z/OS® programs and data reside in virtual storage that, when necessary, is backed by central storage. Most programs and data do not depend on their real addresses. Some z/OS programs, however, do depend on real addresses and some require these real addresses to be less than 16 megabytes. z/OS programmers commonly refer to this storage as being "below the 16-megabyte line."

In z/OS, a program's attributes include one called residence mode or RMODE, which specifies whether the program must reside (be loaded) in storage below 16 megabytes. A program with RMODE(24) must reside below 16 megabytes, while a program with RMODE(31) can reside anywhere in virtual storage.

Examples of programs that require below-the-line storage include any program that allocates a data control block (DCB). Those programs, however, often can be 31-bit residency mode or RMODE(31) as they can run in 31-bit addressing mode or AMODE(31). z/OS reserves as much central storage below 16 megabytes as it can for such programs and, for the most part, handles their central storage dependencies without requiring them to make any changes.

Thousands of programs in use today are AMODE(24) and therefore RMODE(24). Every program written before MVS/XA™ was available, and not subsequently changed, has that characteristic. There are relatively few reasons these days why a new program might need to be AMODE(24), so a new application likely has next to nothing that is RMODE(24).