OS

Memory Management
  Virtual memory
    Page - virtual and physical memorys are both divided into pages (4KB on Intel X86)
    Page Frame Number
    PFN(0:11) + offset(12:) for 4KB page read by process
    Page tables: Processor translates PFN+offset to physical PFN
      Three level in linux
      Page table entry
        Valid flag: if page default
        Physical PFN,
        Access control information
    Demanding Page: When a process attempts to access a virtual address that is not currently in memory the processor cannot find a page table entry for the virtual page referenced, the operating system must bring the appropriate page into memory from the image on disk
    Swapping:
      Dirty page: if the page has been modified, the operating system must preserve the contents of that page so that it can be accessed at a later time
      LRU paging
      Thrashing: is a condition in which excessive paging operations are taking place
    
  Protection
    Each process has its own virtual memory space
Process