AIX version 4.2.1 introduced the EXTSHM environment variable that allows a process to attach to more than 11 shared memory segments. When this environment variable is set to "ON", it is possible to attach multiple shared memory segments within the same 256 MB region. Rather than beginning on a region boundary, these shared memory segments begin on a (typically 4K) page boundary, which allows the creation of thousands of shared memory segments per region. Only those programs that require access to more than 11 shared memory segments should set the EXTSHM variable, since this causes reduced performance for memory access.
64-bit processes do not need to set this variable since a very large number of segments is available
For more details on AIX memory addressing and the EXTSHM environment variable:
See the AIX shmat() man page
General Programming Concepts, Writing and Debugging Programs (AIX documentation can be found on IBM's Web site).