In computing, the kernel is a computer program that manages input/output requests from software and translates them into data processing instructions for the central processing unitand other electronic components of a computer. The kernel is a fundamental part of a modern computer's operating system.
When a computer program (in this case called a process) makes requests of the kernel, the request is called a system call. Various kernel designs differ in how they manage system calls (time-sharing) and resources. For example, a monolithic kernel executes all the operating system instructions in the same address space to improve the performance of the system. A microkernel runs most of the operating system's background process in user space to make the operating system more modular and, therefore, easier to maintain.
Kernel basic facilities
The kernel's primary function is to manage the computer's hardware and resources and allow other programs to run and use these resources. Typically, the resources consist of:
The Central Processing Unit. This is the most central part of a computer system, responsible for running or executing programs. The kernel takes responsibility for deciding at any time which of the many running programs should be allocated to the processor or processors (each of which can usually run only one program at a time)
The computer's memory. Memory is used to store both program instructions and data. Typically, both need to be present in memory in order for a program to execute. Often multiple programs will want access to memory, frequently demanding more memory than the computer has available. The kernel is responsible for deciding which memory each process can use, and determining what to do when not enough is available.
Any Input/Output (I/O) devices present in the computer, such as keyboard, mouse, disk drives, USB devices, printers, displays, network adapters, etc. The kernel allocates requests from applications to perform I/O to an appropriate device (or subsection of a device, in the case of files on a disk or windows on a display) and provides convenient methods for using the device (typically abstracted to the point where the application does not need to know implementation details of the device).
Key aspects necessary in resource management are the definition of an execution domain (address space) and the protection mechanism used to mediate the accesses to the resources within a domain.
Kernels also usually provide methods for synchronization and communication between processes called inter-process communication(IPC).
A kernel may implement these features itself, or rely on some of the processes it runs to provide the facilities to other processes, although in this case it must provide some means of IPC to allow processes to access the facilities provided by each other.
Finally, a kernel must provide running programs with a method to make requests to access these facilities.
