The kernel is the central part of an operating system, that directly controls the computer hardware. Usually, the kernel is the first of the user-installed software on a computer, booting directly after the BIOS. Operating system kernels are specific to the hardware on which they are running, thus most operating systems are distributed with different kernel options that are configured when the system is installed. Changing major hardware components such as the motherboard, processor, or memory, often requires a kernel update. Additionally, often new kernels are offered that improve system security or performance. The two major types of kernels competing in today's computer markets are the Windows kernel and the unix-like kernels.
The Windows kernel is available only with the Microsoft Windows series of operating systems. It is proprietary software, developed and distributed by Microsoft Corporation. Introduced in Windows/386, it's many incarnations have since gone by several different names, and some had no names at all. The latest version of the Windows kernel was introduced in Windows NT, and has had many of it's functions removed and placed in user-mode software for Windows Vista. This leads to increased system stability and security. In Vista, application-level software exploits have much less access to the core functions of the operating system, and application crashes will not bring down the OS.
Unix-like kernels are a family of operating system kernels that are based upon, or operate similar to, the original Bell Labs UNIX operating system. Common examples of unix-like kernels are the Linux kernel, BSD, Mac OS, and Solaris. While many of these kernels were developed with original Bell Labs code as part of the software, not all of them have direct lineage to Bell. Linux, for instance, was developed as a free alternative to Minix, itself an independently developed variation of UNIX. Although originally running an original kernel design, Mac OS was outfitted with a unix-like kernel in 1988 with the introduction of A/UX. All subsequent Apple operating systems have unix-like kernels, including the current Mac OS-X's BSD-derived kernel.
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