The Symbian OS System Model contains the following layers, from top to bottom:

* UI Framework Layer
* Application Services Layer
o Java ME
* OS Services Layer
o generic OS services
o communications services
o multimedia and graphics services
o connectivity services
* Base Services Layer
* Kernel Services & Hardware Interface Layer

The Base Services Layer is the lowest level reachable by user-side operations, it includes the File Server and User Library, the Plug-In Framework which manages all plug-ins, Store, Central Repository, DBMS, and cryptographic services. It also includes the Text Window Server and the Text Shell, the two basic services from which a completely functional port can be created without the need for any higher layer services.

Symbian OS has a microkernel architecture, which means that the minimum necessary is within the kernel to improve robustness, availability, and responsiveness. It contains a scheduler, memory management, and device drivers, but other services like networking, telephony, or filesystem support are placed in the OS Services Layer or Base Services Layer. The inclusion of device drivers means the kernel is not a true microkernel. The EKA2 real-time kernel has been termed a nanokernel, containing only the most basic primitives and supporting an extended kernel to implement any other abstractions.

Symbian OS is designed to emphasize compatibility with other devices, especially removable media file systems. Early development of EPOC led to the adoption of FAT as the internal file system and this remains in the Symbian OS but an object-orientated persistence model has been placed over the underlying FAT, providing a POSIX-style interface and a streaming model. The internal data formats rely on using the same API that create the data to run all file manipulations – this has created the problems of data-dependence and associated difficulties with changes and data migration.

There is a large networking and communication subsystem, which has three main servers – ETEL (EPOC telephony), ESOCK (EPOC sockets) and C32 (responsible for serial communication). Each of these has a plug-in scheme. For example ESOCK allows different “.PRT” protocol modules, implementing different types of networking protocol scheme. The subsystem also contains code that pertains to short-range communication links too, such as Bluetooth, IrDA and USB.

There is also a large volume of ‘User Interface (UI) Code’. For the most part actual user interfaces are maintained by third parties. However the base classes and substructure are contained within the Symbian OS. This component is known as UIKON. The Symbian OS also contains the graphics, text layout, and font rendering libraries.

All Symbian applications are built up from three classes defined by the Application Architecture: an application class, a document class, and an application user interface class. These classes create the fundamental application behavior. The remaining required functions, the application view, data model, and data interface, are created independently and interact solely through their APIs with the other classes. UIQ and S60 both extend this approach, in two different ways.

There are, of course, many other things that do not yet fit into this model – for example, SyncML, Java ME providing another set of APIs on top of most of the OS and multimedia. Quite a few of these are frameworks, and vendors are expected to supply plug-ins to these frameworks from third parties (for example, Helix player for multimedia codecs). This has the advantage that the APIs to such areas of functionality are the same on many phone models, and that vendors get a lot of flexibility, but means that phone vendors need to do a great deal of integration work to make a Symbian OS phone.

Symbian OS device manufacturers also get supplied with an example user interface layer called TechView. This is very similar to the user interface from a Psion Series 5 personal organiser, so isn’t used for any given phone user interface, but provides a basis to start customisation. It is also the environment in which a lot of Symbian OS test code and example code runs.

Symbian OS is a proprietary operating system, designed for mobile devices, with associated libraries, user interface frameworks and reference implementations of common tools, produced by Symbian Ltd. It is a descendant of Psion’s EPOC and runs exclusively on ARM processors.

Symbian is currently owned by Nokia (47.9%), Ericsson (15.6%), Sony Ericsson (13.1%), Panasonic (10.5%), Siemens AG (8.4%) and Samsung (4.5%). Although BenQ acquired the mobile phone subsidiary of Siemens AG, the Siemens AG stake in Symbian did not pass to BenQ.

Design

Symbian OS, with its roots in Psion Software’s EPOC, is structured like many desktop operating systems with pre-emptive multitasking and memory protection. EPOC was inspired by a VMS-like approach to multitasking with server-based asynchronous serialised access based on events.

Symbian OS was built to follow three design rules – the integrity and security of user data is paramount, user time must not be wasted, and all resources are scarce. This led to a continuation of the use of servers; a microkernel; a request and callback approach to all services; an absolute division of user interfaces from system or application services; reuse and openness; extensibility; and robust management and resource recovery to support extended always-on operation. For hardware the OS is optimised for low-power battery-based devices and for ROM-based systems (e.g. features like XIP and re-entrancy in shared libraries). Applications, and the OS, follow an object orientated design, MVC.

Later OS iterations diluted this approach in response to market demands, notably the introduction of a real-time kernel and a platform security model in versions 8 and 9.

There is a strong emphasis on conserving resources, using Symbian-specific programming idioms such as descriptors and a cleanup stack. There are similar techniques for conserving disk space (though the disks on Symbian devices are usually flash memory). Furthermore, all Symbian OS programming is event-based, and the CPU is switched off when applications are not directly dealing with an event. This is achieved through a programming idiom called active objects. Similarly the OS approach to threads vs. processes is driven by reducing overheads.

Competition

Symbian OS is the leading OS in the ’smart mobile device’ market. Statistics published February 2007 showed that Symbian OS had a 67% share of the ’smart mobile device’ market, with Microsoft having 14% through Windows CE and Windows Mobile and RIM having 7%.[1] Other competitors include Palm OS, Linux, Qualcomm’s Brew, and SavaJe.

Symbian OS EKA2 supports sufficiently-fast real-time response that it is possible to build a single-core phone around it – that is, a phone in which a single processor core executes both the user applications and the signalling stack. This is not a feature that is available from Linux. This has allowed SymbianOS EKA2 phones to become smaller, cheaper and more power efficient

Thnx to Wiki