Architecture
Internet Explorer uses a componentized architecture built on the Component Object Model (COM) technology. It consists of several major components, each of which is contained in a separate Dynamic-link library (DLL) and exposes a set of COM programming interfaces hosted by the Internet Explorer main executable, iexplore.exe:
- WinInet.dll is the protocol handler for HTTP, HTTPS and FTP. It handles all network communication over these protocols.
- URLMon.dll is responsible for MIME-type handling and download of web content, and provides a thread-safe wrapper around WinInet.dll and other protocol implementations.
- MSHTML.dll houses the Trident rendering engine introduced in Internet Explorer 4, which is responsible for displaying the pages on-screen and handling the Document Object Model of the web pages. MSHTML.dll parses the HTML/CSS file and creates the internal DOM tree representation of it. It also exposes a set of APIs for runtime inspection and modification of the DOM tree. The DOM tree is further processed by a layout engine which then renders the internal representation on screen.
- IEFrame.dll contains the user interface and window of IE in Internet Explorer 7 and above.
- ShDocVw.dll provides the navigation, local caching and history functionalities for the browser.
- BrowseUI.dll is responsible for the browser user interface, including the browser chrome, which houses all the menus and toolbars.
Internet Explorer does not include any native scripting functionality. Rather, MSHTML.dll exposes an API that permit a programmer to develop a scripting environment to be plugged-in and to access the DOM tree. Internet Explorer 8 includes the bindings for the Active Scripting engine, which is a part of Microsoft Windows and allows any language implemented as an Active Scripting module to be used for client-side scripting. By default, only the JScript and VBScript modules are provided; third party implementations like ScreamingMonkey (for ECMAScript 4 support) can also be used. Microsoft also makes available the Microsoft Silverlight runtime that allows CLI languages, including DLR-based dynamic languages like IronPython and IronRuby, to be used for client-side scripting.
Internet Explorer 8 introduces some major architectural changes, called Loosely Coupled IE (LCIE). LCIE separates the main window process (frame process) from the processes hosting the different web applications in different tabs (tab processes). A frame process can create multiple tab processes, each of which can be of a different integrity level; each tab process can host multiple web sites. The processes use asynchronous Inter-Process Communication to synchronize themselves. Generally, there will be a single frame process for all web sites. In Windows Vista with Protected Mode turned on, however, opening privileged content (such as local HTML pages) will create a new tab process as it will not be constrained by Protected Mode.
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