+values. The \fBoffset\fP must appear subsequent to track, sector and sector
+length values for the sector and track units to work.
+.LP
+Note that it is possible to reserve space between the directory and
+the beginning of data. Although typically data follows the directory,
+some systems used this to store extra data instead of using more
+system tracks (see the fields \fBALV0\fP and \fBALV1\fP in the
+DPB).
+.LP
+There are disk formats that map multiple logical tracks onto a physical
+track, which allows a little bit more capacity in case the system image
+size does not match the physical track capacity well.