Allocating Data Sets with Dynamic Allocation

You can allocate VSAM and non-VSAM data sets using the DYNALLOC macro with the SVC 99 parameter list.

Note: To allocate a VSAM data set using the DYNALLOC macro with the SVC 99 parameter list, specify text unit 800B - RECORG.

For application programs that require 24–bit UCB addresses, the system creates 24–bit UCB addresses for above 16 MB UCBs. This view is known as a captured UCB. With step or batch allocation, the system automatically captures an above 16 MB UCB address at allocation and uncaptures the UCB address at deallocation. However, with dynamic allocation, it is possible not to capture a UCB address if the affected program can handle above 16 MB UCBs. The major user of uncaptured UCBs is DB2.

Use the following dynamic allocation options when allocating a VSAM or non-VSAM data set to use uncaptured UCBs above the 16 MB line and reduce storage usage:

XTIOT option (S99TIOEX) - This option requires that your program be APF authorized, in supervisor state, or in a system key.
NOCAPTURE option (S99ACUCB) - This option causes the creation of an XTIOT entry, but it does not require authorization or the coding of the XTIOT option.
DSAB-above-the-line option (S99DSABA) - Specify this option to place the data set association control block (DSAB) above the 16 MB line. You must specify S99TIOEX if you use this option.

Note: The following non-VSAM access methods support the NOCAPTURE, XTIOT, and DSAB-above-the-line options of dynamic allocation: BPAM, BSAM, and QSAM. The following non-VSAM access methods do not support the NOCAPTURE, XTIOT, and DSAB-above-the-line options of dynamic allocation: BDAM.

Related reading: See z/OS MVS Programming: Authorized Assembler Services Guide for more information and for examples of allocating a data set using the DYNALLOC macro.