SG_V
#define SDT_VALID(sdt) (*(sdt) & SG_V)
*sdt = pa | SG_SO | SG_RW | PG_M | SG_V;
*sdt = pa | SG_RW | PG_M | SG_V;
*sdt++ = pa | SG_SO | SG_RW | PG_M | SG_V;