Visualizza wikitesto di Kernel

[[Image:Kernel Layout.svg|thumb|200px|A kernel connects the application software to the hardware of a computer.]]

In [[computing]], the '''kernel''' is the central component of most computer [[operating system]]s; it is a bridge between applications and the actual data processing done at the hardware level. The kernel's responsibilities include managing the system's resources (the communication between [[Computer hardware|hardware]] and [[Computer software|software]] components).<ref name="Wulf74"/> Usually as a basic component of an operating system, a kernel can provide the lowest-level [[abstraction layer]] for the resources (especially [[Central processing unit|processors]] and [[Input/output|I/O devices]]) that application software must control to perform its function. It typically makes these facilities available to [[Application software|application]] [[process (computing)|processes]] through [[inter-process communication]] mechanisms and [[system call]]s.

Operating system tasks are done differently by different kernels, depending on their design and implementation. While [[monolithic kernel]]s execute all the operating system code in the same [[address space]] to increase the performance of the system, [[microkernel]]s run most of the operating system services in [[user space]] as servers, aiming to improve maintainability and modularity of the operating system.<ref name="mono-micro">Roch 2004</ref> A range of possibilities exists between these two extremes.

==Overview==
[[Image:Computer abstraction layers.svg|thumb|200px|A typical vision of a [[computer architecture]] as a series of abstraction layers: [[hardware]], [[firmware]], [[Assembly language|assembler]], kernel, [[operating system]] and [[Process (computing)|applications]] (see also [http://www.pearsonhighered.com/educator/academic/product/0,,0131485210,00%2ben-USS_01DBC.html ''Structured Computer Organization'', by Andrew S. Tanenbaum])]]
On the definition of "kernel", [[Jochen Liedtke]] said that the word is "traditionally used to denote the part of the operating system that is mandatory and common to all other software."<ref name="Liedtke95">Liedtke 95</ref> Most operating systems rely on this concept of the kernel. The existence of a kernel is a natural consequence of designing a computer system as a series of [[abstraction layer]]s,<ref name="Tanenbaum79">Tanenbaum 79, chapter 1</ref> each relying on the functions of layers beneath it. The kernel, from this viewpoint, is simply the name given to the lowest level of abstraction that is implemented in [[software]]. In order to avoid having a kernel, one would have to design all the software on the system not to use abstraction layers; this would increase the complexity of the design to such a point that only the simplest systems could feasibly be implemented.

While it is today mostly called the ''kernel'', originally the same part of the operating system was also called the ''nucleus'' or ''core''<ref name="Wulf74">Wulf 74 pp.337–345</ref><ref name="Deitel82">Deitel 82, p.65-66 cap. 3.9</ref><ref name="kernelnames">Lorin 81 pp.161–186, Schroeder 77, Shaw 75 pp.245–267</ref><ref name="Hansen70">Brinch Hansen 70 pp.238–241</ref>, and was conceived as containing only the essential support features of the operating system. However the term ''core'' has also been used to refer to the primary memory of a computer system, because some early computers used a form of memory called [[core memory]].

In most cases, the [[boot loader]] starts executing the kernel in [[supervisor mode]].<ref name="supervisor">The highest privilege level has various names throughout different architectures, such as supervisor mode, kernel mode, CPL0, DPL0, Ring 0, etc. See [[Ring (computer security)]] for more information.</ref> The kernel then initializes itself and starts the first process. After this, the kernel does not typically execute directly, only in response to external events (''e.g.'', via system calls used by applications to request services from the kernel, or via [[interrupt]]s used by the hardware to notify the kernel of events). Additionally, the kernel typically provides a loop that is executed whenever no processes are available to run; this is often called the ''idle process''.

Kernel development is considered one of the most complex and difficult tasks in programming.<ref name="bkerndev">[http://osdever.net/bkerndev/index.php?the_id=90 Bona Fide OS Development – Bran's Kernel Development Tutorial], by Brandon Friesen</ref> Its central position in an operating system implies the necessity for good performance, which defines the kernel as a critical piece of software and makes its correct design and implementation difficult. For various reasons, a kernel might not even be able to use the [[Abstraction (computer science)|abstraction]] mechanisms it provides to other software. Such reasons include [[memory management]] concerns (for example, a user-mode function might rely on memory being subject to [[demand paging]], but as the kernel itself provides that facility it cannot use it, because then it might not remain in memory to provide that facility) and lack of [[Reentrant (subroutine)|reentrancy]], thus making its development even more difficult for software engineers.

A kernel will usually provide features for low-level [[scheduling (computing)|scheduling]]<ref name="Deitel82sched">For low-level scheduling see Deitel 82, ch. 10, pp. 249–268.</ref> of processes (dispatching), [[inter-process communication]], process [[Synchronization (computer science)|synchronization]], [[context switch]]ing, manipulation of [[process control block]]s, [[interrupt]] handling, process creation and destruction, and process suspension and resumption (see [[process states]]).<ref name="Deitel82" /><ref name="Hansen70" />

==Kernel basic facilities==
The kernel's primary purpose is to manage the computer's resources and allow other programs to run and use these resources.<ref name="Wulf74"/> Typically, the resources consist of:
* The [[Central Processing Unit]] (CPU, the processor). This is the most central part of a computer system, responsible for ''running'' or ''executing'' programs on it. The kernel takes responsibility for deciding at any time which of the many running programs should be allocated to the processor or processors (each of which can usually run only one program at a time)
* The computer's [[Random-access memory|memory]]. Memory is used to store both program instructions and data. Typically, both need to be present in memory in order for a program to execute. Often multiple programs will want access to memory, frequently demanding more memory than the computer has available. The kernel is responsible for deciding which memory each process can use, and determining what to do when not enough is available.
* Any [[input/output|Input/Output (I/O)]] devices present in the computer, such as keyboard, mouse, disk drives, printers, displays, etc. The kernel allocates requests from applications to perform I/O to an appropriate device (or subsection of a device, in the case of files on a disk or windows on a display) and provides convenient methods for using the device (typically abstracted to the point where the application does not need to know implementation details of the device).

Key aspects necessary in resource managements are the definition of an execution domain ([[address space]]) and the protection mechanism used to mediate the accesses to the resources within a domain.<ref name="Wulf74" />

Kernels also usually provide methods for [[synchronization (computer science)|synchronization]] and [[inter-process communication|communication]] between processes (called ''inter-process communication'' or IPC).

A kernel may implement these features itself, or rely on some of the processes it runs to provide the facilities to other processes, although in this case it must provide some means of IPC to allow processes to access the facilities provided by each other.

Finally, a kernel must provide running programs with a method to make requests to access these facilities.

===Process management===
{{Main|Process_management_(computing)}}
The main task of a kernel is to allow the execution of applications and support them with features such as hardware abstractions. A process defines which memory portions the application can access.<ref name="Levy84">Levy 1984, p.5</ref> (For this introduction, process, application and program are used as synonyms.)<!-- a clear introductory definition of process is missing -->  Kernel [[process management]] must take into account the hardware built-in equipment for [[memory protection]].<ref>Needham, R.M., Wilkes, M. V. ''[http://comjnl.oxfordjournals.org/cgi/content/abstract/17/2/117 Domains of protection and the management of processes]'', Computer Journal, vol. 17, no. 2, May 1974, pp 117–120.</ref>

To run an application, a kernel typically sets up an [[address space]] for the application, loads the file containing the application's code into memory (perhaps via [[demand paging]]), sets up a [[Call stack|stack]] for the program and branches to a given location inside the program, thus starting its execution.<ref name="OS-Concepts">Silberschatz 1990</ref>

[[Computer multitasking|Multi-tasking]] kernels are able to give the user the illusion that the number of processes being run simultaneously on the computer is higher than the maximum number of processes the computer is physically able to run simultaneously. Typically, the number of processes a system may run simultaneously is equal to the number of CPUs installed (however this may not be the case if the processors support [[simultaneous multithreading]]).

In a [[pre-emptive multitasking]] system, the kernel will give every program a slice of time and switch from process to process so quickly that it will appear to the user as if these processes were being executed simultaneously. The kernel uses [[scheduling algorithm]]s to determine which process is running next and how much time it will be given. The algorithm chosen may allow for some processes to have higher priority than others. The kernel generally also provides these processes a way to communicate; this is known as [[inter-process communication]] (IPC) and the main approaches are [[shared memory]], [[message passing]] and [[remote procedure call]]s (see [[concurrent computing]]).

Other systems (particularly on smaller, less powerful computers) may provide [[co-operative multitasking]], where each process is allowed to run uninterrupted until it makes a special request that tells the kernel it may switch to another process. Such requests are known as "yielding", and typically occur in response to requests for interprocess communication, or for waiting for an event to occur. Older versions of [[Microsoft Windows|Windows]] and [[Mac OS]] both used co-operative multitasking but switched to pre-emptive schemes as the power of the computers to which they were targeted grew{{Citation needed|date=May 2008}}.

The operating system might also support [[multiprocessing]] ([[Symmetric multiprocessing|SMP]] or [[Non-Uniform Memory Access]]); in that case, different programs and threads may run on different processors. A kernel for such a system must be designed to be re-entrant, meaning that it may safely run two different parts of its code simultaneously. This typically means providing [[synchronization (computer science)|synchronization]] mechanisms (such as [[spinlock]]s) to ensure that no two processors attempt to modify the same data at the same time.

===Memory management===
The kernel has full access to the system's memory and must allow processes to safely access this memory as they require it. Often the first step in doing this is [[virtual addressing]], usually achieved by [[paging]] and/or [[Segmentation (memory)|segmentation]]. Virtual addressing allows the kernel to make a given physical address appear to be another address, the virtual address. Virtual address spaces may be different for different processes; the memory that one process accesses at a particular (virtual) address may be different memory from what another process accesses at the same address. This allows every program to behave as if it is the only one (apart from the kernel) running and thus prevents applications from crashing each other.<ref name="OS-Concepts"/>

On many systems, a program's virtual address may refer to data which is not currently in memory. The layer of indirection provided by virtual addressing allows the operating system to use other data stores, like a [[hard drive]], to store what would otherwise have to remain in main memory (RAM). As a result, operating systems can allow programs to use more memory than the system has physically available. When a program needs data which is not currently in RAM, the CPU signals to the kernel that this has happened, and the kernel responds by writing the contents of an inactive memory block to disk (if necessary) and replacing it with the data requested by the program. The program can then be resumed from the point where it was stopped. This scheme is generally known as [[demand paging]].

Virtual addressing also allows creation of virtual partitions of memory in two disjointed areas, one being reserved for the kernel ([[kernel space]]) and the other for the applications ([[user space]]). The applications are not permitted by the processor to address kernel memory, thus preventing an application from damaging the running kernel. This fundamental partition of memory space has contributed much to current designs of actual general-purpose kernels and is almost universal in such systems, although some research kernels (e.g. [[Singularity (operating system)|Singularity]]) take other approaches.

===Device management===
To perform useful functions, processes need access to the [[peripheral]]s connected to the computer, which are controlled by the kernel through [[device driver]]s. For example, to show the user something on the screen, an application would make a request to the kernel, which would forward the request to its display driver, which is then responsible for actually plotting the character/pixel.<ref name="OS-Concepts"/>

A kernel must maintain a list of available devices. This list may be known in advance (e.g. on an embedded system where the kernel will be rewritten if the available hardware changes), configured by the user (typical on older PCs and on systems that are not designed for personal use) or detected by the operating system at run time (normally called [[plug and play]]).

In a plug and play system, a device manager first performs a scan on different [[hardware bus]]es, such as [[Peripheral Component Interconnect]] (PCI) or [[Universal Serial Bus]] (USB), to detect installed devices, then searches for the appropriate drivers.

As device management is a very OS-specific topic, these drivers are handled differently by each kind of kernel design, but in every case, the kernel has to provide the [[Input/Output|I/O]] to allow drivers to physically access their devices through some port or memory location. Very important decisions have to be made when designing the device management system, as in some designs accesses may involve [[context switch]]es, making the operation very CPU-intensive and easily causing a significant performance overhead.{{Citation needed|date=July 2007}}

===System calls===
<!-- This seems a very "narrow perspective" section, a more general approach would help -->
To actually perform useful work, a process must be able to access the services provided by the kernel. This is implemented differently by each kernel, but most provide a [[C library]] or an [[Application Programming Interface|API]], which in turn invokes the related kernel functions.<ref>{{cite book |last= Tanenbaum|first=Andrew S.|authorlink=Andrew S. Tanenbaum |title=Modern Operating Systems |edition=3rd |series= |date= |year=2008 |publisher=Prentice Hall |isbn=0-13-600663-9 |pages=50–51 |quote=. . . nearly all system calls [are] invoked from C programs by calling a library procedure . . . The library procedure . . . executes a TRAP instruction to switch from user mode to kernel mode and start execution . . . }}</ref>

The method of invoking the kernel function varies from kernel to kernel. If memory isolation is in use, it is impossible for a user process to call the kernel directly, because that would be a violation of the processor's access control rules. A few possibilities are:
* Using a software-simulated [[interrupt]]. This method is available on most hardware, and is therefore very common.
* Using a [[call gate]]. A call gate is a special address stored by the kernel in a list in kernel memory at a location known to the processor. When the processor detects a call to that address, it instead redirects to the target location without causing an access violation. This requires hardware support, but the hardware for it is quite common.
* Using a special system call instruction. This technique requires special hardware support, which common architectures (notably, [[x86]]) may lack. System call instructions have been added to recent models of x86 processors, however, and some operating systems for PCs make use of them when available.
* Using a memory-based queue. An application that makes large numbers of requests but does not need to wait for the result of each may add details of requests to an area of memory that the kernel periodically scans to find requests.

==Kernel design decisions==
===Issues of kernel support for protection===
An important consideration in the design of a kernel is the support it provides for protection from faults ([[fault tolerance]]) and from malicious behaviors ([[computer security|security]]). These two aspects are usually not clearly distinguished, and the [[Separation of protection and security|adoption of this distinction]] in the kernel design leads to the rejection of a [[hierarchical protection domains|hierarchical structure for protection]].<ref name="Wulf74" />

The mechanisms or policies provided by the kernel can be classified according to several criteria, as: static (enforced at [[compile time]]) or dynamic (enforced at [[Run time (computing)|runtime]]); preemptive or post-detection; according to the protection principles they satisfy (i.e. [[Peter J. Denning|Denning]]<ref name="Denning76">Denning 1976</ref><ref name="Swift05Denning76">Swift 2005, p.29 quote: "isolation, resource control, decision verification (checking), and error recovery."</ref>); whether they are hardware supported or language based;  whether they are more an open mechanism or a binding policy; and many more.

====Fault tolerance====
A useful measure of the level of fault tolerance of a system is how closely it adheres to the [[principle of least privilege]].<ref>Cook, D.J. ''[http://portal.acm.org/citation.cfm?id=803220 Measuring memory protection]'', accepted for 3rd International Conference on Software Engineering, Atlanta, Georgia, May 1978.</ref> In cases where multiple programs are running on a single computer, it is important to prevent a fault in one of the programs from negatively affecting the other. Extended to malicious design rather than a fault, this also applies to [[computer security|security]], where it is necessary to prevent processes from accessing information without being granted permission.

The two major hardware approaches<ref name="Swift05">Swift 2005 p.26</ref> for protection (of sensitive information) are [[Hierarchical protection domains]] (also called [[Ring (computer security)|ring]] architectures, segment architectures or [[supervisor mode]]),<ref>Intel Corporation 2002</ref> and [[Capability-based addressing]].<ref>Houdek et al. 1981</ref>

[[Image:Priv rings.svg|250px|thumb|right|[[Privilege ring]]s, such as in the [[x86]], are a common implementation  of [[Hierarchical protection domains]] used in many commercial systems to have some level of fault tolerance.]]
Hierarchical protection domains are much less flexible, as is the case with every kernel with a hierarchical structure assumed as global design criterion.<ref name="Wulf74" /> In the case of protection it is not possible to assign different privileges to processes that are at the same privileged level, and therefore is not possible to satisfy [[Peter J. Denning|Denning's]] four principles for fault tolerance<ref name="Denning76" /><ref name="Swift05Denning76" /> (particularly the Principle of least privilege). Hierarchical protection domains also have a major performance drawback, since interaction between different levels of protection, when a process has to manipulate a data structure both in 'user mode' and 'supervisor mode', always requires message copying (transmission [[Call-by-value|by value]]).<ref name="Hansen73SupervisorMode">Hansen 73, section 7.3 p.233 "''interactions between different levels of protection require transmission of messages by value''"</ref> A kernel based on capabilities, however, is more flexible in assigning privileges, can satisfy Denning's fault tolerance principles,<ref name="LindenCapabilityAddressing">Linden 76</ref> and typically doesn't suffer from the performance issues of copy by value.

Both approaches typically require some hardware or firmware support to be operable and efficient. The hardware support for hierarchical protection domains<ref name="Schroeder72">Schroeder 72</ref> is typically that of "[[CPU modes]]." An efficient and simple way to provide hardware support of capabilities is to delegate the [[Memory management unit|MMU]] the responsibility of checking access-rights for every memory access, a mechanism called [[capability-based addressing]].<ref name="LindenCapabilityAddressing" /> Most commercial computer architectures lack MMU support for capabilities.
An alternative approach is to simulate capabilities using commonly-supported hierarchical domains; in this approach, each protected object must reside in an address space that the application does not have access to; the kernel also maintains a list of capabilities in such memory.  When an application needs to access an object protected by a capability, it performs a system call and the kernel performs the access for it.  The performance cost of address space switching limits the practicality of this approach in systems with complex interactions between objects, but it is used in current operating systems for objects that are not accessed frequently or which are not expected to perform quickly.<ref name="EranianMosberger">Stephane Eranian & David Mosberger, [http://www.informit.com/articles/article.aspx?p=29961 Virtual Memory in the IA-64 Linux Kernel], Prentice Hall PTR, 2002</ref><ref>Silberschatz & Galvin, Operating System Concepts, 4th ed, pp445 & 446</ref>
Approaches where protection mechanism are not firmware supported but are instead simulated at higher levels (e.g. simulating capabilities by manipulating page tables on hardware that does not have direct support), are possible, but there are performance implications.<ref name="HochBrowne">{{cite journal | last = Hoch  | first = Charles  | coauthors = J. C. Browne (University of Texas, Austin)  | year = 1980
 | month = July
 | title = An implementation of capabilities on the PDP-11/45
 | journal = ACM SIGOPS Operating Systems Review
 | volume = 14
 | issue = 3
 | pages = 22–32
 | doi = 10.1145/850697.850701
 | id =
 | url = http://portal.acm.org/citation.cfm?id=850701&dl=acm&coll=&CFID=15151515&CFTOKEN=6184618
 | language =
 | format = PDF
 | accessdate = 2007-01-07
 }}</ref> Lack of hardware support may not be an issue, however, for systems that choose to use language-based protection.<ref name="Schneider">[http://www.cs.cmu.edu/~rwh/papers/langsec/dagstuhl.pdf A Language-Based Approach to Security], Schneider F., Morrissett G. (Cornell University) and Harper R. (Carnegie Mellon University)</ref>

An important kernel design decision is the choice of the abstraction levels where the security mechanisms and policies should be implemented. Kernel security mechanisms play a critical role in supporting security at higher levels.<ref name="LindenCapabilityAddressing" /><ref name="Loscocco98"/><ref>J. Lepreau et al. ''[http://doi.acm.org/10.1145/504450.504477 The Persistent Relevance of the Local Operating System to Global Applications]''. Proceedings of the 7th ACM SIGOPS Eur</noinclude>cshelf/book001/book001.html Information Security: An Integrated Collection of Essays], IEEE Comp. 1995.</ref><ref>J. Anderson, ''[http://csrc.nist.gov/publications/history/ande72.pdf Computer Security Technology Planning Study], Air Force Elect. Systems Div., ESD-TR-73-51, October 1972.</ref><ref>* {{ cite journal| author = Jerry H. Saltzer, Mike D. Schroeder| title = The protection of information in computer systems| journal = Proceedings of the IEEE| volume = 63
| issue = 9| pages = 1278–1308| date = September 1975| url = http://web.mit.edu/Saltzer/www/publications/protection/| doi = 10.1109/PROC.1975.9939 }}</ref>

One approach is to use firmware and kernel support for fault tolerance (see above), and build the security policy for malicious behavior on top of that (adding features such as [[cryptography]] mechanisms where necessary), delegating some responsibility to the [[compiler]]. Approaches that delegate enforcement of security policy to the compiler and/or the application level are often called ''language-based security''.

The lack of many critical security mechanisms in current mainstream operating systems impedes the implementation of adequate security policies at the application [[abstraction level]].<ref name="Loscocco98">P. A. Loscocco, S. D. Smalley, P. A. Muckelbauer, R. C. Taylor, S. J. Turner, and J. F. Farrell. ''[http://www.jya.com/paperF1.htm The Inevitability of Failure: The Flawed Assumption of Security in Modern Computing Environments]''. In Proceedings of the 21st National Information Systems Security Conference, pages 303–314, Oct. 1998. [http://csrc.nist.gov/nissc/1998/proceedings/paperF1.pdf].</ref> In fact, a common misconception in computer security is that any security policy can be implemented in an application regardless of kernel support.<ref name="Loscocco98"/>

====Hardware-based protection or language-based protection====
Typical computer systems today use hardware-enforced rules about what programs are allowed to access what data. The processor monitors the execution and stops a program that violates a rule (e.g., a user process that is about to read or write to kernel memory, and so on). In systems that lack support for capabilities, processes are isolated from each other by using separate address spaces.<ref>{{cite journal|
url=http://portal.acm.org/citation.cfm?doid=319151.319163|
title=EROS: a fast capability system|
author=Jonathan S. Shapiro; Jonathan M. Smith; David J. Farber|
journal=Proceedings of the seventeenth ACM symposium on Operating systems principles}}</ref> Calls from user processes into the kernel are regulated by requiring them to use one of the above-described system call methods.

An alternative approach is to use language-based protection. In a [[Language-based system|language-based protection system]], the kernel will only allow code to execute that has been produced by a trusted language [[compiler]]. The language may then be designed such that it is impossible for the programmer to instruct it to do something that will violate a security requirement.<ref name="Schneider"/>

Advantages of this approach include:
* No need for separate address spaces. Switching between address spaces is a slow operation that causes a great deal of overhead, and a lot of optimization work is currently performed in order to prevent unnecessary switches in current operating systems. Switching is completely unnecessary in a language-based protection system, as all code can safely operate in the same address space.
* Flexibility. Any protection scheme that can be designed to be expressed via a programming language can be implemented using this method. Changes to the protection scheme (e.g. from a hierarchical system to a capability-based one) do not require new hardware.

Disadvantages include:
* Longer application start up time. Applications must be verified when they are started to ensure they have been compiled by the correct compiler, or may need recompiling either from source code or from [[bytecode]].
* Inflexible [[type system]]s. On traditional systems, applications frequently perform operations that are not [[type safety|type safe]]. Such operations cannot be permitted in a language-based protection system, which means that applications may need to be rewritten and may, in some cases, lose performance.

Examples of systems with language-based protection include [[JX (operating system)|JX]] and [[Microsoft]]'s [[Singularity (operating system)|Singularity]].

===Process cooperation===
[[Edsger Dijkstra]] proved that from a logical point of view, [[Atomicity (programming)|atomic]] [[Lock (computer science)|lock]] and unlock operations operating on binary [[Semaphore (programming)|semaphores]] are sufficient primitives to express any functionality of process cooperation.<ref name="Dijkstra65">Dijkstra, E. W. ''Cooperating Sequential Processes''. Math. Dep., Technological U., Eindhoven, Sept. 1965.</ref> However this approach is generally held to be lacking in terms of safety and efficiency, whereas a [[message passing]] approach is more flexible.<ref name="Hansen70" />

===I/O devices management===
The idea of a kernel where I/O devices are handled uniformly with other processes, as parallel co-operating processes, was first proposed and implemented by [[Brinch Hansen]] (although similar ideas were suggested in 1967<ref>{{cite web | title=SHARER, a time sharing system for the CDC 6600 | work= | url=http://portal.acm.org/citation.cfm?id=363778&dl=ACM&coll=GUIDE&CFID=11111111&CFTOKEN=2222222 | accessdate=2007-01-07}}</ref><ref>{{cite web | title=Dynamic Supervisors – their design and construction | work= | url=http://portal.acm.org/citation.cfm?id=811675&dl=ACM&coll=GUIDE&CFID=11111111&CFTOKEN=2222222 | accessdate=2007-01-07}}</ref>). In Hansen's description of this, the "common" processes are called ''internal processes'', while the I/O devices are called ''external processes''.<ref name="Hansen70" />

==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.<ref>Baiardi 1988</ref><ref name="Levin75">Levin 75</ref> 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,<ref name="Levin75" /> 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.).<ref name="Wulf74" /><ref name="Hansen70" /> A monolithic kernel instead tends to include many policies, therefore restricting the rest of the system to rely on them.

[[Per Brinch Hansen]] presented [[Cogency|cogent]] arguments in favor of separation of mechanism and policy.<ref name="Wulf74" /><ref name="Hansen70" /> The failure to properly fulfill this separation, is one of the major causes of the lack of substantial innovation in existing operating systems,<ref name="Wulf74" /> a problem common in computer architecture.<ref name="Denning80">Denning 1980</ref><ref name="Nehmer91">Jürgen Nehmer ''[http://portal.acm.org/citation.cfm?id=723612 The Immortality of Operating Systems, or: Is Research in Operating Systems still Justified?]'' Lecture Notes In Computer Science; Vol. 563. Proceedings of the International Workshop on Operating Systems of the 90s and Beyond. pp. 77–83 (1991) ISBN 3-540-54987-0 [http://www.sigmod.org/dblp/db/conf/dagstuhl/os1991.html] quote: "The past 25 years have shown that research on operating system architecture had a minor effect on existing main stream systems." [http://www.soe.ucsc.edu/~brucem/soft_ins/dissert.html]</ref><ref>Levy 84, p.1 quote: "Although the complexity of computer applications increases yearly, the underlying hardware architecture for applications has remained unchanged for decades."</ref> 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;<ref name="Levy84privilegedmode">Levy 84, p.1 quote: "Conventional architectures support a single privileged mode of
operation. This structure leads to monolithic design; any module needing protection must be part of the single operating system kernel. If, instead, any module could execute within a protected domain, systems could be built as a collection of independent modules extensible by any user."</ref> in fact, every module needing protection is therefore preferably included into the kernel.<ref name="Levy84privilegedmode"/> This link between monolithic design and "privileged mode" can be reconducted to the key issue of mechanism-policy separation;<ref name="Wulf74"/> 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<ref name="Wulf74"/> (see [[Separation of protection and security]]).

While [[monolithic kernel]]s execute all of their code in the same address space ([[kernel space]]) [[microkernel]]s try to run most of their services in user space, aiming to improve maintainability and modularity of the codebase.<ref name="mono-micro"/> Most kernels do not fit exactly into one of these categories, but are rather found in between these two designs. These are called [[hybrid kernel]]s. More exotic designs such as [[nanokernel]]s and [[exokernel]]s are available, but are seldom used for production systems. The [[Xen]] hypervisor, for example, is an exokernel.

[[Image:kernel-simple.png|thumb|130px|Diagram of Monolithic kernels]]
===Monolithic kernels===
{{Main|Monolithic kernel}}

In a monolithic kernel, all OS services run along with the main kernel thread, thus also residing in the same memory area. This approach provides rich and powerful hardware access. Some developers, such as UNIX developer Ken Thompson, maintain that it is "easier to implement a monolithic kernel"<ref name="Linuxisobsoletedebate">[http://oreilly.com/catalog/opensources/book/appa.html Open Sources: Voices from the Open Source Revolution]</ref> than microkernels. The main disadvantages of monolithic kernels are the dependencies between system components — a bug in a device driver might crash the entire system — and the fact that large kernels can become very difficult to maintain.

[[Image:Kernel-microkernel.svg|thumb|260px|In the [[microkernel]] approach, the kernel itself only provides basic functionality that allows the execution of [[Server (computing)|servers]], separate programs that assume former kernel functions, such as device drivers, GUI servers, etc.]]

===Microkernels===<!-- This section is linked from [[Unix philosophy]] -->
{{Main|Microkernel}}

The microkernel approach consists of defining a simple abstraction over the hardware, with a set of primitives or [[system call]]s to implement minimal OS services such as [[memory management]], [[Computer multitasking|multitasking]], and [[inter-process communication]]. Other services, including those normally provided by the kernel, such as [[Computer networking|networking]], are implemented in user-space programs, referred to as ''servers''. Microkernels are easier to maintain than monolithic kernels, but the large number of system calls and [[context switch]]es might slow down the system because they typically generate more overhead than plain function calls.

A microkernel allows the implementation of the remaining part of the operating system as a normal application program written in a [[high-level language]], and the use of different operating systems on top of the same unchanged kernel.<ref name="Hansen70" /> It is also possible to dynamically switch among operating systems and to have more than one active simultaneously.<ref name="Hansen70" />

===Monolithic kernels vs. microkernels===
As the computer kernel grows, a number of problems become evident. One of the most obvious is that the [[memory footprint]] increases. This is mitigated to some degree by perfecting the [[virtual memory]] system, but not all [[computer architecture]]s have virtual memory support.<ref>Virtual addressing is most commonly achieved through a built-in [[memory management unit]].</ref> To reduce the kernel's footprint, extensive editing has to be performed to carefully remove unneeded code, which can be very difficult with non-obvious interdependencies between parts of a kernel with millions of lines of code.

By the early 1990s, due to the various shortcomings of monolithic kernels versus microkernels, monolithic kernels were considered obsolete by virtually all operating system researchers. As a result, the design of [[Linux (kernel)|Linux]] as a monolithic kernel rather than a microkernel was the topic of a famous debate between [[Linus Torvalds]] and [[Andrew S. Tanenbaum|Andrew Tanenbaum]].<ref name="TorvaldsTanenbaum">Recordings of the debate between Torvalds and Tanenbaum can be found at [http://www.dina.dk/~abraham/Linus_vs_Tanenbaum.html dina.dk], [http://groups.google.com/group/comp.os.minix/browse_thread/thread/c25870d7a41696d2/f447530d082cd95d?tvc=2#f447530d082cd95d groups.google.com], [http://www.oreilly.com/catalog/opensources/book/appa.html oreilly.com] and [http://www.cs.vu.nl/~ast/reliable-os/ Andrew Tanenbaum's website]</ref> There is merit on both sides of the argument presented in the [[Tanenbaum–Torvalds debate]].

====Performances====
[[Monolithic kernel]]s are designed to have all of their code in the same address space ([[kernel space]]), which some developers argue is necessary to increase the performance of the system.<ref name="MatthewRussell"/> Some developers also maintain that monolithic systems are extremely efficient if well-written.<ref name="MatthewRussell">{{cite web|
url=http://oreilly.com/pub/a/mac/2005/09/27/what-is-darwin.html?page=2|
title=What Is Darwin (and How It Powers Mac OS X)|
author=[[Matthew Russell]]|
publisher=[[O'Reilly Media]]}} quote: "The tightly coupled nature of a monolithic kernel allows it to make very efficient use of the underlying hardware [...] Microkernels, on the other hand, run a lot more of the core processes in userland. [...] Unfortunately, these benefits come at the cost of the microkernel having to pass a lot of information in and out of the kernel space through a process known as a context switch. Context switches introduce considerable overhead and therefore result in a performance penalty." This statements are not part of a peer reviewed article.</ref>{{Verify credibility|date=December 2008}} The monolithic model tends to be more efficient{{Citation needed|date=July 2007}} through the use of shared kernel memory, rather than the slower IPC system of microkernel designs, which is typically based on [[message passing]].{{Citation needed|date=July 2007}}

The performance of microkernels constructed in the 1980s and early 1990s was poor.<ref name="Liedtke95"/><ref name="Hartig97">Härtig 97</ref> Studies that empirically measured the performance of these microkernels did not analyze the reasons of such inefficiency.<ref name="Liedtke95"/> The explanations of this data were left to "folklore"<!-- Someone needs to clarify what is meant by folklore -->, with the assumption that they were due to the increased frequency of switches from "kernel-mode" to "user-mode",<ref name="Liedtke95"/> to the increased frequency of [[inter-process communication]]<ref name="Liedtke95"/> and to the increased frequency of [[context switch]]es.<ref name="Liedtke95"/> <!-- Remains to be covered in this section the impact (particularly on context switch frequency) of the implementation of device drivers as processes or procedures -->

In fact, as guessed in 1995, the  reasons for the poor performance of microkernels might as well have been: (1) an actual inefficiency of the whole microkernel ''approach'', (2) the particular ''concepts'' implemented in those microkernels, and (3) the particular ''implementation'' of those concepts.<ref name="Liedtke95"/> Therefore it remained to be studied if the solution to build an efficient microkernel was, unlike previous attempts, to apply the correct construction techniques.<ref name="Liedtke95"/>

On the other end, the [[hierarchical protection domains]] architecture that leads to the design of a monolithic kernel<ref name="Levy84privilegedmode"/> has a significant performance drawback each time there's an interaction between different levels of protection (i.e. when a process has to manipulate a data structure both in 'user mode' and 'supervisor mode'), since this requires message copying [[Call-by-value|by value]].<ref name="Hansen73SupervisorMode" />

By the mid-1990s, most researchers had abandoned the belief that careful tuning could reduce this overhead dramatically,{{Citation needed|date=July 2007}} but recently, newer microkernels, optimized for performance, such as [[L4 microkernel family|L4]]<ref name="l4">[http://os.inf.tu-dresden.de/L4/overview.html The L4 microkernel family – Overview]</ref> and [[K42]] have addressed these problems.{{Verify source|date=July 2007}}

[[Image:Kernel-hybrid.svg|thumb|260px|The [[hybrid kernel]] approach combines the speed and simpler design of a monolithic kernel with the modularity and execution safety of a microkernel.]]

===Hybrid kernels===
{{Main|Hybrid kernel}}

Hybrid kernels are a compromise between the monolithic and microkernel designs. This implies running some services (such as the [[network stack]] or the [[filesystem]]) in kernel space to reduce the performance overhead{{Citation needed|date=July 2007}} of a traditional microkernel, but still running kernel code (such as device drivers) as servers in user space.

===Nanokernels===
{{Main|Nanokernel}}

A nanokernel delegates virtually all services — including even the most basic ones like [[Programmable Interrupt Controller|interrupt controllers]] or the [[timer]] — to [[device driver]]s to make the kernel memory requirement even smaller than a traditional microkernel.<ref>[http://www.cis.upenn.edu/~KeyKOS/NanoKernel/NanoKernel.html KeyKOS Nanokernel Architecture]</ref>

===Exokernels===
{{Main|Exokernel}}

An exokernel is a type of kernel that does not abstract hardware into theoretical models. Instead it allocates physical hardware resources, such as processor time, memory pages, and disk blocks, to different programs. A program running on an exokernel can link to a ''library operating system'' that uses the exokernel to simulate the abstractions of a well-known OS, or it can develop application-specific abstractions for better performance.<ref>[http://pdos.csail.mit.edu/exo.html MIT Exokernel Operating System]</ref>

==History of kernel development==
===Early operating system kernels===
{{Main|History of operating systems}}

Strictly speaking, an operating system (and thus, a kernel) is not ''required'' to run a computer. Programs can be directly loaded and executed on the "bare metal" machine, provided that the authors of those programs are willing to work without any hardware abstraction or operating system support. Most early computers operated this way during the 1950s and early 1960s, which were reset and reloaded between the execution of different programs. Eventually, small ancillary programs such as [[program loader]]s and [[debugger]]s were left in memory between runs, or loaded from [[Read-only memory|ROM]]. As these were developed, they formed the basis of what became early operating system kernels. The [[Bare metal|"bare metal"]] approach is still used today on some [[video game console]]s and [[embedded system]]s, but in general, newer computers use modern operating systems and kernels.

In 1969 the [[RC 4000 Multiprogramming System]] introduced the system design philosophy of a small nucleus "upon which operating systems for different purposes could be built in an orderly manner",<ref name="Hansen2001RC4k">Hansen 2001 (os), pp.17–18</ref> what would be called the microkernel approach.

===Time-sharing operating systems===
{{Main|Time-sharing}}

In the decade preceding [[Unix]], computers had grown enormously in power — to the point where computer operators were looking for new ways to get people to use the spare time on their machines. One of the major developments during this era was [[time-sharing]], whereby a number of users would get small slices of computer time, at a rate at which it appeared they were each connected to their own, slower, machine.<ref>[http://cm.bell-labs.com/cm/cs/who/dmr/cacm.html BSTJ version of C.ACM Unix paper]</ref>

The development of time-sharing systems led to a number of problems. One was that users, particularly at universities where the systems were being developed, seemed to want to [[Hacker (computer security)|hack]] the system to get more [[Central processing unit|CPU]] time. For this reason, [[Computer security|security]] and [[access control]] became a major focus of the [[Multics]] project in 1965.<ref>[http://www.multicians.org/fjcc1.html Introduction and Overview of the Multics System], by F. J. Corbató and V. A. Vissotsky.</ref> Another ongoing issue was properly handling computing resources: users spent most of their time staring at the screen and thinking instead of actually using the resources of the computer, and a time-sharing system should give the CPU time to an active user during these periods. Finally, the systems typically offered a [[memory hierarchy]] several layers deep, and partitioning this expensive resource led to major developments in [[virtual memory]] systems

===Amiga===
{{Main|AmigaOS}}

The [[Commodore International|Commodore]] [[Amiga]] was released in 1985, and was among the first (and certainly most successful) home computers to feature a microkernel operating system.  The Amiga's kernel, ''exec.library'', was small but capable, providing fast pre-emptive multitasking on hardware similar to the cooperatively-multitasked Apple Macintosh, and an advanced [[dynamic linking]] system that allowed for easy expansion.<ref>{{cite web|
url=http://www.atarimagazines.com/creative/v11n9/34_What_makes_it_so_great.php|
accessdate=2006-02-05|
title=What makes it so great! (Commodore Amiga)|
author=Sheldon Leemon|
publisher=Creative Computing}}</ref>

===Unix===
{{Main|Unix}}
[[Image:Unix-history.svg|300px|thumb|A diagram of the predecessor/successor family relationship for [[Unix-like]] systems.]]

During the design phase of [[Unix]], programmers decided to model every high-level [[Device file|device as a file]], because they believed the purpose of computation was data transformation.<ref name="unix">[http://www.unix.org/what_is_unix/single_unix_specification.html The UNIX System — The Single Unix Specification]</ref> For instance, printers were represented as a "file" at a known location — when data was copied to the file, it printed out. Other systems, to provide a similar functionality, tended to virtualize devices at a lower level — that is, both devices ''and'' files would be instances of some lower level concept. Virtualizing the system at the file level allowed users to manipulate the entire system using their existing file management utilities and concepts, dramatically simplifying operation. As an extension of the same paradigm, Unix allows programmers to manipulate files using a series of small programs, using the concept of [[pipeline (Unix)|pipes]], which allowed users to complete operations in stages, feeding a file through a chain of single-purpose tools. Although the end result was the same, using smaller programs in this way dramatically increased flexibility as well as ease of development and use, allowing the user to modify their workflow by adding or removing a program from the chain.

In the Unix model, the ''Operating System'' consists of two parts; first, the huge collection of utility programs that drive most operations, the other the kernel that runs the programs.<ref name="unix"/> Under Unix, from a programming standpoint, the distinction between the two is fairly thin; the kernel is a program, running in supervisor mode,<ref name="supervisor"/> that acts as a program loader and supervisor for the small utility programs making up the rest of the system, and to provide [[lock (software engineering)|locking]] and [[Input/output|I/O]] services for these programs; beyond that, the kernel didn't intervene at all in [[user space]].

Over the years the computing model changed, and Unix's treatment of everything as a file or byte stream no longer was as universally applicable as it was before. Although a [[computer terminal|terminal]] could be treated as a file or a byte stream, which is printed to or read from, the same did not seem to be true for a [[graphical user interface]]. [[Computer networking|Networking]] posed another problem. Even if network communication can be compared to file access, the low-level packet-oriented architecture dealt with discrete chunks of data and not with whole files. As the capability of computers grew, Unix became increasingly cluttered with code. It is also because the modularity of the Unix kernel is extensively scalable.<ref>[http://www.asymco.com/2010/09/29/unixs-revenge/ Unix’s Revenge by Horace Dediu]</ref> While kernels might have had 100,000 [[Source lines of code|lines of code]] in the seventies and eighties, kernels of modern Unix successors like [[Linux kernel|Linux]] have more than 13 million lines.<ref>[http://www.dwheeler.com/essays/linux-kernel-cost.html Linux Kernel 2.6: It's Worth More!], by David A. Wheeler, October 12, 2004</ref>

Modern Unix-derivatives are generally based on module-loading monolithic kernels. Examples of this are the [[Linux]] kernel in its many [[Linux distributions|distributions]] as well as the [[Berkeley software distribution]] variant kernels such as [[FreeBSD]], [[DragonflyBSD]], [[OpenBSD]] and [[NetBSD]]. Apart from these alternatives, amateur developers maintain an active [[Operating system development|operating system development community]], populated by self-written hobby kernels which mostly end up sharing many features with Linux, FreeBSD, DragonflyBSD, OpenBSD or NetBSD kernels and/or being compatible with them.<ref>This community mostly gathers at [http://www.osdever.net Bona Fide OS Development],  [http://www.mega-tokyo.com/forum The Mega-Tokyo Message Board] and other operating system enthusiast web sites.</ref>

===Mac OS===
{{Main|History of Mac OS}}

[[Apple Computer]] first launched [[Mac OS]] in 1984, bundled with its [[Apple Macintosh]] [[personal computer]]. For the first few releases, Mac OS (or System Software, as it was called) lacked many essential features, such as multitasking and a hierarchical filesystem. With time, the OS evolved and eventually became Mac OS 9 and had many new features added, but the kernel basically stayed the same.{{Citation needed|date=October 2007}} Against this, [[Mac OS X]] is based on [[Darwin (operating system)|Darwin]], which uses a hybrid kernel called [[XNU]], which was created combining the [[BSD#4;SD|4.3BSD]] kernel and the [[Mach (kernel)|Mach kernel]].<ref>[http://www.kernelthread.com/mac/osx/arch_xnu.html XNU: The Kernel]</ref>

===Microsoft Windows===
{{Main|History of Microsoft Windows}}

[[Microsoft Windows]] was first released in 1985 as an add-on to [[MS-DOS]].  Because of its dependence on another operating system, initial releases of Windows, prior to Windows 95, were considered an [[operating environment]] (not to be confused with an  [[operating system]]).  This product line continued to evolve through the 1980s and 1990s, culminating with release of the [[Windows 9x]] series (upgrading the system's capabilities to 32-bit addressing and pre-emptive multitasking) through the mid 1990s and ending with the release of [[Windows Me]] in 2000. Microsoft also developed [[Windows NT]], an operating system intended for high-end and business users. This line started with the release of [[Windows NT 3.1]] in 1993, and has continued through the years of 2000 with [[Windows 7]] and [[Windows Server 2008]].

The release of [[Windows XP]] in October 2001 brought these two product lines together, with the intent of combining the stability of the NT kernel with consumer features from the 9x series.<ref>[http://www.linuxworld.com.au/index.php/id;940707233;fp;2;fpid;1 LinuxWorld IDC: Consolidation of Windows won't happen]</ref> The [[architecture of Windows NT]]'s kernel is considered a hybrid kernel because the kernel itself contains tasks such as the Window Manager and the IPC Manager, but several subsystems run in user mode.<ref>[http://www.microsoft.com/windows/WinHistoryDesktop.mspx Windows History: Windows Desktop Products History]</ref>  The precise breakdown of user mode and kernel mode components has changed from release to release, but with the introduction of the [[User Mode Driver Framework]] in Windows Vista, and user-mode thread scheduling in [[Windows 7]],<ref>{{cite web
|url=http://www.osnews.com/story/20931/Windows_7_Gets_User_Mode_Scheduling
|title=Windows 7 Gets User Mode Scheduling
|last=Holwerda
|first=Thom
|date=February 7, 2009
|accessdate=2009-02-28
|publisher=OSNews
}}</ref> have brought more kernel-mode functionality into user-mode processes.

===Development of microkernels===
Although [[Mach (kernel)|Mach]], developed at [[Carnegie Mellon University]] from 1985 to 1994, is the best-known general-purpose microkernel, other microkernels have been developed with more specific aims. The [[L4 microkernel family]] (mainly the L3 and the L4 kernel) was created to demonstrate that microkernels are not necessarily slow.<ref name="l4"/> Newer implementations such as [[Fiasco (L4 clone)|Fiasco]] and [[Pistachio (L4 clone)|Pistachio]] are able to run [[Linux]] next to other L4 processes in separate address spaces.<ref>[http://os.inf.tu-dresden.de/fiasco/overview.html The Fiasco microkernel – Overview]</ref><ref>[http://www.l4ka.org L4Ka – The L4 microkernel family and friends]</ref>

[[QNX]] is a [[real-time operating system]] with a minimalistic microkernel design that has been developed since 1982, having been far more successful than Mach in achieving the goals of the microkernel paradigm.<ref>[http://www.qnx.com/products/rtos/microkernel.html QNX Realtime Operating System Overview]</ref> It is principally used in [[embedded system]]s and in situations where software is not allowed to fail, such as the robotic arms on the [[space shuttle]] and machines that control grinding of glass to extremely fine tolerances, where a tiny mistake may cost hundreds of thousands of dollars.