Z/Architecture

z/Architecture

Designer	IBM
Bits	64-bit
Introduced	2000;24 years ago (2000)
Version	ARCHLVL 2 and ARCHLVL 3 (2008)
Design	CISC
Type	Register–Register Register–Memory Memory–Memory
Encoding	Variable (2, 4 or 6 bytes long)
Branching	Condition code, indexing, counting
Endianness	Big
Predecessor	ESA/390
Registers
General-purpose	16× 64-bit
Floating point	16× 64-bit
Vector	32× 128-bit

z/Architecture, initially and briefly called ESA Modal Extensions (ESAME), is IBM's 64-bit complex instruction set computer (CISC) instruction set architecture, implemented by its mainframe computers. IBM introduced its first z/Architecture-based system, the z900, in late 2000. ^[1] Later z/Architecture systems include the IBM z800, z990, z890, System z9, System z10, zEnterprise 196, zEnterprise 114, zEC12, zBC12, z13, z14, z15 and z16.

z/Architecture retains backward compatibility with previous 32-bit-data/31-bit-addressing architecture ESA/390 and its predecessors all the way back to the 32-bit-data/24-bit-addressing System/360. The IBM z13 is the last z Systems server to support running an operating system in ESA/390 architecture mode. ^[2] However, all 24-bit and 31-bit problem-state application programs originally written to run on the ESA/390 architecture will be unaffected by this change.

Operating system support

IBM's operating systems z/OS, z/VSE, z/TPF, and z/VM are versions of MVS, VSE, Transaction Processing Facility (TPF), and VM that support z/Architecture. Older versions of z/OS, z/VSE, and z/VM continued to support 32-bit systems; z/OS version 1.6 and later, z/VSE Version 4 and later, and z/VM Version 5 and later require z/Architecture.

Linux also supports z/Architecture with Linux on IBM Z.

z/Architecture supports running multiple concurrent operating systems and applications even if they use different address sizes. This allows software developers to choose the address size that is most advantageous for their applications and data structures.

On July 7, 2009, IBM on occasion of announcing a new version of one of its operating systems implicitly stated that Architecture Level Set 4 (ALS 4) exists, and is implemented on the System z10 and subsequent machines. ^{[3] [4]} The ALS 4 is also specified in LOADxx as ARCHLVL 3, whereas the earlier z900, z800, z990, z890, System z9 specified ARCHLVL 2. Earlier announcements of System z10 simply specified that it implements z/Architecture with some additions: 50+ new machine instructions, 1 MB page frames, and hardware decimal floating point unit (HDFU). ^{[5] [6]}

Most^{[ citation needed ]} operating systems for the z/Architecture, including z/OS, generally restrict code execution to the first 2 GB (31 address bits, or 2³¹ addressable bytes) of each virtual address space for reasons of efficiency and compatibility rather than because of architectural limits. Linux on IBM Z allows code to execute within 64-bit address ranges.

z/OS

Each z/OS address space, called a 64-bit address space, is 16 exabytes in size.

Code (or mixed) spaces

The z/OS implementation of the Java programming language is an exception. The z/OS virtual memory implementation supports multiple 2 GB address spaces, permitting more than 2 GB of concurrently resident program code.

Data-only spaces

Data-only spaces are memory regions that can be read from and written to, but not used as executable code. (Similar to the NX bit on other modern processors.) By default, the z/Architecture memory space is indexed by 64-bit pointers, allowing up to 16 exabytes of memory to be visible to an executing program.

Dataspaces and hiperspaces

Applications that need more than a 16  exabyte data address space can employ extended addressability techniques, using additional address spaces or data-only spaces. The data-only spaces that are available for user programs are called:

• dataspaces (sometimes referred to as "data spaces") ^{[7] [8]} and
• hiperspaces (High performance space). ^{[9] [10]}

These spaces are similar in that both are areas of virtual storage that a program can create, and can be up to 2  gigabytes. Unlike an address space, a dataspace or hiperspace contains only user data; it does not contain system control blocks or common areas. Program code cannot run in a dataspace or a hiperspace. ^[11]

A dataspace differs from a hiperspace in that dataspaces are byte-addressable, whereas hiperspaces are page-addressable.

IBM mainframe expanded storage

Traditionally IBM Mainframe memory has been byte-addressable. This kind of memory is termed "Central Storage". IBM Mainframe processors through much of the 1980s and 1990s supported another kind of memory: Expanded Storage. It was first introduced with the IBM 3090 high-end mainframe series in 1985. ^[12]

Expanded Storage is 4KB-page addressable. When an application wants to access data in Expanded Storage it must first be moved into Central Storage. Similarly, data movement from Central Storage to Expanded Storage is done in multiples of 4KB pages. Initially page movement was performed using relatively expensive instructions, by paging subsystem code.

The overhead of moving single and groups of pages between Central and Expanded Storage was reduced with the introduction of the MVPG (Move Page) instruction and the ADMF (Asynchronous Data Mover Facility) capability.

The MVPG instruction and ADMF are explicitly invoked—generally by middleware in z/OS or z/VM (and ACP?)—to access data in expanded storage. Some uses are namely:

• MVPG is used by VSAM Local Shared Resources (LSR) buffer pool management to access buffers in a hiperspace in Expanded Storage.
• Both MVPG and ADMF are used by IBM Db2 to access hiperpools. Hiperpools are portions of a buffer pool located in a hiperspace.
• VM Minidisk Caching.

Until the mid-1990s Central and Expanded Storage were physically different areas of memory on the processor. Since the mid-1990s Central and Expanded Storage were merely assignment choices for the underlying processor memory. These choices were made based on specific expected uses: For example, Expanded Storage is required for the Hiperbatch function (which uses the MVPG instruction to access its hiperspaces).

In addition to the hiperspace and paging cases mentioned above there are other uses of expanded storage, including:

• Virtual I/O (VIO) to Expanded Storage which stored temporary data sets in simulated devices in Expanded Storage. (This function has been replaced by VIO in Central Storage.)
• VM Minidisk Caching.

z/OS removed the support for Expanded Storage. All memory in z/OS is now Central Storage. z/VM 6.4 fulfills Statement of Direction to drop support for all use of Expanded Storage.

MVPG and ADMF

MVPG

IBM described MVPG as "moves a single page and the central processor cannot execute any other instructions until the page move is completed." ^[13]

The MVPG mainframe instruction ^[14] (MoVe PaGe, opcode X'B254') has been compared to the MVCL (MoVe Character Long) instruction, both of which can move more than 256 bytes within main memory using a single instruction. These instructions do not comply with definitions for atomicity, although they can be used as a single instruction within documented timing and non-overlap restrictions. ^{[15] : Note 8, page 7–27  [16]}

The need to move more than 256 bytes within main memory had historically been addressed with software ^[17] (MVC loops), MVCL, ^[18] which was introduced with the 1970 announcement of the System/370, and MVPG, patented ^[19] and announced by IBM in 1989, each have advantages. ^[20]

ADMF

ADMF (Asynchronous Data Mover Facility), which was introduced in 1992, goes beyond the capabilities of the MVPG (Move Page) instruction, which is limited to a single page, ^[21] and can move groups of pages between Central and Expanded Storage.

A macro instruction named IOSADMF, which has been described as an API that avoids "direct, low-level use of ADMF", ^[22] can be used to read ^{[NB 1]} or write data to or from a hiperspace. ^[23] Hiperspaces are created using DSPSERV CREATE.

To provide reentrancy, IOSADMF is used together with a "List form" and "Execute form." ^[24]

Non-IBM implementations

Platform Solutions Inc. (PSI) previously marketed Itanium-based servers which were compatible with z/Architecture. IBM bought PSI in July 2008, and the PSI systems are no longer available. ^[25] FLEX-ES, zPDT and the Hercules emulator also implement z/Architecture. Hitachi mainframes running newer releases of the VOS3 operating system implement ESA/390 plus Hitachi-unique CPU instructions, including a few 64-bit instructions. While Hitachi formally collaborated with IBM on the z900-G2/z800 CPUs introduced in 2002, Hitachi's machines are not z/Architecture-compatible.

Notes

1. AREAD - transfer data from a hiperspace to the program's primary address space.

References

1. Development and Attributes of z/Architecture Archived 2013-12-12 at the Wayback Machine , IBM Journal of Research and Development, 2002.
2. "Accommodate functions for the z13 server to be discontinued on future servers". IBM . 25 June 2015. Archived from the original on 2017-09-15. Retrieved 2017-09-18.
3. Preview: IBM z/VM V6.1 - Foundation for future virtualization growth Archived 2021-10-28 at the Wayback Machine , IBM United States Software Announcement 209-207, dated July 7, 2009
4. ALS 1 was 9672 G2; ALS 2 was 9672 G5; ALS 3 was the original z/Architecture: "IBM CMOS Processor Table". 18 November 2008. Archived from the original on 10 December 2013. Retrieved 18 October 2012.
5. "IBM System z10 Business Class (z10 BC) Reference Guide" (PDF). 2008. Archived (PDF) from the original on 2011-03-04. Retrieved 2012-10-18.
6. "z/Architecture Principles of Operation" (PDF). Archived (PDF) from the original on 2020-11-30. Retrieved 2016-01-15.
7. Hoskins, Jim; Frank, Bob (2002). Exploring IBM Eserver Zseries and S/390 Servers. Maximum Press. p. 26. ISBN   1885068913. Archived from the original on 2021-04-27. Retrieved 2017-10-19. VM Data Spaces architecture is standard on all System/390 processors.
8. "CA Defends VSE Policy". InformationWeek. October 21, 1991. p. 15. Computer Associates International is now providing data space technology to VSE/ESA or System/370 users.
9. "Analyzing data in memory". IBM.
10. Hemanth Nandas (October 15, 2007). "What is hiperspace? Which was the first OS to support hiperspace?". Newsgroup:  ibmmainframes.com. Archived from the original on February 2, 2017. Retrieved January 25, 2017. HIGH PERFORMANCE SPACE or "High Performance Dataspace" (author Anuj Dhawan, same date)
11. "CheatSheet #54 zTidBits z/OS Extended Addressing" (PDF). Retrieved 2022-07-17.
12. Sakaki, M.; Samukawa, H.; Honjou, N. (1988). "Effective utilization of IBM 3090 large virtual storage in the numerically intensive computations of ab initio molecular orbitals". IBM Systems Journal. 27 (4): 528–540. doi:10.1147/sj.274.0528. ISSN   0018-8670.
13. US 5442802  Asynchronous co-processor data mover method and means
14. "HLASM - MVPG = MoVe PaGe". Archived from the original on 2013-10-06. Retrieved 2017-01-24.
15. MOVE LONG, note 8. "GA22-7000-10, IBM System/370, Principles of Operation" (PDF). Archived (PDF) from the original on 2021-04-11. Retrieved 2021-10-11.
16. "things are done immediately, and there is no chance of the instruction being half-completed or of another being interspersed. Used especially to convey that an operation cannot be interrupted." "Atomic from FOLDOC".
17. "$MVCL – Move more than 256 bytes of storage". IBM . 20 September 2014. Archived from the original on 2 February 2017. Retrieved 24 January 2017.
18. "Move Long". Archived from the original on 2017-04-27. Retrieved 2017-01-24.
19. US 5237668  Process using virtual addressing in a non-privileged instruction to control the copying of a page of data in or between multiple media
20. "MVPG faster than MVCL for aligned pages?". IBM-MAIN (Mailing list). Archived from the original on 2011-01-22. Retrieved 2017-01-24.
21. IBM's patent EP0549924A1 describes MVPG as "moves a single page."
22. Celestini, Art (Aug 20, 1997). "admf". IBM-MAIN (Mailing list). Archived from the original on 2011-01-22. Retrieved 2017-01-24– via Google Groups.
23. z/OS MVS Programming: Extended Addressability Guide - SA23-1394-00
24. "IOSADMF — Transfer hiperspace data". IBM . 7 February 2015. Archived from the original on 2 February 2017. Retrieved 24 January 2017.
25. "IBM Acquires Platform Solutions" (Press release). IBM. 2008-07-02. Archived from the original on 2008-09-05. Retrieved 2008-09-06.

Further reading

• Preshing on Programming - Atomic vs. Non-Atomic Operations
• Principles of Computer Design - Atomicity