The Windows NT design included many of the same objectives as Mach, the archetypal microkernel system, one of the most important being its structure as a collection of modules that communicate via well-known interfaces, with a small microkernel limited to core functions such as first-level interrupt handling, thread scheduling and synchronization primitives. This allows for the possibility of using either direct procedure calls or interprocess communication (IPC) to communicate between modules, and hence for the potential location of modules in different address spaces (for example in either kernel space or server processes). Other design goals shared with Mach included support for diverse architectures, a kernel with abstractions general enough to allow multiple operating system personalities to be implemented on top of it and an object-oriented organisation.
The reason NT is not a micro-kernel system is that nearly all of the subsystems providing system services, including the entire Executive, run in kernel mode (in the same address space as the microkernel itself), rather than in user-mode server processes, as would be the case with a microkernel design. This is an attribute NT shares with early versions of Mach, as well as all commercial systems based on Mach, and stems from the superior performance offered by using direct procedure calls in a single memory space, rather than IPC, for communication amongst subsystems.
In describing NT, the list of which subsystems do not run in kernel mode is far shorter than the list of those that do. The user-mode subsystems on NT include one or more emulation subsystems, each of which provides an operating system personality to applications, the Session Manager Subsystem (smss.exe), which starts the emulation subsystems during system startup and the Local Security Authority Subsystem Service (lsass.exe), which enforces security on the system. The subsystems are not written to a particular OS personality, but rather to the native NT API (or Native API).
The primary operating system personality on Windows is the Windows API, which is always present. The emulation subsystem which implements the Windows personality is called the Client/Server Runtime Subsystem (csrss.exe). On versions of NT prior to 4.0, this subsystem process also contained the window manager, graphics device interface and graphics device drivers. For performance reasons, however, in version 4.0 and later, these modules (which are often implemented in user mode even on monolithic systems, especially those designed without internal graphics support) run as a kernel-mode subsystem.
As of 2007, one other operating system personality, UNIX, is offered as an optionally installed system component on certain versions of Windows Vista and Windows Server 2003 R2. The associated subsystem process is the Subsystem for UNIX-Based Applications (psxss.exe), which was formerly part of a Windows add-on called Windows Services for UNIX. An OS/2 subsystem (os2ss.exe) was supported in older versions of Windows NT, as was a very limited POSIX subsystem (psxss.exe). The POSIX subsystem was supplanted by the UNIX subsystem, hence the identical executable name.
Applications that run on NT are written to one of the OS personalities (usually the Windows API), and not to the native NT API for which documentation is not publicly available (with the exception of routines used in device driver development). An OS personality is implemented via a set of user-mode DLLs (see Dynamic-link library), which are mapped into application processes' address spaces as required, together with an emulation subsystem server process (as described previously). Applications access system services by calling into the OS personality DLLs mapped into their address spaces, which in turn call into the NT run-time library (ntdll.dll), also mapped into the process address space. The NT run-time library services these requests by trapping into kernel mode to either call kernel-mode Executive routines or make Local Procedure Calls (LPCs) to the appropriate user-mode subsystem server processes, which in turn use the NT API to communicate with application processes, the kernel-mode subsystems and each other.
Other articles related to "description":
... creating a dominant impression, using descriptive language, and organizing the description are the rhetorical choices to be considered when using a description ... A description is usually arranged spatially but can also be chronological or emphatic ... The focus of a description is the scene ...
... Universal Description, Discovery and Integration (UDDI, pronounced Yu-diː) is a platform-independent, Extensible Markup Language (XML)-based registry by ...
... Unlike the keywords attribute, the description attribute is supported by most major search engines, like Yahoo! and Bing, while Google will fall back on this tag when ... The description attribute provides a concise explanation of a Web page's content ... This allows the Web page authors to give a more meaningful description for listings than might be displayed if the search engine was unable to automatically create its own ...
... He gives a vivid and accurate description of the last colony of the European Beaver in Wales on the River Teifi, but spoils it by repeating the legend that beavers castrate themselves to avoid danger ... Likewise he gives a good description of an Osprey fishing, but adds the mythical detail that the bird has one webbed foot ... His description of Irish wildlife was harshly called "worthless" the better view perhaps is that despite its faults it gives a valuable glimpse of Irish fauna in the 1180s ...
Famous quotes containing the word description:
“Whose are the truly labored sentences? From the weak and flimsy periods of the politician and literary man, we are glad to turn even to the description of work, the simple record of the months labor in the farmers almanac, to restore our tone and spirits.”
—Henry David Thoreau (18171862)
“Everything to which we concede existence is a posit from the standpoint of a description of the theory-building process, and simultaneously real from the standpoint of the theory that is being built. Nor let us look down on the standpoint of the theory as make-believe; for we can never do better than occupy the standpoint of some theory or other, the best we can muster at the time.”
—Willard Van Orman Quine (b. 1908)
“A sound mind in a sound body, is a short, but full description of a happy state in this World: he that has these two, has little more to wish for; and he that wants either of them, will be little the better for anything else.”
—John Locke (16321704)