MBR_BPB_OFFSET
ATF_CHECK_EQ(MBR_BPB_OFFSET, offsetof(struct mbr_sector, mbr_bpb));
bpbsize = bootstrapbuf[0] + 2 - MBR_BPB_OFFSET;
(void)memcpy(bootstrapbuf + MBR_BPB_OFFSET, &mbr.mbr_bpb,