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In computing, a "kernel" is essential system software which is the heart of the Operating System. In UNIX and other operating systems the term "kernel-space" is used to refer to the continuum of software that operates in the kernel, and "user-space" the software that runs outside of the kernel.

In most operating systems the kernel is responsible for interfacing with the hardware on behalf of software, regulating software and providing basic services such as file-systems.

The most common design for a kernel is a "monolithic kernel" in which the kernel is responsible for all basic services, but additional basic services may be added by loading "modules" into the kernel. The monolithic kernel architecture is responsible for the endemic instability of many modern operating systems, if any portion of the kernel runs into trouble (for example, a module crashes) the entire system will crash because the kernel is a single point of failure.

"Microkernels" were developed to address this deficiency by moving many functions outside of the kernel, into user-space programs. Thus, if one of these programs ran into trouble, the kernel could kill it and start a new one seamlessly. "Nanokernels" took this one step further, essentially reducing the kernel to "glue" that holds the various kernel-like user-space programs together.

Microkernels never really caught on because they suffered from severe performance problems. One famous attempt at a Microkernel was MINIX, which is still available as a hobby operating system to this day.

Attempts to hybridize microkernels and monolithic kernels have attracted considerable interest but have been partially successful. The Mac OS X operating system's hybridized kernel is based on the "Mach" microkernel developed at Carnegie-Mellon University, with parts of the FreeBSD non-kernel operating system in the user-space. The latest versions of Windows have endeavored to use a hybrid kernel, however it took many years for Microsoft to stabilize the effort. Computer science is not yet decided on the micro-kernel versus Monolithic-kernel debate.

Categories: Software

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