Editing Memory management (section)

==Memory management in Burroughs/Unisys MCP systems{{r|Unisys-MCP-Memory}}==

An operating system manages various resources in the computing system. The memory subsystem is the system element for managing memory. The memory subsystem combines the hardware memory resource and the MCP OS software that manages the resource.

The memory subsystem manages the physical memory and the virtual memory of the system (both part of the hardware resource). The virtual memory extends physical memory by using extra space on a peripheral device, usually disk. The memory subsystem is responsible for moving code and data between main and virtual memory in a process known as overlaying. Burroughs was the first commercial implementation of virtual memory (although developed at Manchester University for the Ferranti Atlas computer) and integrated virtual memory with the system design of the B5000 from the start (in 1961) needing no external memory management unit (MMU).<ref name="Waychoff 1979">{{cite web |last1=Waychoff |first1=Richard |title=Stories About the B5000 and People Who Were There |url= https://archive.computerhistory.org/resources/access/text/2016/06/102724640-05-01-acc.pdf |website=Computer History Museum}}</ref>{{rp|48}}

The memory subsystem is responsible for mapping logical requests for memory blocks to physical portions of memory (segments) which are found in the list of free segments. Each allocated block is managed by means of a segment descriptor,<ref name="MCP Descriptor">{{cite book |title=The Descriptor |url=http://www.bitsavers.org/pdf/burroughs/LargeSystems/B5000_5500_5700/5000-20002-P_The_Descriptor_-_A_Definition_of_the_B_5000_Information_Processing_System_196102.pdf |publisher=[[Burroughs Corporation]] |date=February 1961}}</ref> a special control word containing relevant metadata about the segment including address, length, machine type, and the p-bit or ‘presence’ bit which indicates whether the block is in main memory or needs to be loaded from the address given in the descriptor.

[[Burroughs large systems descriptors|Descriptors]] are essential in providing memory safety and security so that operations cannot overflow or underflow the referenced block (commonly known as buffer overflow). Descriptors themselves are protected control words that cannot be manipulated except for specific elements of the MCP OS (enabled by the UNSAFE block directive in [[NEWP]]).

Donald Knuth describes a similar system in Section 2.5 ‘Dynamic Storage Allocation’ of [[The Art of Computer Programming|‘Fundamental Algorithms’]].{{disputed inline|talk=Memory management#Burroughs/Unisys MCP memory management system discussed in Knuth?|date=November 2024}}