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 3.2.1. Main Storage Attributes

 The main storage of a system typically includes both general-purpose and nonvolatile storage. It also includes special-purpose hardware registers or arrays used for functions such as system configuration, data-transfer synchronization, memory-mapped I/O, and I/O subsystems.

 Table 3.1, “Sizes of Main Storage Address Spaces” lists the sizes of address spaces in main storage.



Table 3.1. Sizes of Main Storage Address Spaces

Address Space



  Real Address Space

 2m bytes

 where m ≤ 60

  Effective Address Space

 2 64 bytes

 An effective address is translated to a virtual address using the segment lookaside buffer (SLB).

  Virtual Address Space

 2n bytes

 where 65 ≤ n ≤ 78

 A virtual address is translated to a real address using the page table.

 Real Page (Base)

 2 12 bytes


 Virtual Page

 2p bytes

 where 12 ≤ p ≤ 28

 Up to eight page sizes can be supported simultaneously. A small 4 KB (p = 12) page is always supported. The number of large pages and their sizes are implementation dependent.

 Segment Size

 2s bytes

 where s = 28 or 40

 The number of virtual segments is 2(n - s) where 65 ≤ n ≤ 78

 Note: The values of "m,"; "n,"; and "p"; are implementation dependent.

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