DADSM allocate and extend routines assign tracks and cylinders on direct access volumes for data sets. The DADSM extend routine gets additional space for a data set that has exceeded its primary allocation. The DADSM scratch and partial release routines release space that is no longer needed on a direct access volume.

The DADSM routines allocate and release space by adding, deleting, and modifying the DSCBs and updating the VTOC index. When space is released, the scratch routines free the DSCBs of the deleted data set or data space.

Every data set occupies an integral number of tracks. Unused space on a track cannot be used for another data set unless the whole track is released. The minimum amount of space allocated to a data set is zero tracks but a PDS with no tracks can never contain a member.

After a certain place on the volume, the system rounds up each primary or secondary space request so it is a multiple of 21 cylinders. Currently that place is after the first 65520 cylinders. The space within the first 65520 cylinders is called the track-managed space, even if data sets are allocated in units of cylinders. The space after the first 65520 cylinders is called the cylinder-managed space. If the volume capacity is 65520 cylinders or less, then all of the space is track-managed.