mfmblkencode
rp = mfmblkencode(&info, rp, &hcksum, 1);
rp = mfmblkencode(mfmnull, rp, &hcksum, 4);
rp = mfmblkencode(&hcksum, rp, NULL, 1);
rp = mfmblkencode(dp, rp + 2, &dcksum, FDSECLWORDS);
crp = mfmblkencode(&dcksum, crp, NULL, 1);
u_long *mfmblkencode(u_long *, u_long *, u_long *, int);