# MOS 6502 emulator

A portable NMOS MOS 6502 emulator in C with an Easy6502-compatible virtual
machine and an original Snake game.

The reusable core implements all 56 documented instructions (151 official
opcode encodings), NMOS decimal arithmetic, IRQ/NMI/BRK, stack behavior,
addressing quirks, and instruction cycle counts. Unsupported opcodes stop with
an explicit error.

## Build and test

```sh
make                 # core and command-line emulator
make test            # unit/integration checks
make test-functional # official Klaus Dormann functional suite
make test-decimal    # exhaustive NMOS BCD accumulator/flag suite
make test-interrupt  # IRQ/NMI/BRK integration suite
make test-vectors    # 2.36M independent single-instruction vectors (large download)
make debug            # strict -Werror debug build and tests
make sanitize         # AddressSanitizer/UBSan build and tests
```

The core has no external dependencies. The graphical frontend uses SDL2 and
SDL2_ttf. Building the included game also needs `ca65` and `ld65` from cc65.
On Debian/Ubuntu install `libsdl2-dev`, `libsdl2-ttf-dev`, `fonts-dejavu-core`,
and `cc65`. Set `MOS6502_FONT` to another TrueType font path if desired.

```sh
make snake
```

The startup menu offers Start/Resume, New Game, Reset High Score, and Quit. Use
Up/Down or W/S to select and Enter or Space to activate. During play, WASD or
the arrow keys steer, Space or P pauses, R restarts after game over, and Escape
returns to the menu. D-pad and Start controls are supported on SDL game
controllers. The window title shows the current
score and persistent high score. The high score is stored in
`snake.highscore` in the working directory.

F5 saves the current game and F9 restores it. Food and game-over events have
lightweight synthesized sound effects. Save states are stored in `snake.state`.

The in-game arcade layout includes a framed pixel display, live score and high
score, control legend, responsive resizing, and a full playfield pause overlay.

Run another flat binary with:

```sh
bin/mos6502 program.bin --load 0x0600 --start 0x0600
```

Useful options are `--hz`, `--scale`, `--seed`, `--headless`, and `--trace`.

An interactive debugger provides stepping, continue, breakpoints, registers,
memory examination/modification, disassembly, and save/load commands:

```sh
make bin/debugger
bin/debugger program.bin 0x0600 0x0600
```

External functional ROMs can be run without modifying the application:

```sh
make test-rom ROM=6502_functional_test.bin LOAD=0 START=0x400 SUCCESS=0x3469
```

## Virtual machine

The machine provides 64 KiB RAM, a game-state byte at `$00FB`, a high-score byte
at `$00FC`, a game-score byte at `$00FD`, a random byte at `$00FE`, the latest keyboard character at `$00FF`,
and a 32x32 16-color framebuffer at `$0200-$05FF`.
This is deliberately a small game-oriented computer, not an NES emulator.
Original arcade Pac-Man used a Z80; running that ROM would require a different
CPU and hardware platform.

## Apple I

The Apple I terminal target implements the keyboard/display PIA registers at
`$D010-$D013`. Supply a legally obtained 256-byte Woz Monitor image and press
Ctrl-C to exit:

```sh
make bin/apple1
bin/apple1 WOZMON.bin
```

## Nintendo Entertainment System

The NES target implements the Ricoh 2A03 CPU behavior, iNES parsing, mappers 0
(NROM), 2 (UxROM), and 3 (CNROM), mirrored CPU RAM and PPU registers, controller shift register, OAM DMA,
vblank/NMI timing, background scrolling/attributes, sprites, palette rendering,
the pulse/triangle/noise APU channels, and an SDL video/audio frontend:

```sh
make bin/nes
bin/nes game.nes
```

Controls are arrows, Z (B), X (A), Right Shift (Select), Enter (Start), and
Escape. Use legally obtained ROMs. `make test-nestest` verifies all 8,991
canonical CPU trace lines and cumulative cycle counts.

See [the design](docs/design.md) and [project status](docs/plan.md).

## Validation status

The automated suite passes Klaus Dormann's functional and interrupt ROMs,
Bruce Clark's exhaustive decimal test, all 8,991 canonical `nestest` trace
lines, an authentic Woz Monitor interaction, and 2,360,000 SingleStepTests
vectors. Downloaded fixtures are checksum-verified or cached locally rather
than vendored.