With improved control interval access, you can specify that control blocks are to be fixed in real storage (ACB MACRF=(CFX,...)). If you so specify, your I/O buffers must also be fixed in real storage. Having your control blocks fixed in real storage, but not your I/O buffers, can cause physical errors or unpredictable results. If you specify MACRF=CFX without ICI, VSAM ignores CFX. NFX is the default; it indicates that buffers are not fixed in real storage, except for an I/O operation. A program must be authorized to fix pages in real storage, either in supervisor state with protection key 0 - 7, or link-edited with authorization. (The authorized program facility (APF) is described in z/OS MVS Programming: Authorized Assembler Services Guide). An unauthorized request is ignored.

You can use 64-bit real storage for all VSAM data sets, whether they are extended-format data sets. You can obtain buffer storage from any real address location available to the processor. The location can have a real address greater than 2 gigabytes or can be in 31-bit real storage with a real address less than 2 gigabytes.