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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==History==

The [[Burroughs Corporation]] [[Burroughs large systems|B5000]] computer was one of the first to implement segmentation, and "perhaps the first commercial computer to provide virtual memory"<ref>{{cite web |last=Mayer |first=Alastair J. W. |title=The Architecture of the Burroughs B5000 - 20 Years Later and Still Ahead of the Times? |url=http://www.smecc.org/The%20Architecture%20%20of%20the%20Burroughs%20B-5000.htm |access-date=2012-03-15}}</ref> based on segmentation. The B5000 is equipped with a segment information table called the Program Reference Table (PRT) which is used to indicate whether the corresponding segment resides in the main memory, to maintain the [[base address]] and the size of the segment.<ref>{{cite book |title=Computer Architecture and Organization |last=Hayes |first=John P. |isbn=0-07-027363-4 |year=1978 |page=371|publisher=McGraw-Hill }}</ref> The later [[Burroughs large systems#B6500|B6500]] computer also implemented segmentation; a version of its architecture is still in use today on the Unisys ClearPath Libra servers.{{Citation needed|date=April 2024}}

The [[GE 645]] computer, a modification of the [[GE-600 series|GE-635]] with segmentation and paging support added, was designed in 1964 to support [[Multics]].

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==Examples==

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

{{seealso|Burroughs large systems descriptors}}

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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====

Descriptors in the [[B5000]], B5500 and B5700 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.

====B6500B5000, 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

| id = 5000-21005A

| edition = A

| year = 1962

| url = https://bitsavers.org/pdf/burroughs/LargeSystems/B5000_5500_5700/5000-21005_B5000_operChar_1963.pdf

| ___location = Detroit

| publisher = Burroughs

}}

Descriptors</ref> in[[Burroughs thelarge [[B5000systems descriptors|Descriptors]], B5500have andthe uppermost bit set in the word. B5700They 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}}

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 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===