IBM 990 - Manual (Page 67)

Chapter 2. System structure and design 55
unlikely, memory should fail, it is technically feasible to Power-on Reset the system with the
remaining memory resources (see “Partial Memory Restart” on page 54). After Power-on
Reset, the memory distribution across the books is now different, as is the amount of
memory.
Capacity Upgrade on Demand (CUoD) for memory can be used to order more memory than
needed on the initial model, but that is required on the target model; see “Memory upgrades”
on page 29. For more information about CUoD for memory, refer to “CUoD for memory” on
page 193.
Processor memory, even though physically the same, can be configured as both Central
storage and Expanded storage.
Central storage (CS)
Central storage (CS) consists of main storage, addressable by programs, and storage not
directly addressable by programs. Non-addressable storage includes the Hardware System
Area (HSA). Central storage provides:
Data storage and retrieval for the PUs and I/O
Communication with PUs and I/O
Communication with and control of optional expanded storage
Error checking and correction
Central storage can be accessed by all processors, but cannot be shared between logical
partitions. Any system image (logical partition) must have a central storage size defined. This
defined central storage is allocated exclusively to the logical partition during partition
activation.
A logical partition can have more than 2 GB defined as central storage, but 31-bit operating
systems cannot use central storage above 2 GB; refer to 2.2.8, “Storage operations” on
page 67 for more detail.
Expanded storage (ES)
Expanded storage (ES) can optionally be defined on z990 servers. Expanded storage is
physically a section of processor storage. It is controlled by the operating system and
transfers 4 KB pages to and from central storage.
Except for z/VM, z/Architecture operating systems do not use expanded storage. As they
operate in 64-bit addressing mode, they can have all the required storage capacity allocated
as central storage. z/VM is an exception since, even when operating in 64-bit mode, it can
have guest virtual machines running in 31-bit addressing mode, which can use expanded
storage.
It is not possible to define expanded storage to a Coupling Facility image. However, any other
image type can have expanded storage defined, even if that image runs a 64-bit operating
system and does not use expanded storage.
The z990 only runs in LPAR mode. Storage is placed into a single storage pool called LPAR
Single Storage Pool, which can be dynamically converted to expanded storage and back to
central storage as needed when partitions are activated or de-activated.
LPAR single storage pool
In LPAR mode, storage is not split into central storage and expanded storage at Power-on
Reset. Rather, the storage is placed into a single central storage pool that is dynamically
assigned to Expanded Storage and back to Central Storage, as needed.