Memory segmentation: Difference between revisions

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{{Short description|The divisionDivision of computer's primary memory into separately relocatable segments or sections}}

{{About|segmented computer memory|segments in object code|Object file}}

{{Use dmy dates|date=July 2022|cs1-dates=y}}

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===[[Burroughs Large Systems|Burroughs large system architectures]]===

{{seealso|Burroughs large systems descriptors}}

Burroughs introduced segmentation on the [[B5000]], followed by the compatible B5500 and B5700.

The later [[B6500]] replaced the Program Reference Table (PRT) with a [[Saguaro stack]], changed control word formats, changed [[Burroughs large systems descriptors|descriptor]] formats and changed the mechanism for referring to a control word or descriptor.

====B5000, B5500 and B5700====

{{see also|B5000 instruction set|Burroughs large systems descriptors#B5000, B5500 and B5700}}

Words in the [[Burroughs Large Systems#B5000, B5500, and B5700|B5000, B5500 and B5700]] are 48 bits long.<ref name=oper>{{Citation

| title = The Operational Characteristic of the Processors for the Burroughs B 5000

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</ref> [[Burroughs large systems descriptors|Descriptors]] have the uppermost bit set in the word. They reside in either the Program Reference Table (PRT) or the stack, and contain a ''presence bit'' indicating whether the data are present in memory. There are distinct data and program descriptors.<ref name=oper />{{rp|pages = 4{{hyp}}2-4{{hyp}}4}}

====B6500, B7500 and successors====

{{see also|Burroughs B6x00-7x00 instruction set|Burroughs large systems descriptors#B6500, B7500 and successors}}

The B6000 line, starting with the B6500, was the same architecture as the B5500, but larger and faster machines in the same family.

Words in the B6500 and its successors have 48 bits of data and 3 tag bits.<!--4 bits in later machines?--><ref name="b6500-ref-man">{{cite book |url=http://www.bitsavers.org/pdf/burroughs/LargeSystems/B6500_6700/1043676_B6500_RefMan_Sep69.pdf |title=B6500 Information Processing Systems Reference Manual |publisher=Burroughs |date=September 1969}}</ref>{{rp|page=2{{hyp}}1}} The tag bits indicate the type of data contained in the word; there are several descriptor types, indicated by different tag bit values.<ref name="b6500-ref-man" />{{rp|pages=6{{hyp}}5-6{{hyp}}10}} Control words and descriptors reside in the Saguaro stack. Array segments may be paged.

The line continued until the B6900, then the A series of Unisys and continuing until the current Clearpath MCP systems (Libra).

The line includes the B6500, B6700, B7700, B6800, B6900, B5900, the A-series Burroughs and Unisys machines, and the current Clearpath MCP systems (Libra). While there have been a few enhancements over the years, particularly hardware advances, the architecture has changed little. The segmentation scheme has remained the same, see [[Virtual Memory#Segmented_virtual_memory|Segmented memory]].

===Multics architectures===