IBM z/OS - Manual (Page 9)

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Each book can support up to 64 GB of Memory, delivered
on two memory cards, and 12 STIs giving a total of 256 GB
of memory and 48 STIs on the D32 model. The memory is
delivered on 8 GB, 16 GB or 32 GB memory cards which
can be purchased in 8 GB increments. The minimum
memory is 16 GB. The two memory cards associated with
each book must be the same size. Each book has 3 MBAs
and each MBA supports 4 STIs.
All books are interconnected with a super-fast bi-direc-
tional redundant ring structure which allows the system to
be operated and controlled by PR/SM operating in LPAR
mode as a symmetrical, memory coherent, multiproces-
sor. PR/SM provides the ability to confi gure and operate
as many as 30 Logical Partitions which may be assigned
processors, memory and I/O resources from any of the
available books. The z990 supports LPAR mode only (i.e.
basic mode is no longer supported).
The MultiChip Module is the technology cornerstone for
fl exible PU deployment in the z990 models. For most
models, the ability of the MCM to have inactive PUs allows
such features as Capacity Upgrade on Demand (CUoD),
Customer Initiated Upgrades (CIU), and the ability to add
CPs, ICFs, IFLs, and zAAPs dynamically providing nondis-
ruptive upgrade of processing capability. Also, the ability
to add CPs lets a z990 with spare PU capacity become a
backup for other systems in the enterprise; expanding the
z990 system to meet an emergency outage situation. This
is called Capacity BackUp (CBU). The greater capacity of
the z990 offers customers even more fl exibility for using
this feature to backup critical systems in their enterprise.
In order to support the highly scalable multi-book system
design the I/O SubSystem has been enhanced by intro-
ducing a new Logical Channel SubSystem (LCSS) which
provides the capability to install up to 1024 CHPIDs across
three I/O cages (256 per operating system image). I/O
improvements in the Parallel Sysplex Coupling Link archi-
tecture and technology support faster and more effi cient
transmission between the Coupling Facility and production
systems. HiperSockets ™ provides high-speed capability to
communicate among virtual servers and Logical Partitions;
this is based on high-speed TCP/IP memory speed trans-
fers and provides value in allowing applications running
in one partition to communicate with applications running
in another without dependency on an external network.
Industry standard and openness are design objectives for
I/O in z990. The improved I/O subsystem is delivering new
horizons in I/O capability and has eliminated the 256 limit
to I/O attachments for a mainframe.