Changing Existing BSAM and QSAM Applications

For existing BSAM, and QSAM applications, in most cases, programs do not have to be changed or recompiled to take advantage of sequential data striping. However, you can choose to update programs to more fully exploit new functions. To improve performance, you will want to have more buffers. You also can have the buffers be above the 16 MB line.

If you use BSAM, you can set a larger NCP value or have the system calculate an NCP value by means of the DCBE macro MULTSDN parameter. You can also request accumulation by means of the DCBE macro MULTACC parameter. DCBENSTR in the DCBE macro tells the number of stripes for the current data set.

If you use QSAM, you can request more buffers using the BUFNO parameter. Your program can calculate BUFNO according to the number of stripes. Your program can test DCBENSTR in the DCBE during the DCB open exit routine.

Existing programs need to be changed and reassembled if you want any of the following:

To switch from 24-bit addressing mode to 31-bit mode SAM.
To ask the system to determine an appropriate NCP value. Use the MULTSDN parameter of the DCBE macro.
To get maximum benefit from BSAM performance chaining. You must change the program by adding the DCBE parameter to the DCB macro and including the DCBE macro with the MULTACC parameter. If the program uses WAIT or EVENTS or a POST exit (instead of, or in addition to, the CHECK macro), your program must issue the TRUNC macro whenever the WAIT or EVENTS macro is about to be issued or the POST exit is depended upon to get control.

Related reading: For more information, see DASD and Tape Performance and the DCBE and IHADCBE macros in z/OS DFSMS Macro Instructions for Data Sets.