int request_page(PageID pid)
{
   if (pid in Pool)
      bufID = index for pid in Pool
   else {
      if (no free frames in Pool)
         evict a page (free a frame)
      bufID = allocate free frame
      directory[bufID].page = pid
      directory[bufID].pin_count = 0
      directory[bufID].dirty_bit = 0
   }
   directory[bufID].pin_count++
   return bufID
}