In computer science, a microkernel (also known as μ-kernel or Samuel kernel) is the near-minimum amount of software that can provide the mechanisms needed to implement an operating system (OS). These mechanisms include low-level address space management, thread management, and inter-process communication (IPC). If the hardware provides multiple rings or CPU modes, the microkernel is the only software executing at the most privileged level (generally referred to as supervisor or kernel mode).[citation needed] Traditional operating system functions, such as device drivers, protocol stacks and file systems, are removed from the microkernel to run in user space.[citation needed] In source code size, microkernels tend to be under 10,000 lines of code, as a general rule. MINIX's kernel, for example has fewer than 6,000 lines of code.[1]
Microkernels were developed in the 1980s as a response to changes in the computer world, and to several challenges adapting existing "mono-kernels" to these new systems. New device drivers, protocol stacks, file systems and other low-level systems were being developed all the time. This code was normally located in the monolithic kernel, and thus required considerable work and careful code management to work on. Microkernels were developed with the idea that all of these services would be implemented as user-space programs, like any other, allowing them to be worked on monolithically and started and stopped like any other program. This would not only allow these services to be more easily worked on, but also separated the kernel code to allow it to be finely tuned without worrying about unintended side effects. Moreover, it would allow entirely new operating systems to be "built up" on a common core, aiding OS research.