FIRSTAVDP
avdp = FIRSTAVDP * fsbsize / DEV_BSIZE;
err = verifytag(tp, FIRSTAVDP, tp, UD_ANCH_VOL_DESC);
rdfs(FIRSTAVDP, sectorsize, (char *)avp);
err = verifytag(tp, FIRSTAVDP, tp, UD_ANCH_VOL_DESC);
for (i = MAXBSIZE / sectorsize; i < FIRSTAVDP; i++) {
part_start = FIRSTAVDP + AVDPLEN;
if (nextblock > FIRSTAVDP) {
nextblock = FIRSTAVDP;