Browser-Based Simulation

RHDL includes a browser-based simulator that compiles the IR simulation backends to WebAssembly and renders live VCD waveforms using p5.js visualization.

Overview

The web simulator lets you run RHDL designs directly in the browser — no installation required. It supports multiple execution backends, preconfigured runner presets, and interactive debugging tools.

Features

Multiple backends — interpreter, JIT, and compiler execution modes
Live waveforms — VCD signal visualization rendered with p5.js
Preconfigured presets — ready-made runners for Generic, CPU, MOS 6502, Apple II, and Game Boy architectures
Debugging tools — value breakpoints and memory inspection
Interactive schematic — component visualization using ELK.js layout and WebGL 2.0

Architecture

The build process has two stages:

Ruby-side artifact generation — produces WASM binaries and metadata from RHDL IR
Bun bundling — packages JavaScript, LitElement components, and runtime assets into web/dist/

The application uses:

LitElement components for the UI
Redux for state management
ELK.js for schematic layout
WebGL 2.0 for interactive rendering
SharedArrayBuffer for multi-threaded WASM execution

User Interface

The workspace is organized into tabbed panels:

Panel	Purpose
I/O	Set inputs, read outputs, control simulation
VCD	Signal analysis with waveform viewer
Memory	Browse and inspect memory contents
Components	Component tree with drill-down
Schematic	Interactive circuit visualization

Running the Simulator

Prerequisites

Rust toolchain with wasm-pack
Bun (JavaScript bundler)

Build

# Build WASM artifacts
rake web:build
 
# Bundle and serve
cd web && bun run dev

CORS Requirements

The simulator requires specific HTTP headers for SharedArrayBuffer:

Cross-Origin-Opener-Policy: same-origin
Cross-Origin-Embedder-Policy: require-corp

These are automatically configured for local development. For deployment, the build includes a service worker fallback.

Presets

The simulator ships with preconfigured presets for demonstration:

Preset	Description	Components
Generic	Basic component testing	Any single component
CPU	8-bit CPU datapath	ALU, registers, PC, decoder
MOS 6502	Full 6502 processor	CPU, memory, I/O
Apple II	Apple II system	6502, video, keyboard, memory
Game Boy	Game Boy system	SM83 CPU, PPU, APU, memory

Deployment

GitHub Actions workflows automate deployment to GitHub Pages:

# Build for production
rake web:dist
 
# Deploy (via CI)
gh workflow run deploy-web

Next Steps

RTL Simulation — Ruby behavioral simulation
Performance Tuning — backend comparison
Building a 6502 — try the MOS 6502 preset

CIRCT

Explorer