CHIP-8 Interpreter

A functional CHIP-8 interpreter written in Rust. This project emulates the hardware specifications of the 1970s CHIP-8 virtual machine, allowing it to run classic games like Pong, Tetris, and Blitz.

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Technical Specifications

The interpreter implements the following virtual hardware components:

• Memory: 4096 bytes (4KB) of RAM.
• Registers: 16 general-purpose 8-bit registers ($V0$ to $VF$). $VF$ is also used as a flag for carry, borrows, and collision detection.
• Index Register ($I$): A 16-bit register used for memory addresses.
• Program Counter ($PC$): A 16-bit register pointing to the current instruction.
• Display: A $64 \times 32$ monochrome display (2048 pixels total).
• Timers: Two 8-bit timers (Delay and Sound) that decrement at 60Hz.
• Stack: Used to store return addresses when calling subroutines.
• Input: A 16-key hexadecimal keypad ($0$ through $F$).

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Instruction Set

The CHIP-8 uses 35 opcodes, each 2 bytes long. In the table below:

• NNN: Address (12 bits)
• NN: 8-bit constant
• X, Y: 4-bit register identifiers

Opcode	Description
00E0	Clears the display.
00EE	Returns from a subroutine.
1NNN	Jumps to address NNN.
2NNN	Calls subroutine at NNN.
3XNN	Skips next instruction if VX == NN.
4XNN	Skips next instruction if VX != NN.
5XY0	Skips next instruction if VX == VY.
6XNN	Sets VX to NN.
7XNN	Adds NN to VX (Carry flag unchanged).
8XY0	Sets VX to the value of VY.
8XY1	Bitwise OR (`VX
8XY2	Bitwise AND (VX &= VY). Resets VF to 0.
8XY3	Bitwise XOR (VX ^= VY). Resets VF to 0.
8XY4	Adds VY to VX. VF is set to 1 if there is a carry.
8XY5	Subtracts VY from VX. VF is set to 0 if there is a borrow.
8XY6	Stores LSB of VX in VF then shifts VX right by 1.
8XY7	Sets VX to VY - VX. VF is set to 0 if there is a borrow.
8XYE	Stores MSB of VX in VF then shifts VX left by 1.
9XY0	Skips next instruction if VX != VY.
ANNN	Sets I to address NNN.
BNNN	Jumps to address NNN + V0.
CXNN	Sets VX to result of rand() & NN.
DXYN	Draws a sprite at (VX, VY) with width 8 and height N.
EX9E	Skips next instruction if key in VX is pressed.
EXA1	Skips next instruction if key in VX is not pressed.
FX07	Sets VX to the value of the delay timer.
FX0A	Awaits a key press and stores the result in VX.
FX15	Sets the delay timer to VX.
FX18	Sets the sound timer to VX.
FX1E	Adds VX to I.
FX29	Sets I to the location of the sprite for character in VX.
FX33	Stores the BCD representation of VX in memory at I, I+1, I+2.
FX55	Stores registers V0 through VX in memory starting at I.
FX65	Fills registers V0 through VX from memory starting at I.

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How to Run

Prerequisites

• Rust compiler and Cargo.
• Dependencies for Macroquad (system libraries for graphics).

Execution

To run a specific ROM, pass the path as an argument:

cargo run -- path/to/rom

By default, if no path is provided, the interpreter attempts to load a test opcode ROM.

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Input Mapping

The original CHIP-8 used a 4x4 keypad. This interpreter maps the keys to a standard QWERTY keyboard as follows:

Keyboard	CHIP-8
1 2 3 4	1 2 3 C
Q W E R	4 5 6 D
A S D F	7 8 9 E
Z X C V	A 0 B F

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Sources

• Wikipedia: CHIP-8
• Multigesture: How to write an emulator