Draft Document for Review April 7, 2004 6:15 pm

6947ch02.fm

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 bits, 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 bits, 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 bits 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’s Coupling Facility Control Code can run in Coupling Facility mode.

Linux Only mode

In Linux Only mode, storage addressing can be 31 or 64 bits, depending on the operating system architecture and the operating system configuration, in exactly the same way as in ESA/390 mode.

Only Linux and z/VM operating systems can run in Linux Only mode:

￿Linux for zSeries uses 64-bit addressing and operates in the z/Architecture mode.

￿Linux for S/390 uses 31-bit addressing and operates in the ESA/390 Architecture mode.

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IBM 990 manual ESA/390 TPF mode, Coupling Facility mode, Linux Only mode, Architecture mode, ESA/390 architecture mode
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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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