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An operating system is difficult to define, [6] but has been called "the layer of software that manages a computer's resources for its users and their applications". [7] Operating systems include the software that is always running, called a kernel—but can include other software as well.
The presentation layer handles protocol conversion, data encryption, data decryption, data compression, data decompression, incompatibility of data representation between operating systems, and graphic commands. The presentation layer transforms data into the form that the application layer accepts, to be sent across a network.
Another version of a software stack is operating system, middleware, database, ... mixing both the hardware and software in layers of support. [4] [5] ...
This is the mode in which the operating system usually runs." [14] In a monolithic kernel, the operating system runs in supervisor mode and the applications run in user mode. Other types of operating systems, like those with an exokernel or microkernel, do not necessarily share this behavior. Some examples from the PC world:
This article lists protocols, categorized by the nearest layer in the Open Systems Interconnection model.This list is not exclusive to only the OSI protocol family.Many of these protocols are originally based on the Internet Protocol Suite (TCP/IP) and other models and they often do not fit neatly into OSI layers.
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 . As a result, operating systems can allow programs to use more memory than the system has physically available.
Additional diagram of macOS architecture (2017 update) The architecture of macOS describes the layers of the operating system that is the culmination of Apple Inc.'s decade-long research and development process to replace the classic Mac OS.
The article "Usage share of operating systems" provides a broader, and more general, comparison of operating systems that includes servers, mainframes and supercomputers. Because of the large number and variety of available Linux distributions , they are all grouped under a single entry; see comparison of Linux distributions for a detailed ...