Operating-System Structure

Operating-System Structure

An operating system is a construct that allows the user application programs to interact with the system hardware. Since the operating system is such a complex structure, it should be created with utmost care so it can be used and modified easily. An easy way to do this is to create the operating system in parts. Each of these parts should be well defined with clear inputs, outputs and functions.

 Simple Structure

There are many operating systems that have a rather simple structure. These started as small systems and rapidly expanded much further than their scope. A common example of this is MS-DOS.

Layered Structure

One way to achieve modularity in the operating system is the layered approach. In this, the bottom layer is the hardware and the topmost layer is the user interface.

One problem with the layered structure is that each layer needs to be carefully defined. This is necessary because the upper layers can only use the functionalities of the layers below them.

Microkernels

• The basic idea behind micro kernels is to remove all non-essential services from the kernel, and implement them as system applications instead, thereby making the kernel as small and efficient as possible.
• Most microkernels provide basic process and memory management, and message passing between other services, and not much more.
• Security and protection can be enhanced, as most services are performed in user mode, not kernel mode.
• System expansion can also be easier, because it only involves adding more system applications, not rebuilding a new kernel.
• Mach was the first and most widely known microkernel, and now forms a major component of Mac OSX.
• Windows NT was originally microkernel, but suffered from performance problems relative to Windows 95. NT 4.0 improved performance by moving more services into the kernel, and now XP is back to being more monolithic.
• Another microkernel example is QNX, a real-time OS for embedded systems.

Modules

• Modern OS development is object-oriented, with a relatively small core kernel and a set of modules which can be linked in dynamically.
• Modules are similar to layers in that each subsystem has clearly defined tasks and interfaces, but any module is free to contact any other module, eliminating the problems of going through multiple intermediary layers.
• The kernel is relatively small in this architecture, similar to microkernels, but the kernel does not have to implement message passing since modules are free to contact each other directly.

Hybrid Systems

Most Operating Systems today do not strictly adhere to one architecture, but are hybrids of several.

Example: Mac OS X, ios, Andriod