IBM 990 - Manual (Page 81)

Chapter 2. System structure and design 69
An ESA/390 mode image is always initiated in 31-bit addressing mode. During its
initialization, a z/Architecture operating system can change it to 64-bit addressing mode and
operate in the z/Architecture mode.
Some z/Architecture operating systems, like z/OS, will always change this addressing mode
and operate in 64-bit mode. The z/OS Bimodal Migration Accommodation Offering allows for
a limited amount of time to run z/OS in 31-bit mode. This offering provides fallback support to
31-bit mode in the event it is required during migration to z/OS in 64-bit mode. Beginning with
z/OS V1.5, the z/OS Bimodal Migration Accommodation Offering is no longer available.
Other z/Architecture operating systems, like the z/VM and the OS/390 Version 2 Release 10,
can be configured to change to 64-bit mode or to stay in 31-bit mode and operate in the
ESA/390 architecture mode.
z/Architecture mode
In z/Architecture mode, storage addressing is 64-bit, allowing for an addressing range of up to
16 exabytes (16 EB). The 64-bit architecture allows a maximum of 16 EB to be used as
central storage. However, the current z990 definition limit for logical partitions is 128 GB of
central storage.
Expanded storage can also be configured to an image running an operating system in
z/Architecture mode. However, only z/VM is able to use expanded storage. Any other
operating system running in z/Architecture mode (like a z/OS or a Linux for zSeries image)
will not address the configured expanded storage. This expanded storage remains
configured to this image and is unused .
ESA/390 architecture mode
In ESA/390 architecture mode, storage addressing is 31-bit, allowing for an addressing range
of up to 2 GB. A maximum of 2 GB can be used for central storage. Since the processor
storage can be configured as central and expanded storage, memory above 2 GB may be
configured as expanded storage. In addition, this mode permits the use of either 24-bit or
31-bit addressing, under program control, and permits existing application programs to run
with existing control programs.
Since an ESA/390 mode image can be defined with up to 128 GB of central storage, the
central storage above 2 GB will
not be used, but remains configured to this image.
ESA/390 TPF mode
In ESA/390 TPF mode, storage addressing follows the ESA/390 architecture mode, to run the
TPF/ESA operating system in the 31-bit addressing mode.
Coupling Facility mode
In Coupling Facility mode, storage addressing is 64-bit for a Coupling Facility image running
CFLEVEL 12 or above, allowing for an addressing range up to 16 EB. However, the current
z990 definition limit for logical partitions is 128 GB of storage.
Expanded storage cannot be defined for a Coupling Facility image.
Only IBM Coupling Facility Control Code can run in Coupling Facility mode.
Linux Only mode
In Linux Only mode, storage addressing can be 31- or 64-bit, depending on the operating
system architecture
and the operating system configuration, in exactly the same way as in
ESA/390 mode.