Fundamental Concepts of Operating Systems and Computer Architecture

Operating System (OS) : Manages computer hardware and software resources, handling memory allocation, process scheduling, I/O control, file systems, and providing user interfaces.

Shell : A program that receives commands from the keyboard and passes them to the OS for execution; historically the primary user interface on Unix-like systems.

GUI (Graphical User Interface) : Allows users to interact with the system via graphical icons and visual indicators instead of text commands.

Kernel mode (supervisor mode) : Executes with unrestricted access to hardware, enabling privileged instructions and direct memory access; failures in this mode can crash the entire system.

User mode : Runs regular applications with limited privileges; transitions to kernel mode occur via system calls or interrupts.

Computer architecture : Defines the functional organization of a computer system, including instruction set, memory management, I/O, and bus structures.

SATA (Serial ATA) : A bus standard for data transfer between the motherboard and storage devices such as hard drives and optical drives.

Multiplexing : Shares limited resources (time or space) among multiple programs or users, allowing them to use the resource in turn.

Mainframe : Large, high‑capacity computers known for reliability and used by enterprises for critical data processing.

Batch system : Executes jobs submitted on offline media without direct user interaction, grouping similar tasks for efficiency.

OS/360 : IBM’s historic batch operating system for the System/360 mainframe, now discontinued.

Multitasking : Enables a computer to run multiple programs concurrently by interleaving execution slices.

Time‑sharing : Allows many users to share a computer simultaneously by rapidly switching between tasks.

Compatible Time‑Sharing System (CTSS) : One of the earliest time‑sharing operating systems, developed at MIT.

Cloud computing : Provides on‑demand access to computing resources (storage, processing) over the Internet without direct management by the user.

UNIX : A powerful, multi‑user, multi‑tasking OS supporting many processor architectures.

UNIX System V : A major UNIX variant that introduced many standard system interfaces.

BSD (Berkeley Software Distribution) : A Unix‑derived operating system family.

POSIX : A set of standards defining portable OS interfaces for Unix‑like systems.

MINIX : A minimal Unix‑like OS used for educational purposes.

Linux : A widely used, open‑source Unix‑like operating system kernel.

DOS (Disk Operating System) : Early operating systems that manage disk storage and provide basic file handling.

MS‑DOS : Microsoft’s dominant DOS variant for IBM‑compatible PCs before Windows.

macOS X : Apple’s graphical operating system for Macintosh computers.

Windows NT : Microsoft’s 32‑bit, fully preemptive kernel architecture.

Service Pack (SP) : A collection of updates, fixes, and enhancements distributed as a single installable package.

DRM (Digital Rights Management) : Technologies that restrict the use and distribution of copyrighted content.

x86 : An instruction‑set architecture originally developed by Intel, supporting 16‑, 32‑, and 64‑bit extensions.

FreeBSD : An open‑source Unix‑like operating system derived from BSD.

X Window System : A network‑transparent windowing system for bitmap displays on Unix‑like OSes.

GNOME : A free desktop environment built on top of the X Window System.

Network Operating System (NOS) : Specialized OSes for network devices such as routers and switches.

Distributed Operating System : Software that makes a collection of independent computers appear as a single coherent system.

Program Counter : A CPU register that holds the address of the next instruction to execute.

Stack Pointer : A register that points to the top of the current call stack.

Program Status Word (PSW) : Holds the current state of the processor, including condition codes and interrupt flags.

Pipeline : A technique where multiple instruction processing stages operate in parallel, improving throughput.

Superscalar : CPU architecture that can issue multiple instructions per clock cycle.

System Call : An interface that allows user‑mode programs to request services from the kernel.

Multithreading : Executes multiple threads of execution concurrently within a single process.

CPU Core : An independent processing unit within a CPU that can execute instructions.

GPU (Graphics Processing Unit) : A specialized processor for handling graphics and parallel compute workloads.

Cache hierarchy : Includes L1, L2, and L3 caches that store frequently accessed data to reduce memory latency.

RAM (Random Access Memory) : Volatile main memory used for temporary storage of program data and instructions.

ROM (Read‑Only Memory) : Non‑volatile memory that retains data without power, typically used for firmware.

EEPROM : Electrically erasable programmable read‑only memory that can be rewritten multiple times.

Flash memory : Non‑volatile storage used in SSDs, USB drives, and memory cards.

SSD (Solid State Drive) : Storage device that uses flash memory for fast, durable data access.

Virtual memory : An abstraction that gives processes the illusion of a large, contiguous address space, backed by physical RAM and disk storage.

MMU (Memory Management Unit) : Hardware that translates virtual addresses to physical addresses and enforces protection.

Context switch : Saves the state of a running process and restores the state of another, enabling multitasking.

Device driver : Software that mediates communication between the OS and hardware devices.

Busy waiting (spin) : A technique where a process repeatedly checks a condition, consuming CPU cycles.

Interrupt : An asynchronous signal that causes the CPU to pause current execution and handle an event.

Interrupt vector : A table that maps interrupt requests to their corresponding handler addresses.

DMA (Direct Memory Access) : Allows hardware subsystems to transfer data directly to/from memory without CPU intervention.

Bus : A communication system that transfers data between components inside a computer.

PCIe (Peripheral Component Interconnect Express) : A high‑speed serial expansion bus standard.

DMI (Direct Media Interface) : Intel’s proprietary bus linking the northbridge and southbridge on a motherboard.

USB (Universal Serial Bus) : A widely adopted serial bus for connecting peripherals.

BIOS (Basic Input/Output System) : Firmware that initializes hardware during boot and provides low‑level services to the OS.

Hard real‑time system : Guarantees that critical tasks meet strict deadlines.

Soft real‑time system : Allows occasional deadline misses with degraded performance.

Process : An executing instance of a program, containing its own memory space and resources.

Address space : The range of memory addresses a process can use.

Process table : Kernel data structure that tracks all active processes and their attributes.

Command‑line interpreter : A text‑based interface for entering commands, preceding modern GUIs.

Directory : A container that holds files and sub‑directories.

Path name : A string that uniquely identifies the location of a file or directory.

Root directory : The top‑most directory in a file system hierarchy.

Working directory : The current directory in which a user or process operates.

File descriptor : An abstract handle used by programs to access files or I/O resources.

Inode : A data structure in Unix‑like file systems that stores metadata about a file.

Shared library : A library that can be loaded by multiple programs at runtime.

DLL (Dynamic‑Link Library) : Windows’ implementation of shared libraries.

Client and Server : Roles in a client‑server architecture where clients request services from servers.

Client‑server (C/S) architecture : A network model separating requestors (clients) from providers (servers).

Virtual Machine (VM) : Software that emulates a complete hardware environment, allowing multiple OS instances on a single physical machine.

JVM (Java Virtual Machine) : Executes Java bytecode on any platform by abstracting underlying hardware.

Object file : Contains compiled machine code that is not yet linked into an executable.

C preprocessor : Performs macro expansion, file inclusion, and conditional compilation before C/C++ compilation.