Operating Systems & Generations of Computers

A comprehensive set of vocabulary flashcards derived from academic notes on Operating Systems, covering history, process management, memory structures, and security.

Operating System (OS)

A fundamental software that acts as an intermediary between users and computer hardware, managing resources and providing an environment for program execution.

First Generation Computers

Machines from the 1940s to 1956 that used vacuum tubes for switching and amplification, utilized machine language, and lacked an operating system.

Second Generation Computers

Computers (1956–1963) characterized by the use of transistors, magnetic core memory, and the emergence of simple batch processing OS like FMS.

Third Generation Computers

Computers (1964–1971) based on Integrated Circuits (ICs), introducing semiconductor memory, multiprogramming, and time-sharing operating systems.

Fourth Generation Computers

The era (1971–Present) defined by Very Large Scale Integration (VLSI) and microprocessors, leading to personal computers and modern GUI-based operating systems.

Fifth Generation Computers

Modern and future systems (1980s+) focused on Artificial Intelligence, parallel computing, quantum chips, and natural language processing.

Kernel

The core of the OS and the only part that runs in privileged (kernel) mode, having direct access to computer hardware.

Monolithic Kernel

An architecture where all OS services run in kernel space for speed, though it is harder to maintain and prone to full system crashes.

Microkernel

A design that places only essential services in the kernel and runs the rest in user space to improve stability and modularity.

System Call

A programmatic interface used by user-mode applications to request services from the operating system kernel.

Process

An active entity representing a program in execution, consisting of code, a program counter, stack, data section, and heap.

Process Control Block (PCB)

A data structure used by the OS to store all information about a process, including PID, state, CPU registers, and memory limits.

Thread

The smallest unit of CPU execution within a process that shares address space and resources with other threads in the same process.

Turnaround Time (TAT)

The interval from the time of submission of a process to the time of completion, calculated as $\text{TAT} = \text{Completion Time} - \text{Arrival Time}$.

Waiting Time (WT)

The total amount of time a process spends waiting in the ready queue, calculated as $\text{WT} = \text{Turnaround Time} - \text{Burst Time}$.

Round Robin (RR)

A preemptive CPU scheduling algorithm that allocates a fixed time quantum to each process in a cyclic queue.

First Fit

A memory allocation strategy that assigns the first available hole in memory that is large enough to satisfy the request.

Paging

A memory management scheme that eliminates the need for contiguous allocation of physical memory by dividing it into fixed-size blocks.

Virtual Memory

A technique that allows the execution of processes that are not completely in physical memory by using disk space as an extension of RAM.

Page Fault

An event occurring when a process attempts to access a memory page that is not currently loaded in the physical RAM.

Thrashing

A condition where the system spends more time swapping pages in and out of disk than executing instructions, often caused by excessive page faults.

File Allocation Table (FAT)

A file allocation method where a table stores next-block pointers for linked list allocation, allowing O(1) random access.

Deadlock

A state where a set of processes are each waiting for a resource held by another process in the set, preventing any from proceeding.

Coffman's Conditions

The four necessary conditions for deadlock: Mutual Exclusion, Hold and Wait, No Preemption, and Circular Wait.

Banker’s Algorithm

A deadlock avoidance algorithm that simulates resource allocation to determine if granting a request will maintain the system in a safe state.

Mandatory Access Control (MAC)

A security model where the system enforces access policies based on security labels assigned to data and users.

Direct Memory Access (DMA)

An I/O technique where the CPU is bypassed, allowing a controller to transfer large blocks of data directly between a device and memory.

Critical Section

A segment of code that accesses shared resources and must be protected to ensure only one process executes it at a time.

Semaphore

A synchronization tool consisting of an integer counter used to manage concurrent access to resources through wait() and signal() operations.