fatp_word_t
#define FATP_NTAGS (CACHE_LINE_SIZE / sizeof(fatp_word_t) - 1)
#define FATP_NXT_WIDTH (sizeof(fatp_word_t) * NBBY - FATP_NTAGS)
fatp_word_t inuse : FATP_NTAGS; /* (1+15)*4 == CL_SIZE */
fatp_word_t nxt : FATP_NXT_WIDTH;/* at most 2^17 fat pointers */
fatp_word_t tag[FATP_NTAGS]; /* 15 tags per CL */