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C64 in the Avalonia apps

C64-specific features in the Avalonia Browser and Avalonia Desktop apps. The two runtimes share almost all of this; differences are called out inline as Browser / Desktop tabs or Browser only notes.

For automated startup (loading programs / pasting BASIC from a URL or the command line), see the app pages' startup-parameter reference: Browser URL query parameters / Desktop CLI arguments.

ROMs

The C64 needs the Kernal, Basic and Chargen ROMs. An auto-download option exists (a license acknowledgement is required). See Systems / C64 / ROMs for details.

Upload the ROM binaries via the C64 config UI, or use the auto-download option.

Point the app at a directory containing the ROM files. When no directory override is set, the default is ~/Documents/Highbyte/DotNet6502/roms/C64 (or the Windows Documents equivalent). Desktop user changes are saved to the host-specific appsettings.user.json overlay and can be changed in the UI, or use the auto-download option.

Display / renderers

  • Renderer provider Rasterizer → target Avalonia 2-layer bitmap

    • Character mode (normal and multi-color).
    • Bitmap mode (normal and bitmap mode).
    • Sprites (normal and multi-color).
    • Rendering of raster lines for border and background colors.

  • Renderer provider Video commands → target Skia commands

    • Character mode (normal).

Input

Keyboard uses Avalonia.Input.PhysicalKey (W3C code), so both US and Swedish C64 keyboard layouts work. The layout can be overridden in the C64 config dialog. See Systems / C64 / Keyboard mapping for the full host-agnostic mapping.

Keyboard via Avalonia; gamepad via Highbyte.DotNet6502.Impl.Browser. In Chromium browsers the keyboard layout is auto-detected via navigator.keyboard.getLayoutMap(); other browsers fall through to OS culture, then US.

Keyboard via Avalonia; joystick via Highbyte.DotNet6502.Impl.SilkNet.SDL. The keyboard layout is auto-detected from the host (Win32 KLID / macOS TIS*).

Audio

Audio via NAudio. Two C64 audio providers are available: a sample-based one (good but not perfect accuracy — the default) and a command-stream synthesizer one (low CPU but inaccurate); switch in the C64 config dialog if you need lower CPU. The SID emulation mode (Auto / Fast) is selectable in the same dialog. See C64 audio.

Playback via a WebAudio JS interop wave player.

Playback via OpenAL.

SwiftLink supports both direct raw-byte bridging and a fixed-target Hayes modem workflow for software such as Compunet Reborn. See Systems / C64 / SwiftLink support.

Available through a WebSocket bridge endpoint. The browser C64 defaults target the deployed Cloudflare bridge: wss://ws-tcp-bridge.highbyte.workers.dev/bridge, target id compunet-reborn, transport mode HayesModem, interrupt line NMI. When developing the bridge locally with wrangler dev, temporarily change the bridge URL to ws://127.0.0.1:8787/bridge.

Optional native SwiftLink cartridge support with RawTcp and HayesModem transport modes. This is the native host currently recommended for SwiftLink-based software such as Compunet Reborn.

C64 menu

The C64-specific menu (shared by both runtimes) exposes:

  • Copy / Paste — copy the running machine's current BASIC listing to the clipboard, or paste clipboard text into the running C64 via the keyboard.
  • Disk Drive & .D64 images — attach/detach a .d64 disk image, and Download & Run a curated list of programs (auto-downloads the .d64/.prg and starts it). See Compatible programs.
  • Load / Save — load and save BASIC .prg files, load & start a binary, and load bundled assembly / BASIC example programs.
  • Configuration — active joystick, keyboard-joystick on/off and port, and the full C64 config dialog (ROMs, renderer, audio, SwiftLink, and the Basic AI coding assistant — enable it there; F9 toggles it live while running). See Basic code completion.

Browser only

The browser app can generate a shareable URL that reproduces the current state when opened — the inverse of the URL query parameters.

The Share link… button (below the Copy/Paste buttons) opens a dialog that builds a link for one of:

  • Current BASIC program — shares the live BASIC listing (optionally auto-RUN).
  • Selected "Download & Run" program — shares the remote .prg/.d64 of the program selected in the Disk Drive section.

Options: Auto-RUN after start and Include current settings (audio + keyboard-joystick; on by default). The generated URL always carries the system variant (e.g. C64PAL) so the recipient gets the same machine. The link contains the clean target URL — any CORS proxy is applied by the running emulator at fetch time, never baked into the link.

Inline-BASIC links are length-checked (the binding limit is the host/CDN serving the app, not the browser): a warning past ~8,000 characters, and the dialog refuses to share past ~16,000. For a large BASIC program, host it and share a basicUrl= link instead.