LOCLUS
blkbyt(fs, (b) - LOCLUS))
#define blklsn(fs, b) ((fs)->lsndta + blksec(fs, (b) - LOCLUS))
#define okclus(fs, c) ((c) >= LOCLUS && (c) <= (fs)->xclus)
if (fs->rdcl < LOCLUS)
if ((fs->xclus = secblk(fs, sc - fs->lsndta) + 1) < LOCLUS)