Operating System Architecture

The core software components of an operating system are collectively known as the kernel. The kernel has unrestricted access to all of the resources on the system. In early monolithic systems, each component of the operating system was contained within the kernel, could communicate directly with any other component, and had unrestricted system access. While this made the operating system very efficient, it also meant that errors were more difficult to isolate, and there was a high risk of damage due to erroneous or malicious code.


A monolithic OS architecture

A monolithic OS architecture


As operating systems became larger and more complex, this approach was largely abandoned in favour of a modular approach which grouped components with similar functionality into layers to help operating system designers to manage the complexity of the system. In this kind of architecture, each layer communicates only with the layers immediately above and below it, and lower-level layers provide services to higher-level ones using an interface that hides their implementation. The modularity of layered operating systems allows the implementation of each layer to be modified without requiring any modification to adjacent layers. Although this modular approach imposes structure and consistency on the operating system, simplifying debugging and modification, a service request from a user process may pass through many layers of system software before it is serviced and performance compares unfavourably to that of a monolithic kernel. Also, because all layers still have unrestricted access to the system, the kernel is still susceptible to errant or malicious code. Many of today’s operating systems, including Microsoft Windows and Linux, implement some level of layering.


A layered OS architecture

A layered OS architecture


A microkernel architecture includes only a very small number of services within the kernel in an attempt to keep it small and scalable. The services typically include low-level memory management, inter-process communication and basic process synchronisation to enable processes to cooperate. In microkernel designs, most operating system components, such as process management and device management, execute outside the kernel with a lower level of system access. Microkernels are highly modular, making them extensible, portable and scalable. Operating system components outside the kernel can fail without causing the operating system to fall over. Once again, the downside is an increased level of inter-module communication which can degrade system performance.


A microkernel OS architecture

A microkernel OS architecture