Logically, DFSORT (like any other application) works within a virtual address space. Installation defaults such as TMAXLIM and run-time options such as REGION and MAINSIZE determine the size of this address space. With the exception of dataspace sorting (see Memory object sorting, hipersorting and data space sorting for a discussion of dataspace sorting) and memory object sorting (see Memory object sorting, hipersorting and data space sorting for more information about memory object sorting), this size remains constant throughout most of the sorting process.

DFSORT attempts to make the best use of the virtual storage it has available. If you provide DFSORT with enough virtual storage, it might be able to sort the input data set entirely in virtual storage (an "in-main-storage" sort) without need of work data sets or Hiperspace. This is the preferred method of sorting small data sets up to a few megabytes in size. To sort larger input data sets, DFSORT may use data space or memory objects to perform an "in-main-storage" sort.

When an in-main-storage sort is not possible or practical, DFSORT processes a portion of the input data set at a time, then combines all of these processed portions together into the final sorted data set. It is here that DFSORT excels at allocating virtual storage effectively in order to minimize both the number of portions as well as the time spent combining the portions into the output data set.

With dataspace sorting, DFSORT creates a data space to help carry out its processing. This is a new area of virtual storage, and is in addition to the original (specified or defaulted) virtual storage requested. The size of the data space is sufficient to guarantee an efficient sort (or dataspace sorting is not used).

In addition, DFSORT adjusts the size of the data space during processing, as necessary, in response to system paging levels. When system paging levels rise, DFSORT reduces its use of virtual storage (as long as this reduction does not significantly degrade DFSORT performance).

DFSORT also tries to move as many of its data areas above 16 MB virtual as it can to help provide virtual storage constraint relief for the system.

With z/OS, the MVS™ address space expands up to 16 exabytes in size. The architecture that creates this address space provides 64-bit addresses. The 64-bit address space marks the 2-gigabyte virtual line called "the bar". The bar separates storage below the 2-gigabyte address, called "below the bar", from storage above the 2-gigabyte address, called "above the bar". DFSORT obtains storage in virtual storage above the bar as a "memory object" for sorting.