When you know both the number and the size of the buffers required for a given data set before program assembly, you can reserve an area of the appropriate size to be used as a buffer pool. Any type of area can be used–for example, a predefined storage area or an area of coding no longer needed.

A BUILD macro, issued during execution of your program, uses the reserved storage area to build a buffer pool. The address of the buffer pool must be the same as that specified for the buffer pool control block (BUFCB) in your DCB. The BUFCB parameter cannot refer to an area that resides above the 16 MB line. The buffer pool control block is an 8 byte field preceding the buffers in the buffer pool. The number (BUFNO) and length (BUFL) of the buffers must also be specified. The length of BUFL must be at least the block size.

When the data set using the buffer pool is closed, you can reuse the area as required. You can also reissue the BUILD macro to reconstruct the area into a new buffer pool to be used by another data set.

You can assign the buffer pool to two or more data sets that require buffers of the same length. To do this, you must construct an area large enough to accommodate the total number of buffers required at any one time during execution. That is, if each of two data sets requires 5 buffers (BUFNO=5), the BUILD macro should specify 10 buffers. The area must also be large enough to contain the 8 byte buffer pool control block.

You can issue the BUILD macro in 31-bit mode, but the buffer area cannot reside above the line and be associated with a DCB. In any case, real addresses can point above the 2 GB bar.