Kernel - Revision history

Jnc: link microkernel; move ukernel content there

2022-01-29T12:11:46Z

link microkernel; move ukernel content there

Jnc: +cat

2018-12-16T21:18:00Z

+cat

Jnc: Fmt, minor clarification

2018-06-17T13:58:37Z

Fmt, minor clarification

Jnc: An OK start

2018-06-17T13:55:44Z

An OK start

New page

The '''kernel''' is the core of an [[operating system]]; it is usually permanently resident in [[main memory]], and in [[time-sharing]] systems (which almost all OS's now are), it usually uses a special mode in the [[Central Processing Unit|CPU]] which allows some operations (e.g. changing the configuration of the [[virtual memory]] [[hardware]]) which are forbidden to 'ordinary' [[user]] [[program]]s (usually running in [[process]]es).

In simple systems, such as early versions of [[UNIX]], the kernel is a single monolithic program, which is completely loaded when the system is [[bootstrap|booted]].

In more complex systems such as [[Multics]], there are 'layers' within the kernel, so e.g. only some of the kernel might be always resident in main memory (e.g. [[interrupt]] handlers, and the code to handle 'page [[trap|fault]]s' with virtual memory); other parts of the kernel might be 'paged'. Generally, even the higher layers will still have access to operations which ordinary user code is not allowed to use.

A recent trend is OS work is the '''micro-kernel''', where only the most limited functionality is in the kernel; other functionality which is normally considered part of the kernel (e.g. the [[file system]]) might be run in some other environment. These sub-systems are only loaded, and started, once the micro-kernel is loaded and running.

There is still no general agreement on the use of micro-kernels; although they are generally more robust, and have capabilities that the 'classic' '''monolithic kernels''' do not have (e.g. the ability switch to a new version of the file system code), the division usually imposes some overhead, which is unacceptable to some.

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 In more complex systems such as [[Multics]], there are 'layers' within the kernel, so e.g. only some of the kernel might be always resident in main memory (e.g. [[interrupt]] handlers, and the code to handle 'page [[trap|fault]]s' with virtual memory); other parts of the kernel might be 'paged'. Generally, even the higher layers will still have access to operations which ordinary user code is not allowed to use.
-A recent trend is OS work is the '''micro-kernel''', where only the most limited functionality is in the kernel; other functionality which is normally considered part of the kernel (e.g. the [[file system]]) might be run in some other environment. These sub-systems are only loaded, and started, once the micro-kernel is loaded and running.
+A recent trend is OS work is the [[microkernel]], where only the most limited functionality is in the kernel; other functionality which is normally considered part of the kernel (e.g. the [[file system]]) might be run in some other environment. These sub-systems are only loaded, and started, once the microkernel is loaded and running.
 [[Category: OS Basics]]

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	There is still no general agreement on the use of micro-kernels; although they are generally more robust, and have capabilities that the 'classic' '''monolithic kernels''' do not have (e.g. the ability switch to a new version of the file system code while the system is running), the division usually imposes some overhead, which is un-acceptable to some.		There is still no general agreement on the use of micro-kernels; although they are generally more robust, and have capabilities that the 'classic' '''monolithic kernels''' do not have (e.g. the ability switch to a new version of the file system code while the system is running), the division usually imposes some overhead, which is un-acceptable to some.
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		+	[[Category: OS Basics]]

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+The '''kernel''' is the core of an [[operating system]] (in the modern, expansive sense of that term); it is usually permanently resident in [[main memory]], and in [[time-sharing]] systems (which almost all OS's now are), it usually uses a special mode in the [[Central Processing Unit|CPU]] which allows some operations (e.g. changing the configuration of the [[virtual memory]] [[hardware]]) which are forbidden to 'ordinary' [[user]] [[program]]s (usually running in [[process]]es).
 In simple systems, such as early versions of [[UNIX]], the kernel is a single monolithic program, which is completely loaded when the system is [[bootstrap|booted]].
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 A recent trend is OS work is the '''micro-kernel''', where only the most limited functionality is in the kernel; other functionality which is normally considered part of the kernel (e.g. the [[file system]]) might be run in some other environment. These sub-systems are only loaded, and started, once the micro-kernel is loaded and running.
-There is still no general agreement on the use of micro-kernels; although they are generally more robust, and have capabilities that the 'classic' '''monolithic kernels''' do not have (e.g. the ability switch to a new version of the file system code), the division usually imposes some overhead, which is unacceptable to some.
+There is still no general agreement on the use of micro-kernels; although they are generally more robust, and have capabilities that the 'classic' '''monolithic kernels''' do not have (e.g. the ability switch to a new version of the file system code while the system is running), the division usually imposes some overhead, which is un-acceptable to some.