Memory Management Notes

MEMORY MANAGEMENT

Overview of Memory Management
  • Memory management is crucial in multi-user and multiprogramming environments.
  • It handles the allocation and deallocation of physical memory to processes.
  • Central to the operation of a computer, it supports the "Stored Program" concept.
Four Functions of Memory Management
  1. Allocation Status Tracking:
    • Monitor memory usage to identify free and allocated spaces.
  2. Allocation Policy Determination:
    • Establish methods to decide how much memory each process receives, especially with multiprogramming.
  3. Memory Allocation:
    • Assign specific memory locations to processes based on the allocation policy.
  4. Memory Deallocation:
    • Release memory once a process has finished executing and update the status of memory locations.
Memory Management Techniques

  1. Contiguous Allocation Memory Management

    • Allocates memory in a contiguous block.
      • Types Include:
      • Single Contiguous Allocation
      • Partitioned Allocation
      • Relocatable Partitioned Allocation

  2. Non-Contiguous Allocation Memory Management

    • Allows processes to be allocated non-contiguous memory areas.
      • Implemented Through:
      • Paged Memory Management
      • Virtual Memory Management
Logical vs Physical Address
  • Logical Address:
    • Address generated by the CPU (also called virtual address).
  • Physical Address:
    • Actual address on the storage device used to access data.
Contiguous Allocation Memory Management

  • Single Contiguous Allocation:

    • Simple method requiring minimal hardware support.
    • Memory divided into OS (kernel) and user space; multiple processes are not supported simultaneously.
    • Example: Microsoft DOS.

  • Partitioned Allocation:

    • Divides physical memory into partitions, each assigned to a process.
    • Can be Static (fixed size) or Dynamic (variable size).
    • Requires both Base and Limit registers for hardware support to ensure process isolation.

  • Advantages:

    • Supports multiple processes, minimal hardware required.

  • Disadvantages:

    • Wastage of memory, fragmentation, pathologies from static allocation failure if jobs exceed allocated partitions.
Non-Contiguous Allocation Memory Management
  1. Paged Memory Management:
    • Splits both logical address space and physical memory into equal-sized pieces (pages).
    • Involves a Page Map Table (PMT) for mapping logical pages to physical memory.
Virtual Memory Management
  • Allows the system to use memory more efficiently by allowing processes to execute without needing the entire address space loaded in physical memory at once.
  • Uses techniques like Demand Paging to load pages as needed and Segmentation for modularity.
  • Advantages:
    • Increased efficiency beyond 100% utilization of physical memory, supports larger address spaces without requiring full availability.
  • Disadvantages:
    • Increased hardware costs, potential for thrashing (excessive swapping).
Thrashing and Locality of Reference
  • Thrashing: Occurs when many processes compete for limited memory resources, leading to excessive paging and reduced system throughput.
  • Locality of Reference: Programs tend to access a limited set of memory locations repeatedly within short periods, promoting the effectiveness of caching and paging techniques.
Address Translation in Segmented Demand Paging
  1. Virtual address v(s,p,d):
    • s: Segment number
    • p: Page number within that segment
    • d: Displacement within the page
  2. Protection: Each segment has associated access rights defined in the segment map to control which processes can access it.
Swapping & Overlays
  1. Swapping:
    • Temporary removal of process from memory to disk to make room for other processes.
    • Aimed at maintaining system performance and efficient memory utilization.
  2. Overlays:
    • Used to extend memory capabilities by loading only necessary portions of a program into memory, swapping in and out as needed without requiring large contiguous blocks of memory.
Summary of Memory Management Schemes
  • Contiguous allocation suited for smaller, simpler systems.
  • Paging and segmentation support larger, more complex applications with dynamic memory requirements.
  • Swapping and overlays are techniques to optimize memory usage, especially useful in systems with heavy multitasking.