Memory Management in Operating Systems: Fragmentation, Paging, Segmentation, and Virtual Memory

Posted on May 4, 2024 in Computers

Understanding Memory Fragmentation

External Fragmentation

External fragmentation occurs when free memory is divided into small, non-contiguous blocks, making it difficult to allocate larger blocks for processes.

Example:

| Allocated | Free | Allocated | Free | Allocated | Free | Allocated |

In this example, despite having enough total free memory, allocating a large block is impossible due to the scattered free spaces.

Internal Fragmentation

Internal fragmentation occurs when allocated memory blocks are larger than necessary, wasting space within the allocated block.

Example:

| Process A (50 KB) | Process B (30 KB) | Process C (20 KB) |

Here, each process has some wasted space within its allocated block.

Paging for Efficient Memory Management

Paging divides logical and physical memory into fixed-size blocks called pages and frames, respectively. The operating system uses a page table to map logical pages to physical frames.

Paging Calculations:

Given:

  • Logical Address Space (LAS) = 128KB
  • Physical Address Space (PAS) = 512KB
  • Page size = 16KB

Calculations:

  1. Number of bits for Logical Address (LA) = 17 bits
  2. Number of bits for Physical Address (PA) = 19 bits
  3. Number of Pages in LAS = 8 pages
  4. Number of Frames in PAS = 32 frames
  5. Page Table Size = 40 bits

Segmentation for Logical Memory Division

Segmentation divides logical memory into variable-size segments based on the program’s structure (e.g., code, data, stack).

Segmentation vs. Paging:

  • **Unit of Division:** Paging uses fixed-size pages, while segmentation uses variable-size segments.
  • **Address Translation:** Paging uses a page table, while segmentation uses a segment table.
  • **Fragmentation:** Paging may suffer from internal fragmentation, while segmentation may suffer from external fragmentation.

Virtual Memory: Expanding Memory Capabilities

Virtual memory creates an illusion of having more memory than physically available by swapping data between RAM and disk storage.

Benefits of Virtual Memory:

  • Efficient memory utilization
  • Execution of larger programs
  • Memory protection and isolation

Logical vs. Physical Address Space

Logical address space is the address space seen by a process, while physical address space is the actual memory available in the hardware.

Key Differences:

  • **Perspective:** Logical address space is from the process’s viewpoint, while physical address space is from the hardware’s viewpoint.
  • **Visibility:** Each process has its own logical address space, while all processes share the same physical address space.
  • **Size:** Logical address space can be larger than physical address space.

Page Replacement Algorithms

Page replacement algorithms determine which page to replace when a page fault occurs.

Common Algorithms:

  • **LRU (Least Recently Used):** Replaces the least recently used page.
  • **FIFO (First-In-First-Out):** Replaces the oldest page.
  • **Optimal:** Replaces the page that will not be used for the longest period (theoretical).

Thrashing: Excessive Paging

Thrashing occurs when the system spends more time swapping pages than executing instructions, leading to performance degradation.

Causes of Thrashing:

  • High degree of multiprogramming
  • Insufficient memory allocation
  • Poor memory management

Demand Paging: On-Demand Loading

Demand paging loads pages into memory only when needed, improving memory utilization and reducing startup time.

Fixed vs. Dynamic Partitioning

Fixed partitioning divides memory into fixed-size partitions, while dynamic partitioning divides memory into variable-size partitions.

Key Differences:

  • **Partition Size:** Fixed partitions have predetermined sizes, while dynamic partitions adjust sizes based on process needs.
  • **Fragmentation:** Fixed partitioning may lead to external fragmentation, while dynamic partitioning reduces internal fragmentation.

The Role of the Operating System in Security

The operating system plays a crucial role in ensuring system security through various mechanisms:

  • User authentication and access control
  • Process isolation and privilege separation
  • File system security
  • Network security
  • Malware detection and prevention
  • System resource protection