The GENCB macro can be used to generate an access method control block, an exit list, or a request parameter list when your program is run. Generating the control block at execution time with GENCB has the advantage of requiring no reassembly of the program when you adopt a new version of VSAM in which control block formats might have changed. If you use the ACB, EXLST, and RPL macros to build control blocks, and adopt a subsequent release of VSAM in which the control block format has changed, you have to reassemble your program. GENCB also gives you the ability to generate multiple copies of the ACB, EXLST, or RPL to be used for concurrent requests. The disadvantage of using GENCB is that the path length is longer. It takes more instructions to build a control block using GENCB than to code the control block directly.

You can use the WAREA parameter to provide an area of storage in which to generate the control block. This work area has a 64K (X'FFFF') size limit. If you do not provide storage when you generate control blocks, the ACB, RPL, and EXLST reside below 16 MB unless LOC=ANY is specified.