NRXD
static struct rxdesc rxd[NRXD] __attribute__ ((aligned(4)));
static unsigned char rxb[NRXD * BUFLEN] __attribute__ ((aligned(4)));
for (i=0; i<NRXD; i++) {
rxd[NRXD - 1].buf2addr = (void *)virt_to_bus(&rxd[0]);
if (rxd_tail == NRXD) rxd_tail = 0;
if (rxd_tail == NRXD) rxd_tail = 0;