Design symbianOS

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 an OpenVMS-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-oriented 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.

Symbian OS EKA2 supports sufficiently-fast real-time response such 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 a feature which is not available in Linux. This has allowed SymbianOS EKA2 phones to become smaller, cheaper and more power efficienthttp://en.wikipedia.org/wiki/Symbian_OS#Symbian_OS_v6.0_and_6.1