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on page 29. For more information about CUoD for memory, refer to “CUoD for memory” on page 191.

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 “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.

The Storage Assignment function of a Reset Profile on the Hardware Management Console just shows the total “Installed Storage” and the “Customer Storage”, which is the total installed storage minus the Hardware System Area (HSA). Logical partitions are still defined to have Central Storage and optional Expanded Storage. Activation of logical partitions, as well as dynamic storage reconfiguration, will cause the storage to be converted to the type needed.

Chapter 2. System structure and design 55

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IBM 990 manual Central storage CS, Expanded storage ES, Lpar single storage pool
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990 specifications

The IBM 990 series, often referred to in the context of IBM's pioneering efforts in the realm of mainframe computing, represents a unique chapter in the history of information technology. Introduced in the late 1960s, the IBM 990 series was designed as a powerful tool for enterprise-level data processing and scientific calculations, showcasing the company's commitment to advancing computing capabilities.

One of the main features of the IBM 990 was its architecture, which was built to support a wide range of applications, from business processing to complex scientific computations. The system employed a 32-bit word length, which was advanced for its time, allowing for more flexible and efficient data handling. CPUs in the IBM 990 series supported multiple instructions per cycle, which contributed significantly to the overall efficiency and processing power of the machines.

The technology behind the IBM 990 was also notable for its use of solid-state technology. This provided a shift away from vacuum tube systems that were prevalent in earlier computing systems, enhancing the reliability and longevity of the hardware. The IBM 990 series utilized core memory, which was faster and more reliable than the magnetic drum memory systems that had been standard up to that point.

Another defining characteristic of the IBM 990 was its extensibility. Organizations could configure the machine to suit their specific needs by adding memory, storage, and peripheral devices as required. This modular approach facilitated the growth of systems alongside the technological and operational demands of the business environments they served.

In terms of software, the IBM 990 series was compatible with a variety of operating systems and programming environments, including FORTRAN and COBOL, enabling users to access a broader array of applications. This versatility was a significant advantage, making the IBM 990 an appealing choice for educational institutions, research facilities, and enterprises alike.

Moreover, the IBM 990 was engineered to support multiprocessing, which allowed multiple processes to run simultaneously, further increasing its effectiveness in tackling complex computing tasks.

In summary, the IBM 990 series represents a significant advancement in computing technology during the late 20th century. With a robust architecture, versatile configuration options, and a focus on solid-state technology, the IBM 990 facilitated substantial improvements in data processing capabilities, making it a cornerstone for many businesses and academic institutions of its time. Its impact can still be seen today in the continued evolution of mainframe computing.
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