The SORT statement results in a call to DFSORT with a parameter list that contains a SORT control statement and other information.

DFSORT treats the INPUT PROCEDURE as an E15 user exit which must supply all the input records. DFSORT calls the INPUT PROCEDURE once for each input record. The INPUT PROCEDURE reads the input data set and uses RELEASE to pass each record with a non-zero OMIT field to DFSORT.

DFSORT sorts the records passed to it by the INPUT PROCEDURE as requested by the SORT statement passed to it by the calling program.

DFSORT treats the OUTPUT PROCEDURE as an E35 user exit which must dispose of all the output records. DFSORT calls the OUTPUT PROCEDURE once for each sorted record. The OUTPUT PROCEDURE uses RETURN to obtain the records passed from DFSORT and writes one record with each key to the output data set.