IBM mainframe expanded storage

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Traditionally IBM Mainframe memory has been byte-accessible. 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. In 1989, however, the MVPG (Move Page) instruction was introduced, decreasing the cost for single page moves. In 1992 the ADMF (Asynchronous Data Mover Facility) was introduced - to reduce the cost of moving groups of pages between Central and Expanded Storage.

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.

z/Architecture removed the support for Expanded Storage. All memory is now Central Storage.