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Guy Harris (talk | contribs) Clarify that "extended real addressing" on S/370 only let you attach more main memory; virtual addresses were still only 24-bit. Ask for a citation on "[exploiting] 26-bit mode" - what's there to exploit, in problem state? |
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{{Computer architecture bit widths}}
In [[computer architecture]], '''26-bit''' [[integers]], [[memory address|memory addresses]], or other [[data]] units are those that are 26 bits wide. Two examples of computer processors that featured 26-bit memory addressing are certain second generation IBM [[System/370]] [[mainframe computer]] models introduced in 1981 (and several subsequent models), which had 26-bit physical addresses but had only the same 24-bit virtual addresses as earlier models, and the first generations of [[ARM Limited|ARM]] processors.
== History ==
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=== IBM System/370 ===
As [[data processing]] needs continued to grow, IBM and their customers faced challenges directly addressing larger memory sizes. In what ended up being a short-term "emergency" solution, a pair of IBM's second wave of System/370 models, the 3033 and 3081, introduced 26-bit real memory addressing, increasing the System/370's memory address space by a factor of 4 from the previous [[24-bit]] limits. IBM referred to 26-bit addressing as "extended real addressing," and some subsequent models also included 26-bit support. However, only 2 years later, IBM introduced [[31-bit]] memory addressing with its System/370-XA models, and even the popular 3081 was upgradeable to XA standard.
Given 26-bit's brief history as the state-of-the-art in memory addressing available in IBM's model range, and given that virtual addresses were still limited to 24 bits, [[software]] exploitation of 26-bit mode was limited. The few customers that exploited 26-bit mode eventually adjusted their applications to support 31-bit addressing,{{cn}} and IBM dropped support for 26-bit mode after several years producing models supporting 24-bit, 26-bit, and 31-bit modes. The 26-bit mode is the only addressing mode that IBM removed from its line of mainframe computers descended from the [[System/360]]. All the other addressing modes, including now 64-bit mode, are supported in current model mainframes. === Early ARM Processors ===
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