CS-150 Operating Systems – Vocabulary Flashcards

Operating System Basics

• Operating System (OS): core system software managing hardware & providing user/application interface.
• Application software ⟂ System software (OS + utilities).

Key Roles / Services

• Resource management: CPU, memory, I/O, files.
• User interface (CLI/GUI).
• Program execution & process control.
• Security, authentication, access control.
• Error detection, accounting, part of booting.

OS Types

• Single‐tasking vs multi‐tasking.
• Single‐user vs multi‐user.
• Distributed, Embedded, Real-time.

Process Management

• Process = program in execution.
• OS tracks processes via Process Control Block (PCB): \text{PC},\;\text{registers},\;\text{state},\;\text{priority},\;\text{memory info},\;\text{accounting}.
• States: New → Ready → Running → (Blocked/Suspended) → Terminated.
• Creation: by OS (services) or parent (child processes).
• Termination triggers: normal exit, user log-off, error, parent/OS shutdown.
• Suspension: waiting for resource, debugging, periodic tasks, resource reallocation.
• Context switch (expensive): save current registers to PCB, load next PCB.

CPU Scheduling

• Goal: select ready process for CPU.
• Non-pre-emptive: running process yields CPU.
• Pre-emptive: OS can forcibly switch.

Algorithms

• First-Come First-Served (FCFS).
• Shortest Job Next (SJN) – optimal but needs future knowledge.
• Round Robin – time slice q (e.g., 100 ms).

Evaluation Metrics

• Turnaround time =\text{finish} - \text{arrival}.
• % Delay =\dfrac{\text{turnaround} - \text{service}}{\text{service}} \times 100.
• Processor utilisation = \dfrac{\text{CPU busy time}}{\text{total time}} \times 100.

Memory Management

• Logical address: offset within process space.
• Physical address: actual RAM location.
• OS duties: isolation, allocation/deallocation, protection, long-term storage.

Single Contiguous

• Only OS + one user program; simple, used in tiny embedded systems.
• Address binding: physical = \text{base} + \text{logical}.

Partition Memory Management

• Multiple concurrent processes + OS.

Schemes

• Fixed partitions: pre-set sizes.
• Dynamic partitions: created on demand.

Registers

• Base: start address of current partition.
• Limit: length of current partition.

Allocation Algorithms (fixed)

• Best Fit – smallest sufficient hole.
• First Fit – first sufficient hole.
• Next Fit – continue search from last placement.
• Worst Fit – largest hole (mainly dynamic).

Fragmentation

• Internal: unused space inside allocated partition.
• External: free space scattered into unusable small holes.

Paged Memory Management & Virtual Memory

• Break process into equal-size pages; RAM divided into frames (same size).
• Pages can load into any frame ⇒ eliminates external fragmentation.
• Page Map Table (PMT) maps page → frame.
• Address translation: logical → (page, offset) → frame + offset.
• Demand paging: load page only when referenced; may cause page swap.
• Virtual memory illusion: program larger than RAM.
• Thrashing: excessive page swapping, low CPU efficiency.

File Systems

• File: named collection of related data; text vs binary.
• File system: OS view + operations (create, read, write, delete, copy, rename, permission, metadata).
• File type/format often indicated by extension (e.g., .txt, .mp3, .class).
• Access modes: Sequential vs Direct (random).
• Protection: define who (owner, group, others) & what (read/write/execute) can access.

Directory Trees & Paths

• Directory: named group of files; can nest ⇒ tree hierarchy.
• Root directory at top; parent "..", self "." references.
• Working directory: current location in tree.
• Path: route to file/dir.
  • Absolute: starts at root (e.g., /home/smith/reports/week2.txt).
  • Relative: starts at working dir (e.g., ../smith/reports/week2.txt).