CS 3733 Operating Systems Notes: Win 2000 Virtual Memory

Address Translation

• 32-bit virtual addresses
• Page size = 4K
• Pages not in physical memory are in a paging file on disk.
• Part of the virtual address space (usually 2G) shared by all processes and used for kernel mode.
• Memory allocation done in 2 steps:
  reserve a portion of the virtual address space is reserved
  commit space is allocated in the paging file
• Memory protection:
  read only
  read-write
  execute only
  guard page - generates an exception when accessed
  copy on write - for efficient sharing of memory
• Address translation for a process is in 2 stages:
  A page directory contains 1024 references to page tables (4-byte virtual addresses)
  A page table contains 1024 page table entries (4 bytes each containing a frame number and several dedicated bits)
  Dedicated bits include: dirty, accessed, read-only, valid
  Note how the numbers combine: 1K*1K*4K = 4G
  Virtual address looks like this:

       -------------------------------------------------------------
      |    PDE: 10 bits    |   PTE: 10 bits      |  Offset: 12 bits |
       -------------------------------------------------------------
  

• Only the page directory (1 page) must remain in memory.

• Maintain a substantial number of free pages.
• This means that a page fault can be satisfied by a single disk access, a read.
• The major part of the algorithm is how pages are taken away from processes and given to the free list.
• Each process has a working set described by two parameters, min and max.
• Here the "working set" means the number of frames allocated to the process.
• These are not hard limits and a process may have fewer frames than min or more than max.
• Each process starts with the same min and max, but these may change over time.
• min is typically 20-50 and max is typically 45-345, depending on total ram.
• If a page fault occurs and the working set is smaller than the max, a page is added.
• If it is larger than the max, a page is removed, but stays in memory.
• Once a second, the balance set manager checks if there are enough free frames.
• If not it starts the working set manager to recover more frames, starting with large processes that have been idle for a while.
• Each page is examined.
• Each page has a counter associated with it:
  if reference bit is set, counter is cleared
  if reference bit is clear, counter in incremented
  pages with highest counters are removed
• Reference bit is part of the CPU, so this is only used for uniprocessors.
• On multiprocessors, FIFO is used.
• There are 4 free lists:
  standby: contain clean pages that can be restored to processes
  modified: contain dirty pages that can be restored to processes
  free: contain frames not associated with a process
  zeroed: these are free and contain 0's
  
• There is also a list of physically bad RAM frames
• Standby and modified lists allow for fast (soft) page faults.