Computer Science Operating Systems Concepts

An operating system (OS) is system software that manages computer hardware, software resources, and provides common services for computer programs. It acts as an intermediary between users and the computer hardware, facilitating the execution of applications.

• The main functions of an OS include:
  • Process Management: Handling the creation, scheduling, and termination of processes.
  • Memory Management: Managing the allocation and deallocation of memory space.
  • File System Management: Organizing, storing, retrieving, naming, and protecting files.
  • Device Management: Controlling and managing input/output devices.
  • Security and Access Control: Protecting data and resources from unauthorized access.

• Process: An independent program in execution with its own memory space.
• Thread: A smaller unit of a process that can run concurrently with other threads within the same process. Threads share the same memory space but have their own registers and stack.

The different states of a process include:

• New: The process is being created.
• Ready: The process is waiting to be assigned to a CPU.
• Running: The process is currently being executed.
• Waiting: The process is waiting for some event to occur (such as I/O completion).
• Terminated: The process has finished execution.

• First-Come, First-Served (FCFS): Processes are scheduled in the order they arrive.
• Shortest Job Next (SJN): Processes with the shortest execution time are scheduled first.
• Round Robin (RR): Each process is assigned a fixed time slot (quantum) in a cyclic order.
• Priority Scheduling: Processes are scheduled based on priority.
• Multilevel Queue Scheduling: Processes are divided into multiple queues based on different criteria, and each queue has its own scheduling algorithm.

Virtual memory is a memory management technique that gives an application the impression it has contiguous working memory, while in reality, it may be fragmented and span across multiple physical memory locations. This is achieved using paging or segmentation.

• Paging: Memory is divided into fixed-size pages. Each process is divided into pages of the same size, which are mapped to physical memory frames.
• Segmentation: Memory is divided into variable-sized segments based on the logical divisions of a program. Each segment is mapped to a physical memory address.

A page fault occurs when a program tries to access a page that is not currently in physical memory (RAM). The operating system must then fetch the page from secondary storage (such as a hard disk) into physical memory.

A file system is a method and data structure that the operating system uses to control how data is stored, organized, and retrieved on a storage device. It manages files and directories, providing a way for users and applications to store and retrieve data efficiently.