基於瀏覽器的Linux

文章目錄

• Javascript PC Emulator - News
• Javascript PC Emulator - News
• Javascript PC Emulator - FAQ
• Javascript PC Emulator - Technical Notes

QEMU模擬器項目創始人Fabrice Bellard開發了在JavaScript中啟動並執行模擬器，允許使用者在瀏覽器上(支援Firefox 4和Google Chrome 11)啟動Linux。現在，另一位開發人員修改了Fabrice Bellard的JS/Linux，允許模擬器啟動較新的核心版本linux kernel 3.0.4，同時支援永久儲存。

圖1 最新版本(核心版本為3.0.4)

                 開發人員建立了一個虛擬塊裝置，它能在HTML5本地儲存中儲存資料，塊裝置可以分區和格式化。部分源碼(http://stud.hro.nl/0814604/jslinuxdemo/)地址，運行方法是點擊emulator.html(http://stud.hro.nl/0814604/jslinuxdemo/emulator.html)，登入名稱為root。

圖2 最新版本(支援本機存放區，可以將檔案系統鏡像儲存到本地或上傳)

另一個版本的地址為http://bellard.org/jslinux/ 。

圖2 另一個版本(可通過clipboard裝置在使用者和虛擬機器中Linux傳遞資訊)

             這是一台完全通過JavaScript寫成的、使用QEMU類比x86指令集的、運行在瀏覽器（我的FireFox運行正常）上的虛擬機器。這台機上面運行著一套2.6.20核心的Linux OS，這裡的“運行”和以前一些WebOS或者JavaScript類比DOS的程式並不是一回事，這裡的OS就是完完全全運行在用戶端瀏覽器上的，各種互動並不會傳送到服務端或者委派到cmd.exe這些程式上執行，說白了這就是一台這實實在在啟動並執行Linux虛擬機器，這台虛擬機器的相關資訊可以在作者公布的Tech頁面看到。也可以自己登入到虛擬機器上查一下。

             發現檔案系統是用BusyBox做的，可以使用大多數Linux的命令，還可以用tab來自動補全操作時的命令。

圖3  所有支援的命令

如何?的：

1. 作者 Fabrice Bellard 使用 JavaScript 編寫了一個簡單的 PC 模擬器，包含32位 x86 相容 CPU、8259 可程式化插斷控制器、8254 可程式化中斷計時器
2. 實現 16450 UART 串口裝置
3. 用 JavaScript 實現一個終端
4. 編譯包含 FPU 類比的 Linux 核心鏡像。
5. 使用 Buildroot 建立檔案系統並在啟動時載入 RAM。
6. 添加基本工具集 BusyBox，微型 C 編譯器 TinyCC，以及迷你編輯器 QEmacs

jslinux 簡單分析

jslinux：用JavaScript寫出一個PC模擬器，上面運行Linux (The code is written in pure Javascript using Typed Arrays which are available in recent browsers.)
1、首頁面 (http://bellard.org/jslinux/) 很簡單，主要調用了 如下兩個 js 檔案。

http://bellard.org/jslinux/term.js

http://bellard.org/jslinux/cpux86.js

2、term.js 裡也沒有發現什麼特殊的地方； 
3、cpux86.js 使用到一個 load_binary 的函數，查看了一下實現，是通過 XMLHttpRequest 來取如下三個 bin 檔案(使用Typed Arrays 操作二進位檔案)：

http://bellard.org/jslinux/vmlinux26.bin

http://bellard.org/jslinux/root.bin

http://bellard.org/jslinux/linuxstart.bin

根據作者的描述，該模擬器的初衷是瞭解現在 JavaScript 引擎的工作，尤其是 Firefox 4 的 Jaeger Monkey 和 Chrome 的 V8，這個探索的結果，可以被用來：

• 通過 Linux 的啟動時間反應瀏覽器的 JavaScript 效能。(作者說目前在 Firefox 4 下比 Chrome 11 要快 2倍，Chrome 12 有 Bug 無法運行)。
• 實驗在用 JavaScript 進行用戶端運算時載入 x86 運行庫的可能性。
• 進一步發展從而支援 DOS 遊戲的運行。

圖4  微型 C 編譯器 TinyCC

          home目錄下有個一個hello.c檔案，這是將它編譯後運行後的結果。有了C編譯器，以及QEmacs、vi等文本編譯器，我們可以在這個平台上編寫C程式。

圖5  CPU資訊

圖6 記憶體資訊

下面是虛擬塊裝置和本機存放區的一些介紹：

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(儲存在HTML5本機存放區中), 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:
• May 26, 2011:
• May 20, 2011:
• May 18, 2011:
• May 17, 2011:
• May 16, 2011:

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技術內涵

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 ?

===========================================================================

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 (see jslinux.js file).
• 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 the less command
  • fixed ampersand output in the terminal.
• May 16, 2011:
  • Initial realease.

Javascript PC Emulator - FAQWhich 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 the W3C Typed Arrays. Firefox 4.x and Chrome 11 have such support.

The current version of Chrome 12 beta is reported to have a bug preventing 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 can uuencode it on the PC using (assuming you use Linux):

uuencode myfile myfile > myfile.txt

Then you copy myfile.txt to 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 can uuencode in the VM and uudecode on 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 the technical 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

By Fabrice 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 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:

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/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-20110820.tar.gz.

The disk image is just a ram disk image loaded at boot time. It contains a filesystem generated with Buildroot containing BusyBox. I added my toy C compiler TinyCCand 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.

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 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.