Operating System Structure

(1) Monolithic
Frequently such system started from a small, simple and limited systems and grew beyond the original scope. For example, MSDOS and traditional UNIX.

(2) Layered
With proper hardware supoport, OS can be broken into modules. The main advantage of the layered approach is simplicity of construction and debugging. Once a layer is debugged, the upper layer can be tested easily. The major difficulty with the layered approach involves appropriately defining the various layers. sometimes it is not so obvious. For example, the backing store (paging/swapping) driver would normally be above the CPU schedululer because the driver may need to wait for I/O and CPU can be rescheduled during that time. however, on a large system, the CPU scheduler may have mor einformation about all the active processes than can fit in memory. Therefore, this information may need to be swapped in an dout of memory, requiring the back-store driver routine to be below the CPU scheduler. Another problem with layered approach is that it tends to be less efficient than other types. Each layer adds overhead to the system call. This resulted in the recent trend in OS design to have less number of layers and each with more functionality.

(3) Microkernel
In mid-1980s, researchers in Carnegie Mellon University developed an OS called Mach using microkernel approach. The method removes all non-essential components from the kernel and implementing them as user or kernel processes. This results in a smaller kernel. There is little consensus on what should be included or excluded. Typically, micokernel provides minimal process and memory management, in addition to a communication facility. The main purpose of microkernel is to provide a communication facility between the client program and various services that are also running in user space. Communication is provided via message passing through the microkernel.

One benefit is ease of extending the OS. All new services are added as user services and do not require modifying the kernel. This make the OS easier to port to different hardware platform. The microkernel also provides better security and reliability. If a services fails, the rest of the OS remains intact. Several contemporary OS using this approach. Tur64 UNIX provides a UNIX interface to user but was built with a Mach kernel. The Mach kernel maps UNIX system calls into messages to the appropriate user-level services. The Mac OS X (Darwin) was also based on Mach microkernel.

Unfortuntely, microkernel can suffer from performance decreases due to increased system function overhead. For example, the first release of Windows NT was based on a layered microkernel organization. Windows NT 4.0 partially address the performance problem by moving layers from user space to jernel space and integrating them more closely. by the time windows XP was designed, its architecture was more monolithic than microkernel.

(4) Modules
The best current methodology for operating system design involves using object-oriented programming techniques to create a modular kernel. The jernel has a set of core components and links in additional services either during boot time of run time. It uses dynamically loadable modules. For example, Solaris is organized around a core kernel with seven types of loadable kernel modules - scheduling classes, file systems, loadable system calls, executable formats, STREAMS modules, Miscellaneous, device and bus drivers. such design allows the kernel to provide core services yet also allow certain features to be implemented dynamically. For example, device and bus drive for specific hardware can be added.

The result resemble a layered approach in which each section has a defined, protected interface. But it is more flexible than a layered approach as the sections can called one another. The modular approach is also like the microkernel approach in that the primary module has only core functions and knowledge to load and communicate with other modules. but it is more efficient because module can communicate directly with one another rahter than message passing.