BMAP_BPW_SHIFT
ip = &cp->c_bmp[bn >> BMAP_BPW_SHIFT];
rv = (cp->c_bmp[bn >> BMAP_BPW_SHIFT] & bit) != 0;
#define BMAP_BPW_MASK (~(~0ULL << BMAP_BPW_SHIFT))
ip = &cp->c_bmp[bn >> BMAP_BPW_SHIFT];
rv = (cp->c_bmp[bn >> BMAP_BPW_SHIFT] & bit) != 0;