Mastering Core OS Concepts: Processes, Memory, Files, I/O, and Protection

Process

An operating system’s most universal concept is the process, the dynamic execution instance of a program. Each process has its own address space, registers, and resources.

Program is static code; process is the executing instance. One program can run in multiple processes; a process can execute multiple programs. Program persists on storage; process exists only while running. Process is the OS’s scheduling unit; program resides above the OS.

Process state models

Three‑state model : Ready (all required resources allocated, waiting for CPU), Running (currently executing), Blocked (waiting for an event such as I/O and releases the CPU).

Five‑state model adds Created and Terminated. In the Created state the OS builds the Process Control Block ( PCB) and allocates an address space before moving to Ready. The Terminated state performs resource cleanup and memory reclamation.

PCB: data structure storing a process’s management and control information.

Seven‑state model further adds Suspended‑Ready (process swapped out of main memory but ready) and Suspended‑Blocked (blocked process swapped out).

Address Space

Each process owns an address space that holds its executable code, data, heap, and stack. Modern OSes allow many concurrent address spaces, protected by hardware mechanisms.

If a process’s address space exceeds physical memory, the OS uses virtual memory to map virtual addresses to physical pages, allowing programs to perceive a larger, contiguous memory region.

File System

File systems abstract storage devices and provide a uniform interface for creating, reading, writing, and deleting files. Directories form a hierarchical tree, enabling path‑based navigation.

Mounting attaches an external file system (e.g., a CD‑ROM) to a directory in the existing hierarchy, making its contents accessible under that path.

Special files

Block special files model devices with random‑access blocks (e.g., disks). Character special files model stream‑oriented devices (e.g., keyboards, printers). Pipes are pseudo‑files used for inter‑process communication.

I/O Subsystem

The OS classifies devices into three categories: character device – handles data as a stream of characters (e.g., terminals, printers). block device – transfers data in fixed‑size blocks (e.g., disks, tapes). network device – enables communication over networks (e.g., NICs).

Protection

Protection mechanisms control access to system resources. File permissions are expressed with a 9‑bit code (owner, group, others), each field containing r (read), w (write), and x (execute) bits, e.g., rwxr-x--x.

Virtual Memory

Virtual memory decouples a process’s logical address space from physical RAM using paging and swapping. It provides each process with an isolated, contiguous address space, improves memory utilization, and simplifies memory management.