Kernel-wide Design Approaches
Naturally, the above listed tasks and features can be provided in many ways that differ from each other in design and implementation.
The principle of separation of mechanism and policy is the substantial difference between the philosophy of micro and monolithic kernels. Here a mechanism is the support that allows the implementation of many different policies, while a policy is a particular "mode of operation". For instance, a mechanism may provide for user log-in attempts to call an authorization server to determine whether access should be granted; a policy may be for the authorization server to request a password and check it against an encrypted password stored in a database. Because the mechanism is generic, the policy could more easily be changed (e.g. by requiring the use of a security token) than if the mechanism and policy were integrated in the same module.
In minimal microkernel just some very basic policies are included, and its mechanisms allows what is running on top of the kernel (the remaining part of the operating system and the other applications) to decide which policies to adopt (as memory management, high level process scheduling, file system management, etc.). A monolithic kernel instead tends to include many policies, therefore restricting the rest of the system to rely on them.
Per Brinch Hansen presented arguments in favor of separation of mechanism and policy. The failure to properly fulfill this separation, is one of the major causes of the lack of substantial innovation in existing operating systems, a problem common in computer architecture. The monolithic design is induced by the "kernel mode"/"user mode" architectural approach to protection (technically called hierarchical protection domains), which is common in conventional commercial systems; in fact, every module needing protection is therefore preferably included into the kernel. This link between monolithic design and "privileged mode" can be reconducted to the key issue of mechanism-policy separation; in fact the "privileged mode" architectural approach melts together the protection mechanism with the security policies, while the major alternative architectural approach, capability-based addressing, clearly distinguishes between the two, leading naturally to a microkernel design (see Separation of protection and security).
While monolithic kernels execute all of their code in the same address space (kernel space) microkernels try to run most of their services in user space, aiming to improve maintainability and modularity of the codebase. Most kernels do not fit exactly into one of these categories, but are rather found in between these two designs. These are called hybrid kernels. More exotic designs such as nanokernels and exokernels are available, but are seldom used for production systems. The Xen hypervisor, for example, is an exokernel.
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