state column of x$bh

        0, FREE, no valid block image
        1, XCUR, a current mode block, exclusive to this instance
        2, SCUR, a current mode block, shared with other instances
        3, CR,   a consistent read (stale) block image
        4, READ, buffer is reserved for a block being read from disk
        5, MREC, a block in media recovery mode
        6, IREC, a block in instance (crash) recovery mode