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articles/microkernel.md

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+# Microkernel
+A microkernel is a kernel that has only the bare minimum functionality on kernel space, moving most of
+it to the userland, as servers. Due to having less code running in privileged kernel mode (less attack surface),
+microkernels can be more safer than monolithic ones.
+
+A central concept of microkernels are the *servers*, which run in userland and implement
+the functionality that monolithic kernels do in the kernel side (file systems, drivers, memory management).
+Servers aren't different from regular user processes, only that have additional privileges (for accessing hardware directly, for example)
+and communicate with the kernel and with other servers using [interprocess communication (IPC)](ipc.md) mechanisms.
+
+Since servers run in userspace, a crash in a server don't necessarily bring the whole system down, but may affect
+the consistency of the whole system, and reliability is not exactly inherent to the microkernel design.
+
+One of the first microkernels, Mach, had a disappointing performance. This gave microkernels a bad reputation, which
+however where demonstrated to not be an inherent design characteristic, but an implementation one. The L4 family
+of microkernels demonstrated that IPC can be as fast as regular syscalls on a monolithic kernel.
+
+Some notable microkernels are:
+- Mach
+- L4 and its descendants, such as seL4; known for it high-performance IPC, as fast as a monolithic kernel.
+- Minix: [Unix](unix.md)-like kernel. Most notably, a heavily modified version of it runs in the Intel's backdoor known as
+"Management Engine".
+- QNX
+- ...