In this chapter, we examined what an operating system is, how it works, and the role it plays in running application programs on the computer. As the system software layer between the computer hardware and application programs, the OS efficiently manages the computer hardware and implements abstractions that make the computer easier to use. Operating systems implement two abstractions, processes and virtual memory, to support multiprogramming (allowing more than one program running on the computer system at a time). The OS keeps track of all the processes in the system and their state, and it implements context switching of processes running on the CPU cores. The OS also provides a way for processes to create new processes, to exit, and to communicate with one another. Through virtual memory, the OS implements the abstraction of a private virtual memory space for each process. The virtual memory abstraction protects processes from seeing the effects of other processes sharing the computer’s physical memory space. Paging is one implementation of virtual memory that maps individual pages of each process’s virtual address space to frames of physical RAM space. Virtual memory is also a way in which the OS makes more efficient use of RAM; by treating RAM as a cache for disk, it allows pages of virtual memory space to be stored in RAM or on disk.

Our focus in this chapter on the operating system’s role in running a program, including the abstractions and mechanisms it implements to efficiently run programs, is in no way complete. There are many other implementation options and details and policy issues related to processes and process management, and to virtual memory and memory management. Additionally, operating systems implement many other important abstractions, functionality, and policies for managing and using the computer. For example, the OS implements filesystem abstractions for accessing stored data, protection mechanisms and security policies to protect users and the system, and scheduling policies for different OS and hardware resources.

Modern operating systems also implement support for interprocess communication, networking, and parallel and distributed computing. In addition, most operating systems include hypervisor support, which virtualizes the system hardware and allows the host OS to run multiple virtual guest operating systems. Virtualization supports the host OS that manages the computer hardware in booting and running multiple other operating systems on top of itself, each with its own private virtualized view of the underlying hardware. The host operating system’s hypervisor support manages the virtualization, including protection and sharing of the underlying physical resources among the guest operating systems.

Finally, most operating systems provide some degree of extensibility by which a user (often a system administrator) can tune the OS. For example, most Unix-like systems allow users (usually requiring root, or superuser, privileges) to change sizes of OS buffers, caches, swap partitions, and to select from a set of different scheduling policies in OS subsystems and hardware devices. Through these modifications, a user can tune the system for the type of application workloads they run. These types of operating systems often support loadable kernel modules, which are executable code that can be loaded into the kernel and run in kernel mode. Loadable kernel modules are often used to add additional abstractions or functionality into the kernel as well as for loading device driver code into the kernel that is used to handle managing a particular hardware device.

For more breadth and depth of coverage of operating systems, we recommend reading an operating systems textbook, such as "Operating Systems: Three Easy Pieces"¹.