int request_page(PageID pid)
{
   bufID = findInPool(pid)
   if (bufId == NOT_FOUND) {
      if (no free frames in Pool) {
         bufID = findFrameToReplace()
         if (directory[bufID].dirty)
            old = directory[bufID].page
            put_page(old, frames[bufID])
      }
      bufID = index of freed frame
      directory[bufID].page = pid
      directory[bufID].pin_count = 0
      directory[bufID].dirty = 0
      get_page(pid, frames[bufID])
   }
   directory[bufID].pin_count++
   return bufID
}