You can use either 24-bit or 31-bit addressing mode for VSAM programs. You can issue the OPEN and CLOSE macros for any access method in 24-bit or 31-bit addressing mode. VSAM lets you create buffers, user exits, shared resource pools, and some control blocks in virtual storage above 16 MB. Your program must run in 31-bit addressing mode to access these areas above 16 MB. See Using 31-Bit Addressing Mode with VSAM.