Most early operating systems were implemented by means of large monolithic kernels. Loosely speaking, the complete operating system-scheduling, file system, networking, device drivers, memory management, paging, and more -- was packed into a single kernel.
* Microkernels must efficiently support new types of applications that cannot be implemented with good performance on conventional monolithic kernels. This progressive criterion must be satisfied for a real advance in operating-system technology.
Indeed, most early microkernels evolved step by step from monolithic kernels, remaining rich in concepts and large in code size.
