OS as resource manager and extended machine

Operating system as a resource manager

A computer system has many resources: CPU, memory, disks, I/O devices, files and network. Several programs and users demand them at the same time. The OS is the resource manager: it keeps track of the resources, decides who gets what, when and how much, grants and reclaims them, and settles conflicting requests so that resources are used efficiently and fairly.

Resources are shared in two ways:

• Time multiplexing: users take turns using the resource (CPU, printer).
• Space multiplexing: each user gets a part of the resource at the same time (memory, disk).

Example: when two programs print at the same time, the OS queues (spools) the jobs so that their outputs do not mix. Besides allocation, the OS provides protection and accounting. This is the bottom-up view of the OS: it manages the hardware.

Operating system as an extended machine

Raw hardware is complex and difficult to program. The OS hides its details and presents an easier, abstract machine (the extended or virtual machine). Instead of dealing with disk-controller registers, a programmer uses abstractions like files, processes and address spaces through system calls such as open and read. This is the top-down view: it makes the machine convenient and portable for application programmers.

 Application programs
 ---------------------  Extended machine view
 Operating system      (files, processes, sockets)
 ---------------------  Resource manager view
 Hardware               (CPU, memory, disks, I/O)

Therefore the OS has a dual role: it extends the machine for the user and manages the machine's resources.