Unit 13

Cards (12)

  • What is the goal of memory management?

    To make it as simple as possible for programs to find space to be loaded and executed
  • Memory is divided into:
    Logical addresses - an address generated by the CPU
    Physical address - an address that can be seen by the memory unit, it is loaded into the memory address register
  • What is fragmentation
    It is when memory spaces are used in such a way that there are small pieces of memory left unused
  • Virtual memory
    Acts as a backup storage for a computer's main memory. This helps the computer run programs that require more memory than it actually has

    Implemented using demand paging/segmentation
  • What is paging
    When a process is being executed, its pages are loaded into any available memory frame from the backing store

    • In the physical memory, it is a small block sized memory called a frame
    • In the logical memory, it is called a page
  • What is demand paging?

    When a process needs to be executed, it is paged into the memory. The whole process is not paged, only pages that are needed are brought in
  • What is demand segmentation?

    It is when segments of a program are brought into the memory only when they are needed, if they are not used it can be removed from the memory.
    A segment is a logical unit of a programs address space
  • When does a page fault occur?
    A page fault is an error that occurs when a program tries to access a page in the virtual memory that is not currently in the main memory
  • Steps to handling a page fault:

    1. Locate the missing page in the memory
    2. Find free memory frame (if there is no free memory, make a free space)
    3. Copy it into the physical memory
    4. Reset the page table
    5. Restart the instructions
  • What if there are no free frames? PAGE REPLACEMENT

    ...
    • Find a frame that is not being used and free it
    • Use a page replacement algorithm to find a free frame
    How to free a frame:
    • Find a frame that isn't being used
    • Copy its contents to the disk
    • Change the page table to indicate the page is no longer in memory
    • Newly free frame can be used to store the page that resulted in the page fault
  • Page replacement algorithms
    ...
    • First in first out - oldest page is replaced
    • Optimal - replaces the page that will not be used for the longest time
    • Least recently used - replaces the page that has not been used for the longest time
  • What is thrashing?

    A condition where there is high page replacement activity. Process spends more time replacing pages rather than processing, which causes CPU utilization to drop