QEMU模拟器项目创始人Fabrice Bellard开发了在JavaScript中运行的模拟器，允许用户在浏览器上(支持Firefox 4和Google Chrome 11)启动Linux。现在，另一位开发者修改了Fabrice Bellard的JS/Linux，允许模拟器启动较新的内核版本linux kernel 3.0.4，同时支持永久储存。

开发者创建了一个虚拟块设备，它能在HTML5本地储存中储存数据，块设备可以分区和格式化。修改版地址，运行方法是点击emulator.html(http://stud.hro.nl/0814604/jslinuxdemo/emulator.html)，登录名为root。

Javascript PC Emulator - FAQ

Which browser should I use to try the PC emulator ?

What is the expected boot time of Linux with the emulator ?

How can I scroll back in the terminal ?

How can I copy data to the Virtual Machine ?

How can I copy data from the Virtual Machine ?

When will there be network support ?

Is it possible to emulate a VGA card and all the missing devices to launch say FreeDOS or Windows ?

Is there some source code available ?

Is my privacy respected ?

Javascript PC Emulator - Technical Notes

This PC emulator is written in Javascript. The emulated hardware consists in the following devices:

CPU Emulation

Devices

Javascript terminal

Linux distribution

Javascript

What's the use ?

相当于一个在线Linux操作系统，具体有什么用还有待进一步研究。初步发先文件系统是用BusyBox做的，

可以使用大多数Linux的命令，还可以用tab来自动补全操作时的命令。

下面是虚拟块设备和本地存储的一些介绍：

This project allows the virtual machine to access a persistent block device. This means
a user can format and partition this device inside it's virtual machine. Because this disk
is then stored in the HTML5 local storage(存储在HTML5本地存储中), a return to the emulator's web page allows a
user to reopen the block device. All data stored on the disk can therefore be read again.

Schematically, the process works in the following way:

A block device driver is added to the Linux kernel. This allows a user to operate with
a 'device' written in JavaScript. This device is bound to the virtual machine using the
same I/O ports as a floppy disk. The device driver itself uses a temporary testing major
device node, id 250. Theoretically, it allows dynamic device node allocation, but I have
not tested this.

On the JavaScript side, a lot of caching occurs because the local storage interface is
quite slow. I therefore decided to cache the total 'disk' in memory, because it's about
1M anyway. Writes are cached as well, and are 'flushed' once the driver has transferred
a complete sector.

For more details, see the source code.

Javascript PC Emulator - News

• Aug 20, 2011:
  • faster emulation
  • added 16 bit & segmentation support
  • added scroll back in terminal
• May 26, 2011:
  • display boot time
  • faster emulation
  • added some missing instructions (bcd support, XLAT, BOUND)
  • easier customization possible (seejslinux.jsfile).
• May 20, 2011:
  • added support for more browsers including Opera 11.11 and Internet Explorer 9
  • reduced VM RAM size from 32 MB to 16 MB
  • Source code release of the Linux starter utility.

• May 18, 2011:
  • added a clipboard to exchange data between the VM and the host
  • added FAQ and News pages
  • fixed monospace font for some browsers.

• May 17, 2011:
  • fixed binary XHR for Firefox nightly builds
  • fixed terminal height for thelesscommand
  • fixed ampersand output in the terminal.

• May 16, 2011:
  • Initial realease.

Javascript PC Emulator - FAQ

Which browser should I use to try the PC emulator ?

The following browsers are officially supported:

• Firefox >= 4
• Chrome 11
• Opera 11.11
• Internet Explorer 9

For optimal performance, your browser should support theW3C Typed Arrays. Firefox 4.x and Chrome 11 have such support.

The current version of Chrome 12 beta is reported to have abugpreventing to run the emulator.

What is the expected boot time of Linux with the emulator ?

Here are some figures:

Browser	Host	Boot time (s)
Firefox 5.0.1	Core2 Quad 2.3 GHz	4.0
Firefox 6 beta	Samsung Galaxy S2	32.5

How can I scroll back in the terminal ?

Use Ctrl-Up, Ctrl-Down, Ctrl-PageUp and Ctrl-PageDown.

How can I copy data to the Virtual Machine ?

1. Copy your file content to the OS clipboard using your favorite text editor.
2. Paste the data to the JS/Linux clipboard using the mouse (it is the text area on the right of the VM terminal).
3. In the VM Linux shell do:

   cat < /dev/clipboard > /tmp/myfile
   

   Your text file is now copied to/tmp/myfile.

To copy binary data, you canuuencodeit on the PC using (assuming you use Linux):

uuencode myfile myfile > myfile.txt

Then you copymyfile.txtto the Linux VM and uudecode it by doing:

uudecode < /dev/clipboard

How can I copy data from the Virtual Machine ?

The procedure is the reverse of the previous one:

1. Copy your file to the clipboard:

   cat myfile > /dev/clipboard
   

2. Select all the clipboard data and copy it to the host OS clipboard using the mouse over the text area on the right of the VM terminal.
3. Paste the data in your favorite text editor

For binary files, you canuuencodein the VM anduudecodeon the host.

When will there be network support ?

It is relatively easy to add a virtual network interface. The problem is to make it communicate with other hosts, so at this point it is not my priority. For example, due to security restrictions, it is only possible to communicate with the web site hosting the VM or with cooperating servers. Moreover, only specific types of network protocols can be handled because no raw TCP connection nor raw UDP packets can be sent due to API restrictions and security reasons.

Is it possible to emulate a VGA card and all the missing devices to launch say FreeDOS or Windows ?

Yes, it is possible but since the current VM is quite slow, only old OSes (such as Windows 3 or MSDOS) would be really usable.

Is there some source code available ?

In thetechnical notes, an archive is given which explains how to build the various binary images.

The readable source code of the JS PC emulator itself is not yet available. At this point, any mirroring or redistribution of the code needs my explicit permission.

Is my privacy respected ?

Yes. What you type in the Virtual Machine stays in your browser. No data is sent back to the server.

Javascript PC Emulator - Technical Notes

ByFabrice Bellard- May 23, 2011

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.

The code is written in pure Javascript using theW3C Typed Arrays. A slightly slower fallback mode is implemented for browsers missing this support. For the exact list of supported browsers, see theFAQ. 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 inQEMU, 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:

1. No FPU/MMX/SSE
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.

Devices

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/clipboardin the emulator) was added to allow exchange of data between the emulator and the outside world.

Javascript terminal

Although I could have reused the excellenttermlib, 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 describedhere.

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-20110820.tar.gz.

The disk image is just a ram disk image loaded at boot time. It contains a filesystem generated withBuildrootcontainingBusyBox. I added my toy C compilerTinyCCand my unfinished but usable emacs cloneQEmacs. 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 smallself-assembling assembler for MS-DOS.

Javascript

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 ajs-ctypeslike API to call the C/C++ functions from javascript.
• A more advanced version would allow to use old DOS PC software such as games.