typedef char Page[PAGESIZE];
typedef ... PageID;  // defined earlier

typedef struct _FrameData {
   PageID  pid;        // which page is in frame
   int     pin_count;  // how many processes using page
   int     dirty;      // page modified since loaded?
   Time    last_used;  // when page was last accessed
} FrameData;

Page frames[NBUFS];    // actual buffers
FrameData directory[NBUFS];