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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.
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.
Service definitions, like the OSI model, abstractly describe the functionality provided to a layer N by a layer N−1, where N is one of the seven layers of protocols operating in the local host. At each level N , two entities at the communicating devices (layer N peers ) exchange protocol data units (PDUs) by means of a layer N protocol .
Key APIs from the Macintosh Toolbox would be implemented in Mac OS X to run directly on the BSD layers of the operating system instead of in the emulated Macintosh layer. This modified interface, called Carbon , would eliminate approximately 2000 troublesome API calls (of about 8000 total) and replace them with calls compatible with a modern OS.
Layer 1 was concerned with allocating memory to processes. In modern terms, this was the pager. Layer 2 dealt with communication between the operating system and the system console. Layer 3 managed all I/O between the devices attached to the computer. This included buffering information from the various devices. Layer 4 consisted of user programs.
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.
A virtualization layer underneath an operating system, which is more correctly referred to as a hypervisor. A hardware abstraction layer that forms the lowest-level part of a kernel, sometimes used to provide real-time functionality to normal operating systems, like Adeos .
A Commentary on the Sixth Edition UNIX Operating System by John Lions (later reissued as Lions' Commentary on UNIX 6th Edition [1] [2] and commonly referred to as the Lions Book) is a highly influential [3] 1976 publication containing analytical commentary on the source code of the 6th Edition Unix computer operating system "resident nucleus" [4] (i.e., kernel) software, plus copy formatted ...