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CHIP-8 interpreter and virtual machine in Rust
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README.md

Rusty CHIP

CHIP-8 Interpreter in Rust

CHIP-8 Logo in ASCII

CHIP-8

CHIP-8 is an interpreted programming language run on a CHIP-8 virtual machine.

CHIP-8 virtual machine

Memory

There are 4096 memory locations, each 1 byte in size, addressed from 0x000 to 0xFFF.

      FONT  0x000 - 0x200
       ROM  0x200 - 0xFA0
CALL STACK  0xFA0 - 0xFFF
--------------------------
     TOTAL  0x000 - 0xFFF

Registers

Data registers

There are 16 data registers, each 1 byte in size, addressed from 0x0 to 0xF.

The 0xF register is used as a flag for some operations, and should not be written by other programs.

Address registers

There is 1 address register, 2 bytes in size, called I.

Call stack

The call stack stores return addresses when subroutines are called.

Modern interpreters support at least 16 levels of nesting, requiring a call stack size of 32 bytes.

Timers

Timers count down to zero at a rate of 60 Hz.

Delay

Used for timing game events

Sound

Beeps when value is non-zero

Input

CHIP-8 was used with a hexidecimal keypad:

,---,---,---,---,
| 1 | 2 | 3 | C |
|---|---|---|---|
| 4 | 5 | 6 | D |
|---|---|---|---|
| 7 | 8 | 9 | E |
|---|---|---|---|
| A | 0 | B | F |
'---'---'---'---'

Rusty CHIP instead uses ArrowUp, ArrowDown, ArrowLeft, ArrowRight, and hexidecimal characters (0 - F).

Graphics

,---------------------,
|(0, 0)   ...  (63, 0)|
| .                 . |
| .                 . |
| .                 . |
|(0, 31)  ... (63, 31)|
'---------------------'

The monochrome display is 64 pixels wide by 32 pixels high. A sprite is 8 pixels wide by 1 - 15 pixels high. Sprites are drawn to the screen using XOR. The 0xF register flag is set to 1 if any true pixels become false, indicating a collision.

Sound

Rusty CHIP's beep tone is a 1000 Hz sine wave because I don't find it as annoying as the other frequencies I tried.

CHIP-8 instruction set

Opcode Instruction
0NNN Execute machine language subroutine at address NNN
00E0 Clear the screen
00EE Return from a subroutine
1NNN Jump to address NNN
2NNN Execute subroutine starting at address NNN
3XNN Skip the following instruction if the value of register VX equals NN
4XNN Skip the following instruction if the value of register VX is not equal to NN
5XY0 Skip the following instruction if the value of register VX is equal to the value of register VY
6XNN Store number NN in register VX
7XNN Add the value NN to register VX
8XY0 Store the value of register VY in register VX
8XY1 Set VX to VX OR VY
8XY2 Set VX to VX AND VY
8XY3 Set VX to VX XOR VY
8XY4 Add the value of register VY to register VX, Set VF to 01 if a carry occurs, Set VF to 00 if a carry does not occur
8XY5 Subtract the value of register VY from register VX, Set VF to 00 if a borrow occurs, Set VF to 01 if a borrow does not occur
8XY6 Store the value of register VY shifted right one bit in register VX, Set register VF to the least significant bit prior to the shift
8XY7 Set register VX to the value of VY minus VX, Set VF to 00 if a borrow occurs, Set VF to 01 if a borrow does not occur
8XYE Store the value of register VY shifted left one bit in register VX, Set register VF to the most significant bit prior to the shift
9XY0 Skip the following instruction if the value of register VX is not equal to the value of register VY
ANNN Store memory address NNN in register I
BNNN Jump to address NNN + V0
CXNN Set VX to a random number with a mask of NN
DXYN Draw a sprite at position VX, VY with N bytes of sprite data starting at the address stored in I, Set VF to 01 if any set pixels are changed to unset, and 00 otherwise
EX9E Skip the following instruction if the key corresponding to the hex value currently stored in register VX is pressed
EXA1 Skip the following instruction if the key corresponding to the hex value currently stored in register VX is not pressed
FX07 Store the current value of the delay timer in register VX
FX0A Wait for a keypress and store the result in register VX
FX15 Set the delay timer to the value of register VX
FX18 Set the sound timer to the value of register VX
FX1E Add the value stored in register VX to register I
FX29 Set I to the memory address of the sprite data corresponding to the hexadecimal digit stored in register VX
FX33 Store the binary-coded decimal equivalent of the value stored in register VX at addresses I, I+1, and I+2
FX55 Store the values of registers V0 to VX inclusive in memory starting at address I, I is set to I + X + 1 after operation
FX65 Fill registers V0 to VX inclusive with the values stored in memory starting at address I, I is set to I + X + 1 after operation

Source: Mastering CHIP-8 by Matthew Mikolay

Resources

CHIP-8

WebAssembly

Rust + WebAssembly

Acknowledgements

Thank you to the following people who discussed, reviewed, or paired on this project with me!

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