Untitled Quiz

Course Information

• Course Code: CSE3003
• Subject: Operating System
• Student Name: Saaksham Kindra
• Reg. No.: 23BSA10166
• Slot: A21+A22+A23

Memory Management

• Memory management is the process of managing computer memory, specifically the allocation and deallocation of memory spaces to different programs or processes.
• It is a crucial function in an operating system (OS) to ensure efficient use of memory, maintain system stability, and avoid memory leaks or fragmentation.
• In memory management, the operating system keeps track of every byte in a computer's memory and allocates and deallocates memory as needed by processes.

Memory Management Techniques

• 1. Contiguous Memory Allocation:

  • Each process is allocated a single contiguous block of memory.
  • The operating system maintains a table that tracks which parts of memory are in use and which are free.
  • Types:
    • Fixed Partitioning: The memory is divided into fixed-sized partitions. Each partition can hold one process. If a process is smaller than the partition, the leftover space is wasted.
    • Dynamic Partitioning: Memory is divided dynamically based on the size of the process. Each process is allocated only as much memory as it needs.
  • Advantages: Simple to implement and efficient for small systems with simple memory requirements
  • Disadvantages: Internal fragmentation (wasted space within allocated blocks) and external fragmentation (wasted space outside allocated blocks, making it difficult to find contiguous blocks for large processes).

• 2. Paged Memory Allocation:

  • In this technique, both the physical and logical memory are divided into fixed-size blocks called pages and page frames, respectively.
  • A page table is used to map logical addresses (used by programs) to physical addresses (actual locations in memory).
  • Advantages: Eliminates external fragmentation and allows non-contiguous memory allocation, so processes can be loaded in pieces wherever there is free space.
  • Disadvantages: Internal fragmentation and significant overhead if there are many segments.

• 3. Segmented Memory Allocation:

  • Memory is divided into variable-sized segments, such as a code segment, data segment, and stack segment, based on the logical divisions of a program.
  • Each segment has its own base address and limit, allowing a process to use non-contiguous memory blocks.
  • Advantages: More logical as it mimics how a program is structured and reduces fragmentation compared to contiguous memory allocation.
  • Disadvantages: External fragmentation still occurs and the overhead of translation.

• 4. Virtual Memory Allocation:

  • Virtual memory allows programs to use more memory than is physically available by using disk space as additional memory.
  • This is done through techniques like paging or segmentation, and the OS manages this mapping.
  • Advantages: Larger address space, Isolation and protection, simplified memory management, better multitasking, efficiency through paging.
  • Disadvantages: Performance overhead (disk I/O), increased latency, complexity in implementation, memory fragmentation, and limited by disk space.

Swap Space Management

• In systems using virtual memory, when the physical memory is full, processes are moved (swapped) to a reserved space on the disk known as swap space.

Dynamic Memory Allocation

• Dynamic memory allocation involves allocating memory dynamically at runtime, allowing processes to request memory as needed.
  • Memory is typically managed using malloc (memory allocation) and free (deallocation) functions.
  • Types:
  • First Fit: Allocates the first available block of memory that is large enough.
  • Best Fit: Allocates the smallest available block that is large enough, thereby minimizing wasted space.
  • Worst Fit: Allocates the largest available block, maximizing future allocation flexibility.

Paging and Segmentation

• Paging and segmentation are two different techniques for dividing memory into smaller manageable units.

Virtual Memory

• Virtual memory is a memory management technique that allows programs to use more memory than is physically available in the computer.

Page Fault

• A page fault occurs when a process tries to access a page that isn't loaded into RAM.

Thrashing

• Thrashing is a situation where the system spends more time swapping pages in and out of RAM than executing the actual program.

Page Replacement Algorithms

• There are multiple approaches to handling page replacement during thrashing, including:
  • OPT (Optimal Page Replacement): Selects the page that will not be used for the longest period of time in the future.
  • FIFO (First-In, First-Out): Replaces the page that has been in memory the longest.
  • LRU (Least Recently Used): Replaces the page that has not been used for the longest period of time.

Disk Scheduling

• Disk scheduling algorithms determine the order in which disk I/O requests are served to minimize the time it takes to access data.
  • FCFS (First-Come, First-Served): Processes requests in the order they arrive.
  • SSTF (Shortest Seek Time First): Chooses the disk request that is closest to the current head position.
  • SCAN: Moves the disk arm in one direction, servicing requests along the way. Reverses direction at the end of the disk.
  • C-SCAN (Circular SCAN): Similar to SCAN, but instead of reversing direction, the disk arm jumps to the other end of the disk after reaching the end of the disk in the current direction.
  • LOOK: Moves the disk arm in one direction, servicing requests along the way, until it reaches the last request in that direction. Reverses direction without going to the end of the disk.
  • C-LOOK: Similar to LOOK, but jumps to the first request in the opposite direction upon reaching the last request in the current direction.

RAID

• RAID (Redundant Array of Independent Disks) is a method of combining multiple physical disk drives into a single logical unit to improve data storage performance, data redundancy, and storage capacity.
• Multiple levels of RAID each with varying levels of data redundancy.

File System Structure

• A file system is a method and data structure used by an operating system to manage files on a disk. It defines how data is stored, organized, retrieved, and managed, including file attributes like name, size, and location.
• Key Components of File Systems: Files, Directories, File Attributes, Metadata, Inodes, and Allocation Tables (e.g., FAT).

I/O Systems

• An I/O system handles data transfer between a computer system's processor, memory, and external devices.
• The key components of I/O systems include I/O devices (physical hardware), I/O system calls, DMA controllers, memory-mapped I/O, and direct virtual memory addresses.
  • I/O Devices: Includes input devices such as keyboards, mice and output devices such as monitors.
  • I/O System Calls: provides an interface between user programs and operating system which in turn interacts with I/O devices.
  • DMA Controllers: a specialized hardware unit that allows devices to access system memory without involving the CPU for faster transfers.
  • Memory-Mapped I/O: maps I/O device registers into memory addresses so the CPU can interact with them as if accessing memory locations.
  • Direct Virtual Memory Addresses (DVMA): a technique where I/O devices can directly access virtual memory of a computing system.

Banker's Algorithm

• Banker's algorithm is a resource allocation and deadlock avoidance algorithm. In the Banker's algorithm, the resource scheduler acts like a banker who does the allocation of resources and will refuse if a resource request is possible without causing a deadlock. This algorithm aims to prevent a deadlock from occurring when multiple processes request and use the available resources.