When NOFASTSRT is in effect for input processing, the USING or INPUT PROCEDURE causes COBOL to generate a DFSORT E15 user exit routine and pass its address to DFSORT. When an INPUT PROCEDURE is used, COBOL incorporates the INPUT PROCEDURE code into the E15 routine it generates.

DFSORT does not read the input data set directly, but instead obtains all the input records from the E15 routine. The E15 routine generated by COBOL reads the input data set and passes one record to DFSORT each time it is called.

When NOFASTSRT is in effect for output processing, the GIVING or OUTPUT PROCEDURE causes COBOL to generate a DFSORT E35 user exit routine and pass its address to DFSORT. When an OUTPUT PROCEDURE is used, COBOL incorporates the OUTPUT PROCEDURE code into the E35 routine it generates.

DFSORT does not write the output data set directly, but instead passes all the output records to the E35 routine. DFSORT calls the E35 routine generated by COBOL once for each record so the E35 routine can write the record to the output data set.

Input or output processing by COBOL rather than DFSORT can result in degraded performance.