MSDOSFSFREE
error = fatentry(FAT_GET_AND_SET, pmp, cluster, &oldcn, MSDOSFSFREE);
readcn |= MSDOSFSFREE << 4;
readcn |= MSDOSFSFREE & 0xfff;
putushort(bp->b_data + bo, MSDOSFSFREE);
(MSDOSFSFREE & FAT32_MASK) | (cluster & ~FAT32_MASK));