Note to reader
Note to reader This information documents the programming
considerations for running 31-bit addressing mode programs on previous
versions and releases of MVS™. Because this information might
be useful for programmers who maintain or update legacy programs,
the chapter is preserved to reflect the programming environment of
previous MVS versions. If you intend to design and code
a new program to run on z/OS® releases, consider the following:
- Always design a program to run in 31-bit addressing mode, to take
full advantage of the virtual storage capacity of MVS.
- Use the IBM® High Level Assembler, instead of Assembler
H, to assemble the new program. As of MVS/SP 5.2,
Assembler H is not supported.
- Use the program management binder, instead of the linkage editor
and loader, to prepare the program for execution.
End of Note to reader
z/Architecture® supports 64-bit real
and virtual addresses. For compatibility with Enterprise Systems
Architecture (ESA) and 370/Extended Architecture (XA), z/Architecture also
supports 31-bit real and virtual addresses, which provide a maximum
real and virtual address of two gigabytes minus one. For compatibility
with older systems, z/Architecture also supports
24-bit real and virtual addresses.
The basic characteristics of the system that provide for 64-bit
and 31-bit addresses and the continued use of 24-bit addresses are:
- A virtual storage map of two gigabytes with most MVS services
to support programs executing or residing anywhere in the first two
gigabytes of virtual storage. A virtual storage map of a theoretical
16 exabytes to support programs executing in 64-bit mode.
- Two program attributes that specify expected address length on
entry (addressing mode) and intended location in virtual storage (residence
mode).
- Trimodal operation, a capability of the processor that
permits the execution of programs with 24-bit addresses as well as
programs with 31-bit and 64-bit addresses.