BSAM I/O area

The I/O area for spool messages can be very large. It is not uncommon for the area to be 32 KB in length. To reduce the overhead incurred with moving large buffers, IMS attempts to write to the spool data set from the application's I/O area.

BSAM does not support I/O areas in 31-bit storage for SYSOUT files. If IMS finds that the application's I/O area is in 31-bit storage:

• A work area is obtained from 24-bit storage.
• The application's I/O area is moved to the work area.
• The spool data set is written from the work area.

If the application's I/O area can easily be placed in 24-bit storage, the need to move the I/O area can be avoided and possible performance improvements achieved.

Since a record can be written by BSAM directly from the application's I/O area, the area must be in the format expected by BSAM. The format must contain:

• Variable length records
• A Block Descriptor Word (BDW)
• A Record Descriptor Word (RDW)