Fabrice Bellard, who is the author of QEMU, has developed another PC emulator, but this time in Javascript. The JavaScript PC Emulator using the W3C Typed Arrays code is available in some browsers. The program was tested on Firefox and Chrome 11 to 4 on both Linux, Mac and Windows.

It emulates a PC with the following components:
- A 32-bit x86-compatible CPU, near a 486 without FPU
- A programmable interrupt controller 8259,
- A Programmable Timer Interrupt 8254,
- A chip UART 16450,
- A real time clock.
The emulated computer loads a version of Linux 2.6.20, which contains an FPU emulator. To make this functional operating system, a full MMU was implemented.


The emulator now works in Internet Explorer 9.

Recent Changelog:
- faster emulation
- added 16 bit & segmentation support
- added scroll back in terminal

Source: Here