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The operating system provides an interface between an application program and the computer hardware, so that an application program can interact with the hardware only by obeying rules and procedures programmed into the operating system. The operating system is also a set of services which simplify development and execution of application programs.
The application layer is the layer of the OSI model that is closest to the end user, which means both the OSI application layer and the user interact directly with a software application that implements a component of communication between the client and server, such as File Explorer and Microsoft Word.
The Windows NT operating system family's architecture consists of two layers (user mode and kernel mode), with many different modules within both of these layers.. The architecture of Windows NT, a line of operating systems produced and sold by Microsoft, is a layered design that consists of two main components, user mode and kernel mode.
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.
A host operating system kernel could use instructions with full privilege access (kernel mode), whereas applications running on the guest OS in a virtual machine or container could use the lowest level of privileges in user mode.
Modular operating systems such as OS-9 and most modern monolithic-kernel operating systems such as OpenVMS, Linux, FreeBSD, NetBSD, DragonFly BSD, Solaris, and AIX can dynamically load (and unload) executable kernel modules at runtime. This modularity of the operating system is at the binary (image) level and not at the architecture level.
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.
An operating system abstraction layer (OSAL) provides an application programming interface (API) to an abstract operating system making it easier and quicker to develop code for multiple software or hardware platforms. It can make an application less dependent on any one specific operating system.