CHIP-8 JIT

Jonas

TU Wien

2023

CHIP 8 Introduction

• Virtual machine developed in the 70s.
• Describes hardware (input/output) and bytecode instructions.
• Tons of internet resources, differing in some instruction quirks.
• Lots of interpreters, some JITs (none seemed fully working at the time).

Hardware/Memory Specification

Keypad

Display (64 x 32)

• 4kB memory (3584 bytes usable).
• About 500 Hz clock speed is sufficient for most games.
• 16 registers.
• 2 timers (sound, delay).
• 32-byte program stack (for function calls).

Instructions

35 instructions

• Arithmetic (8-bit unsigned).
• Program flow (absolute jump, relative jump, conditional, call, return).
• Special instructions (draw, rnd, bcd, load, store).

Load Fx55 - LD I, Vx

Store registers V0 through Vx in memory starting at location I.

Dxyn - DRW Vx, Vy, nibble

Display n-byte sprite starting at memory location I at (Vx, Vy), set VF = collision.

Sprites are XORed onto the existing screen. If this causes any pixels to be erased, VF is set to 1, otherwise to 0.

Components

• Graphical frontend and input (SFML, threaded).
• Parser:
  • Find blocks (end at jump/function call).
  • Gather used registers in block.
• Compiler (ASMJIT):
  • Register allocation.
  • Calling conventions.
• Dispatcher:
  • Handles dirty blocks (writes within current block).

Memory Layout

Memory Layout