In the spectrum of kernel designs the two extreme points are monolithic kernels and microkernels.

The (classical) Linux kernel for instance is a monolithic kernel (and so is every commercial OS to date as well - though they might claim otherwise);

In that its code is a single C file giving rise to a single process that implements all of the above services.

To exemplify the encapsulation of the Linux kernel we remark that the Linux kernel does not even have access to any of the standard C libraries. Indeed the Linux kernel cannot use rudimentary C library functions such as printf. Instead it implements its own printing function (called prints).



This seclusion of the Linux kernel and self-containment provide Linux kernel with its main advantage: the kernel resides in a single address space1 enabling all features to communicate in the fastest way possible without resorting to any type of message passing. In particular, a monolithic kernel implements all of the device drivers of the system.



This however is the main drawback of a monolithic kernel: introduction of any new unsupported hardware requires a rewrite of the kernel (in the relevant parts), recompilation of it, and re-installing the entire OS.

More importantly, if any device driver crashes the entire kernel suffers as a result. This un-modular approach to hardware additions and hardware crashes is the main argument for supporting the other extreme design approach for kernels. A microkernel is in a sense a minimalistic kernel that houses only the very basic of OS services (like process management and file system management). In a microkernel the device drivers lie outside of the kernel allowing for addition and removal of device drivers while the OS is running and require no alternations of the kernel.