating those dependencies to make the code base even slimmer.

[andrea@manhattan src]$ cloc .
      24 text files.
      24 unique files.                              
       1 file ignored.

github.com/AlDanial/cloc v 1.70  T=0.03 s (780.3 files/s, 63170.2 lines/s)
-------------------------------------------------------------------------------
Language                     files          blank        comment           code
-------------------------------------------------------------------------------
C++                             11            210            110           1163
C/C++ Header                    12             87              7            285
-------------------------------------------------------------------------------
SUM:                            23            297            117           1448
-------------------------------------------------------------------------------

Requirements

LaiNES should run on any Unix system that is compatible with the following tools.

• SConstruct
• C++11 compatible compiler (e.g. clang++)
• SDL2 (including sdl2-ttf and sdl2-image)

Building and running

Install the dependencies:

# Debian-based systems:
sudo apt-get install clang scons libsdl2-dev libsdl2-image-dev libsdl2-ttf-dev

# Mac OS X:
brew install scons sdl2 sdl2_image sdl2_ttf

Compile and run:

git clone --recursive https://github.com/AndreaOrru/LaiNES && cd LaiNES
scons
./laines

Usage

The emulator comes bundled with a simple GUI to navigate the filesystem and set preferences. Use arrow keys and Enter to operate it. ESC toggles between emulation and menu.

The size of the window and the controls are customizable. LaiNES supports multiple controllers and should work with joysticks as well. The default controls for the first player are as follows:

Compatibility

LaiNES implements the most common mappers, which should be enough for a good percentage of the games:

• NROM (Mapper 000)
• MMC1 / SxROM (Mapper 001)
• UxROM (Mapper 002)
• CNROM (Mapper 003)
• MMC3, MMC6 / TxROM (Mapper 004)

You can check the compatibility for each ROM in the following list: http://tuxnes.sourceforge.net/nesmapper.txt

Technical notes

The 6502 CPU and the PPU are implemented in just 219 and 283 lines of code respectively. Some clever meta-programming tricks are used to keep the codebase compact. Here is a good example of how this is achieved:

/* Cycle emulation.
 *     For each CPU cycle, we call the PPU thrice, because it runs at 3 times the frequency. */
#define T   tick()
inline void tick() { PPU::step(); PPU::step(); PPU::step(); ... }
...

/* Addressing modes.
 *     These are all the possible ways instructions can access memory. */
typedef u16 (*Mode)(void);
inline u16 imm() { return PC++; }
...
inline u16 zpx() { T; return (zp() + X) % 0x100; }
...

/* Fetch parameter.
 *     Get the address of the opcode parameter in a, and the value in p. */
#define G  u16 a = m(); u8 p = rd(a)
...

/* Instruction emulation (LDx where x is in registers {A, X, Y}).
 *     upd_nz, not shown, just updates the CPU flags register. */
template<u8& r, Mode m> void ld() { G; upd_nz(r = p); }
...

/* Execute a CPU instruction.
 *     Opcodes are instantiated with the right template parameters
 *     (i.e. register and/or addressing mode).*/
void exec()
{
    switch (rd(PC++))  // Fetch the opcode.
    {
        // Select the right function to emulate the instruction:
         ...
         case 0xA0: return ld<Y,imm>();  case 0xA1: return ld<A,izx>();
         ...
    }
}

Known issues

• The quality of the audio emulation on Linux is very poor. Typing export SDL_AUDIODRIVER=ALSA before running the emulator has been reported to improve it.

Contributors

• Jeff Katz - Mapper 002 & 003, configuration.

References and credits

• Special thanks to Ulf Magnusson for the invaluable [PPU diagram] (http://wiki.nesdev.com/w/images/d/d1/Ntsc_timing.png) and for his excellent implementation which was a big source of inspiration.
• blargg's APU sound chip emulator: http://blargg.8bitalley.com/libs/audio.html#Nes_Snd_Emu
• Complete hardware reference: http://problemkaputt.de/everynes.htm
• Tick-by-tick breakdown of 6502 instructions: http://nesdev.com/6502_cpu.txt