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→Segmented virtual memory: Not all segmented systems have Burroughs Large Systems-style descriptors (the GE 645 and successors didn't).
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Some systems, such as the [[Burroughs Corporation|Burroughs]] B5500,<ref>{{cite book|author=Burroughs|id=1021326|title=Burroughs B5500 Information Processing System Reference Manual|url=http://bitsavers.org/pdf/burroughs/B5000_5500_5700/1021326_B5500_RefMan_May67.pdf|year=1964|publisher=[[Burroughs Corporation]]|access-date=28 November 2013}}</ref> and the current Unisys MCP systems<ref name=“MCP-VM">{{cite web |title=Unisys MCP Virtual Memory |url= https://public.support.unisys.com/aseries/docs/ClearPath-MCP-20.0/86000387-514/section-000023206.html |website=Unisys}}</ref> use segmentation instead of paging, dividing virtual address spaces into variable-length segments. Using segmentation matches the allocated memory blocks to the logical needs and requests of the programs, rather than the physical view of a computer, although pages themselves are an artificial division in memory. The designers of the B5000 would have found the artificial size of pages to be [[Procrustes|Procrustean]] in nature, a story they would later use for the exact data sizes in the [[Burroughs B1700| B1000]].<ref name=“B1000-Procrustes">{{cite web |title=Design of the Burroughs B1700 |url= https://dl.acm.org/doi/10.1145/1479992.1480060 |website=ACM}}</ref>
 
EachIn those systems, each memory segment is described by a master [[Data descriptor|descriptor]] which is a single absolute descriptor which may be referenced by other relative (copy) descriptors, effecting sharing either within a process or between processes. Descriptors are central to the working of virtual memory in MCP systems. Descriptors contain not only the address of a segment, but the segment length and status in virtual memory indicated by the ‘p-bit’ or ‘presence bit’ which indicates if the address is to a segment in main memory or to a secondary-storage block. When a non-resident segment (p-bit is off) is accessed, an interrupt occurs to load the segment from secondary storage at the given address, or if the address itself is 0 then allocate a new block. In the latter case, the length field in the descriptor is used to allocate a segment of that length.
 
A further problem to thrashing in using a segmented scheme is checkerboarding,<ref name="Checkerboarding">{{cite web |title=Checkerboarding |url= https://public.support.unisys.com/aseries/docs/ClearPath-MCP-20.0/86000387-514/section-000023184.html |website=Unisys}}</ref> where all free segments become too small to satisfy requests for new segments. The solution is to perform memory compaction to pack all used segments together and create a large free block from which further segments may be allocated. Since there is a single master descriptor for each segment the new block address only needs to be updated in a single descriptor, since all copies refer to the master descriptor.
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