Notes based on Operating Systems: Three Easy Pieces by Remzi & Andrea Arpaci-Dusseau
Introduction to OS
An Operating System (OS) is a body of software sitting in between software applications and a Von Neumann computer architecture. An OS connects applications to physical hardware. It makes abstractions of the underlying hardware and provides an easy-to-use interface for running portable software on physical hardware.
Abstraction is a great idea in both computer architecture and operating systems. It hides implementation complexity about the underlying layer and exposes a unified model of how to use the underlying layer to the upper layer. Check out the first section of this note.
In the layers of abstractions, we call the operations supported by the lower-layer as mechanisms, and the algorithms/decisions made by the higher-layer on how to use the mechanisms to achieve a goal as policies (or disciplines).
An OS provides support in at least these three general aspects:
Virtualization: taking a possibly limited physical resource (processor, memory, storage, …) and transforms it into a more general, portable, and easy-to-use virtual interface for user applications to use Concurrency: acting as a resource manager which supports multiple user applications (and multiple tasks inside one application) to run concurrently and coordinates among running entities, ensuring correct, fair, and efficient sharing of resources Persistence: data can be easily lost on volatile devices such as DRAM. An OS allows users to talk to external devices - including persistent storage drives - through Input/Output (I/O) A modern operating system also pursues some other goals apart from the above stated. They are:
Performance: minimize the overheads brought by the OS Scalability: with multiprocessors, speed up concurrent accesses Security: protect against bad behavior; provide isolation Reliability: properly recover on fail-stops Connectivity: networking support; connect with the Internet Energy-efficiency, Mobility, … Generally, the most essential part of an OS is the kernel, which is the collection of code that implements the above-mentioned core functionalities. The kernel is typically a piece of static code (hence not a dynamic running entity). A monolithic OS consists of the kernel and some upper-level system applications that provide a more friendly UI. The OS field also covers part of the hardware architecture and external devices.