Javascript PC Emulator – Technical Notes

Javascript PC Emulator – Technical Notes

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Javascript PC Emulator – Technical Notes

By Fabrice Bellard – Aug 30, 2015 This PC emulator is written in Javascript. The emulated hardware consists in the following devices:
  • 32 bit x86 compatible CPU
  • 8259 Programmble Interrupt Controller
  • 8254 Programmble Interrupt Timer
  • 16450 UART
  • Real Time Clock.
  • IDE interface and hard disk.
The code is written in pure Javascript using the W3C Typed Arrays. A slightly slower fallback mode is implemented for browsers missing this support. For the exact list of supported browsers, see the FAQ. In any case, a fast Javascript engine is needed to have good performance.

CPU Emulation

Some of the code is inspired from my x86 dynamic translator present in QEMU, but there are important differences because here it is an interpreter. The CPU is close to a 486 compatible x86 without FPU. The lack of FPU is not a problem when running Linux as Operating System because it contains a FPU emulator. In order to be able to run Linux, a complete MMU is implemented. The exact restrictions of the emulated CPU are:
  2. No segment limit and right checks when accessing memory (Linux does not rely on them for memory protection, so it is not an issue. The x86 emulator of QEMU has the same restriction).
  3. No single-stepping
I added some tricks which are not present in QEMU to be more precise when emulating unaligned load/stores at page boundaries. The condition code emulation is also more efficient than the one in QEMU.


Currently there is no synchronization between the PIT frequency and the real time, so there is a variable drift between the time returned by Linux (try the “date” command) and the real time. The UART (serial port) does not support FIFO mode. Perhaps it could help to improve the display speed. There is no network emulation at this point. A clipboard device (seen as /dev/clipboard in the emulator) was added to allow exchange of data between the emulator and the outside world.

Javascript terminal

Although I could have reused the excellent termlib, I decided to write my own because I was curious to see how it could be done. The main problem is the key handling which is different among browsers and OSes, as described here.

Linux distribution

I compiled a 2.6.20 Linux kernel (I guess any other version would work provided there is still an FPU emulator). The Linux kernel configuration, patch and the source code of the Linux starter (kind of BIOS) are available: linuxstart-20120111.tar.gz. The disk image contains a filesystem generated with Buildroot using BusyBox. I added my toy C compiler TinyCC and my unfinished but usable emacs clone QEmacs. There is also a small MS-DOS .COM launcher I use to test the 16 bit emulation with a tiny .COM program to compute pi and a small self-assembling assembler for MS-DOS.


I happen to be interested by the implementation of Javascript engines these days – but I don’t know yet if I will write my own any time soon ! Anyway, this emulator was a way to learn how to write optimized code for recent Javascript engines, in particular Jaeger Monkey (for Firefox 4) and V8 (for Chrome). A troubling thing is that the PC emulator is slower using V8 than Jaeger Monkey (I used the 32 bit version for both). I have no precise explanation yet because I only looked at the Jeager Monkey code so far.

What’s the use ?

I did it for fun, just because newer 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 ?). For this particular application, efficient handling of 32 bit signed and unsigned integers and of typed arrays is important.
  • Learning to use command line Unix tools without leaving the browser.
  • Client side processing using an x86 library, for example for cryptographic purposes. For such application, the x86 emulator can be modified to provide an API to load x86 dynamic libraries and to provide a js-ctypes like API to call the C/C++ functions from javascript.
  • A more advanced version would allow to use old DOS PC software such as games.

Similar projects

Several other PC or Linux emulators are now available. In particular:
  • jor1k: OpenRISC OR1K CPU emulator.
  • v86: PC emulator.
  • angel: RISC-V CPU emulator.
[Back to the PC emulator] from: –EOF–
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