mfmblkdecode
rp = mfmblkdecode(rp, &info, &hcksum, 1);
rp = mfmblkdecode(rp, mfmnull, &hcksum, 4);
rp = mfmblkdecode(rp, &cktmp, NULL, 1);
rp = mfmblkdecode(rp, &cktmp, NULL, 1);
crp = mfmblkdecode(rp, dp, &dcksum, FDSECLWORDS);
u_long *mfmblkdecode(u_long *, u_long *, u_long *, int);