z/Architecture

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")[3][4] and
• hiperspaces (High performance space).[5][6]

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.[7]

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

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.

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 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.

IBM described MVPG as "moves a single page and the central processor cannot execute any other instructions until the page move is completed."[8]

The MVPG mainframe instruction[9] (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.[10]^{:Note 8, page 7–27}[11]

The need to move more than 256 bytes within main memory had historically been addressed with software[12] (MVC loops), MVCL,[13] which was introduced with the 1970 announcement of the System/370, and MVPG, patented[14] and announced by IBM in 1989, each have advantages.[15]

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,[16] 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,"[17][18] can be used to read[NB 1] or write data to or from a hiperspace.[19] Hiperspaces are created using DSPSERV CREATE.

To provide reentrancy, IOSADMF is used together with a "List form" and "Execute form."[20]