Operating Systems
What is an Operating System?
The concept of an operating system as primarily providing abstractions to application programs is a top-down view. An alternative, bottom-up, view holds that the operating system is there to manage all the pieces of a complex system. Modern computers consist of processors, memories, timers, disks, mice, network interfaces, printers, touch screens, touch pad, and a wide variety of other devices. In the bottom-up view, the job of the operating system is to provide for an orderly and controlled allocation of the processors, memories, and I/O devices among the various programs wanting them.
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Resource management includes multiplexing (sharing) resources in two different ways: in time and in space. When a resource is time multiplexed, different programs or users take turns using it. (…). Determining how the resource is time multiplexed - who goes next and for how long - is the task of the operating system. (…)
The other kind of multiplexing is space multiplexing. Instead of the customers taking turns, each one gets part of the resource. (…). Assuming there is enough memory to hold multiple programs, it is more efficient to hold several programs in memory at once rather than give one of them the entire mem, especially if it only needs a small fraction of the total. Of course, this raises issues of fairness, protection, and so forth, and it is up to the operating system to solve them.
Andrew S. Tanenbaum and Herbert Bos define an Operating System through three distinct lenses:
- Top-Down View (Extended Machine): Provides application programmers with clean abstractions (e.g., processes, threads, files) rather than raw hardware interface details.
- Bottom-Up View (Resource Manager): Manages CPU time, memory space, and I/O devices across competing programs via time- and space-multiplexing.
- Hardware Boundary (Kernel Mode): The privileged software execution layer that runs in supervisor mode, distinct from user-mode software like compilers or shells.