Kernel Theory

In computing, the kernel is the central component of most computer operating systems; it is a bridge between applications and the actual data processing done at the hardware level. The kernel's responsibilities include managing the system's resources (the communication betweenhardware andsoftware components).

Usually as a basic component of an operating system, a kernel can provide the lowest-levelabstraction layer for the resources (especially processors and I/O devices) that application software must control to perform its function. It typically makes these facilities available toapplication processes through inter-process communication mechanisms and system calls.

Type of kernel:

1. Monolithic kernels

   
   
   In a monolithic kernel, all OS services run along with the main kernel thread, thus also residing in the same memory area. This approach provides rich and powerful hardware access. Some developers, such as UNIX developer Ken Thompson, maintain that it is "easier to implement a monolithic kernel than microkernels. The main disadvantages of monolithic kernels are the dependencies between system components — a bug in a device driver might crash the entire system — and the fact that large kernels can become very difficult to maintain.
   
   
   2. Microkernel
   The microkernel approach consists of defining a simple abstraction over the hardware, with a set of primitives or system calls to implement minimal OS services such as memory management, multitasking, and inter-process communication. Other services, including those normally provided by the kernel, such as networking, are implemented in user-space programs, referred to as servers. Microkernels are easier to maintain than monolithic kernels, but the large number of system calls and context switches might slow down the system because they typically generate more overhead than plain function calls.
   A microkernel allows the implementation of the remaining part of the operating system as a normal application program written in a high-level language, and the use of different operating systems on top of the same unchanged kernel. It is also possible to dynamically switch among operating systems and to have more than one active simultaneously.