This is a WebAssembly port of the RPCEmu Risc PC Emulator. It can run the following operating systems in your web browser:

• RISC OS 3.5, 3.6 and 3.7
• RISC OS 4
• RISC OS 5 -- actively developed for systems like the Raspberry Pi
• RISC OS Six

You can try RPCEmu in your browser here. Please note that:

• The original mouse had three buttons. Pressing the mouse wheel should act as the middle (menu) button.
• It is possible to manually sync your files, settings, the guest CMOS, and a custom ROM with your browser's database, so long as you do this before you leave the page.

Here is RPCEmu-WASM running RISC OS 5 and a variety of applications in a web browser:

• RPCEmu's core, including ARM610, ARM710, ARM7500, ARM7500FE and StrongARM SA-110 CPUs
• Video
• Audio
• Keyboard
• Mouse (uncaptured mode)
• Floppy/hard disk/CD-ROM images
• GUI

Networking may also partially work, but this is untested and switched off by default.

• Firefox 79
• Chrome 68
• Edge 79
• Safari 15.2
• Opera 64

Any other browsers that support multi-threaded WebAssembly should work. Many modern mobile browsers based on these will also run the emulator, but the video may not be sized properly unless desktop mode is used. A hardware keyboard and mouse/trackpad is also recommended.

Note that the CPU, sound, video and RPCEmu's GUI are run in multiple threads to improve performance, but they share access to system RAM. In multi-threaded WASM, this shared memory is emulated using a JavaScript SharedArrayBuffer, so a single browser tab can run more than one thread in parallel. This JavaScript feature is a RPCEmu-WASM requirement, and using it can result in high CPU usage.

• RPCEmu-WASM runs entirely in WebAssembly, but the build process can still construct native apps in Linux and Windows.
• The Qt version has been upgraded from the version 5 range to 6.5.
• You can transfer files to and from HostFS. This offers a quick way to load modules, upload archives, and export files.
• You can 'upload' ADF floppy disk and ISO CD-ROM images to the emulator as it runs.
• The screenshot feature can 'download' the screenshot to your local storage.
• HostFS, CMOS, RPCEmu configuration, IDE disc images, and a custom ROM can be attached to the browser's IndexedDB instead of just using the in-memory filesystem. This offers a limited way to keep data between sessions, but for Emscripten performance reasons, you must manually sync to save changes.
• Reintroduced mouse wheel support for PS/2-based systems.

This emulator boots from ROM images like many others, but to save on download size, the standard !Boot sequence is not installed by default. If you like, you can remove the default contents of HostFS, replace the !Boot sequence, and then sync your changes so the system starts in your preferred way the next time.

Due to licence restrictions, I only recommend using RISC OS 5 with RPCEmu-WASM. RISC OS 5 is the same version used on the Raspberry Pi, and the Risc PC version is freely available from RISC OS Open Limited. Other versions (such as those based on RISC OS 3.x) can still be purchased, so I would not recommend using them on a public server. In any case, you should obtain your own legal advice.

To run RISC OS 3.x applications, use the following tools:

• Aemulor - Allows you to run 26-bit applications on 32-bit systems. This compatibility layer can also emulate RISC OS 3.1 behaviour.
• ADFFS - Mounts non-standard (e.g. protected) floppy disks inside the emulator by imaging the disk and replaying I/O calls as though it was a proper disk controller.

These are problems which are specific to RISC OS 5, but not RISC OS 3.7. They also occur on the desktop version of RPCEmu:

• Clicking a floppy drive icon without a disk in the drive results in the misleading message Disc not formatted instead of Drive empty.
• Clicking the CD-ROM drive results in CD-ROM drive not found even if an ISO file is selected.
• IDE discs (native hard disks) are untested, however these are unnecessary if the HostFS filesystem is used.
• Only the instruction interpreter is supported, not the recompiler. However, should a suitable ARM-to-WASM recompiler ever be built, it should be extremely fast!

If you want to build the Windows, Linux or FreeBSD desktop versions of RPCEmu, this version will do that, but you will need Qt 6.5 instead of Qt 5. See https://www.marutan.net/rpcemu/ for instructions on how to build the desktop version of RPCEmu.

This version of RPCEmu has been designed to build WebAssembly code in a similar way to native desktop apps:

• Download, install and activate Emscripten 3.1.25, and set the environment variables as per the on-screen instructions.
• Download and install Qt 6.5. In the Qt Setup ('Maintenance Tool') program, ensure you tick WebAssembly (multi-threaded).

Follow the desktop build instructions, noting:

• Ensure qmake's binary is the version from the wasm_multithread folder instead of gcc_64.
• After running qmake, run make to build the Release version. The HTML/JS/WASM/data files and worker script will be output into the main source folder.