SYSTEM STRUCTURE IN OPERATING SYSTEM

SYSTEM STRUCTURES:

For efficient performance and implementation an OS should be partitioned into separate subsystems, each with carefully defined tasks, inputs, outputs, and performance characteristics. These subsystems can then be arranged in various architectural configurations like:

• Simple Structure
• Layered Approach
• Microkernels

1. Simple Structure:

• Operating systems such as MS-DOS and the original UNIX did not have well-defined structures.
  
• There is no CPU execution mode (user and kernel), and so errors in applications can cause the whole system to crash.
  

2.Layered Approach:

• This approach breaks up the operating system into different layers.
• With the layered approach, the bottom layer is the hardware, while the highest layer is the user interface.
• As long as the external interface of the routines don’t change, developers have more freedom to change the inner workings of the routines.

1. The main advantage is simplicity of construction and debugging.

2. The main difficulty is defining the various layers.

3. The main disadvantage is that the OS tends to be less efficient than other implementations.

3.Micro Kernel:

• 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.
  The benefits of the microkernel are as follows:
  

• Extending the operating system becomes much easier.

• Any changes to the kernel tend to be fewer, since the kernel is smaller.

• The microkernel also provides more security and reliability.

Main disadvantage is poor performance due to increased system overhead from message passing.