History

This emulator has a long history. The current version is based on TinyEMU and compiled to Javascript or WASM with emscripten.

In 2011 I wrote the initial version of JSLinux, the first PC/x86 emulator in Javascript running Linux. I reused parts of another of my emulators (QEMU) for the x86 helpers and the devices.

After some time (2015), I modified it to use the asm.js Javascript subset so that it is faster with the browsers supporting it.

In 2016, after having written TinyEMU, initially a RISC-V emulator, I decided to make a Javascript version from its C code by using emscripten. I added a VirtIO 9P filesystem inspired by the one in jor1k by Sebastian Macke so that it is easier to use remote filesystems and to import or export files.

Then I found interesting to reuse the VirtIO devices for the x86 JS emulator, so I converted the JSLinux asm.js code to C and converted it back to Javascript with emscripten ! With a careful tuning, the new version is now faster than the hand-coded asm.js version.

The next step was to run another operating system than Linux, for example Windows NT. A first release of this emulator ran Windows NT with QEMU inside the emulated Linux system. It was still usable because the x86 emulator supports an x86 virtualization extension (AMD SVM) used by QEMU.

The current version of JSLinux runs Windows NT by emulating the few missing PC devices (PS/2 keyboard and mouse, IDE disk, dummy VGA).

x86 CPU Emulation

A 32 bit or 64 bit x86 CPU with the following features is emulated:

• x87 with bit exact 80 bit floating point numbers

• PAE with NX support

• CMOV instructions

• MMX and SSE2 support

• AMD Virtual Machine extensions (SVM) with Nested Page Table support

• RDPMC support to read the instruction count Known limitations:

• No segment limit and right checks when accessing memory

• No debug support (DRx registers)

Emulated Devices

• 8259 Programmble Interrupt Controller
• 8254 Programmble Interrupt Timer
• 16450 UART (only used to debug)
• Real Time Clock
• PCI bus
• VirtIO console
• VirtIO 9P filesystem
• VirtIO network
• VirtIO block device
• VirtIO input
• Simple framebuffer
• IDE controller (optional)
• PS/2 keyboard and mouse (optional)
• Dummy VGA display (optional)

RISC-V CPU Emulation

A 32 bit or 64 bit RISC-V CPU is emulated with 64 bit FPU and compressed instructions.

Only the RISC-V 64 bit Buildroot and Fedora distributions are now available. The RISC-V 32 bit images are still available but no longer actively maintained:

• buildroot-riscv32 with console.
• buildroot-riscv32 with X Window.

Performance

On a typical desktop PC (2017) with a Firefox browser, the x86 emulator runs about 100 MIPS. Detailed benchmarks are possible with the integrated vmtime utility.

Javascript Terminal

It is an improved version of the original JSLinux one. It has a scrollbar and supports URL highlighting.

Linux distribution

It uses the Alpine Linux and Buildroot distributions. Custom packages are added for vfsync, qemacs, tcc. Optional X Window support is available with the Fluxbox window manager.

Networking

Access to Internet is possible inside the emulator. It uses the websocket VPN offered by Benjamin Burns (see his blog). The bandwidth is capped to 40 kB/s and at most two connections are allowed per public IP address. Please don’t abuse the service.

What’s the use ?

I did it for fun, just because Javascript engines are fast enough to do complicated things. Real use could be:

• Benchmarking of Javascript engines (how much time takes your Javascript engine to boot Linux ?) and use of new browser technologies (asm.js, WASM).
• Learning to use command line Unix tools without leaving the browser.
• Secure file access within the browser (vfsync).
• Running old PC software.

Source code

The source code of the RISC-V version is available in the TinyEMU project. A precompiled easy to install demo is also available.

Similar projects

Several other PC or Linux emulators are available. In particular:

• jor1k: OpenRISC OR1K CPU emulator.
• v86: PC emulator.
• angel: RISC-V CPU emulator.