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memory management operating systems wikipedia jump to content main menu main menu move to sidebar hide navigation main page contents current events random article about wikipedia contact us contribute help learn to edit community portal recent changes upload file special pages search search appearance donate create account log in personal tools donate create account log in contents move to sidebar hide top 1 memory management techniques toggle memory management techniques subsection 1 1 single contiguous allocation 1 2 partitioned allocation 1 3 paged memory management 1 4 segmented memory management 2 rollout rollin 3 see also 4 notes 5 references toggle the table of contents memory management operating systems 4 languages العربية български فارسی русский edit links article talk english read edit view history tools tools move to sidebar hide actions read edit view history general what links here related changes upload file permanent link page information cite this page get shortened url switch to legacy parser print export download as pdf printable version in other projects wikidata item appearance move to sidebar hide from wikipedia the free encyclopedia function of computer operating systems in operating systems memory management is the function responsible for managing the computer s primary memory 1 105 208 the memory management function keeps track of the status of each memory location either allocated or free it determines how memory is allocated among competing processes deciding which gets memory when they receive it and how much they are allowed when memory is allocated it determines which memory locations will be assigned it tracks when memory is freed or unallocated and updates the status this is distinct from application memory management which is how a process manages the memory assigned to it by the operating system memory management techniques edit single contiguous allocation edit single allocation is the simplest memory management technique all the computer s memory usually with the exception of a small portion reserved for the operating system is available to a single application ms dos is an example of a system that allocates memory in this way an embedded system running a single application might also use this technique a system using single contiguous allocation may still multitask by swapping the contents of memory to switch among users early versions of the music operating system used this technique partitioned allocation edit partitioned allocation divides primary memory into multiple memory partitions usually contiguous areas of memory each partition might contain all the information for a specific job or task memory management consists of allocating a partition to a job when it starts and unallocating it when the job ends partitioned allocation usually requires some hardware support to prevent the jobs from interfering with one another or with the operating system the ibm system 360 uses a lock and key technique the univac 1108 pdp 6 and pdp 10 and ge 600 series use base and bounds registers to indicate the ranges of accessible memory partitions may be either static that is defined at initial program load ipl or boot time or by the computer operator or dynamic that is automatically created for a specific job ibm system 360 operating system multiprogramming with a fixed number of tasks mft is an example of static partitioning and multiprogramming with a variable number of tasks mvt is an example of dynamic mvt and successors use the term region to distinguish dynamic partitions from static ones in other systems 2 partitions may be relocatable with base registers as in the univac 1108 pdp 6 and pdp 10 and ge 600 series relocatable partitions are able to be compacted to provide larger chunks of contiguous physical memory compaction moves in use areas of memory to eliminate holes or unused areas of memory caused by process termination in order to create larger contiguous free areas 3 some systems allow partitions to be swapped out to secondary storage to free additional memory early versions of ibm s time sharing option tso swapped users in and out of time sharing partitions 4 a paged memory management edit main article virtual memory paged allocation divides the computer s primary memory into fixed size units called page frames and the program s virtual address space into pages of the same size the hardware memory management unit maps pages to frames the physical memory can be allocated on a page basis while the address space appears contiguous usually with paged memory management each job runs in its own address space however there are some single address space operating systems that run all processes within a single address space such as ibm i which runs all processes within a large address space and ibm os vs1 and os vs2 svs which ran all jobs in a single 16mib virtual address space paged memory can be demand paged when the system can move pages as required between primary and secondary memory segmented memory management edit main article memory segmentation segmented memory is the only memory management technique that does not provide the user s program with a linear and contiguous address space 1 165 segments are areas of memory that usually correspond to a logical grouping of information such as a code procedure or a data array segments require hardware support in the form of a segment table which usually contains the physical address of the segment in memory its size and other data such as access protection bits and status swapped in swapped out etc segmentation allows better access protection than other schemes because memory references are relative to a specific segment and the hardware will not permit the application to reference memory not defined for that segment it is possible to implement segmentation with or without paging without paging support the segment is the physical unit swapped in and out of memory if required with paging support the pages are usually the unit of swapping and segmentation only adds an additional level of security addresses in a segmented system usually consist of the segment id and an offset relative to the segment base address defined to be offset zero the intel ia 32 x86 architecture allows a process to have up to 16 383 segments of up to 4gib each ia 32 segments are subdivisions of the computer s linear address space the virtual address space provided by the paging hardware 5 the multics operating system is probably the best known system implementing segmented memory multics segments are subdivisions of the computer s physical memory of up to 256 pages each page being 1k 36 bit words in size resulting in a maximum segment size of 1mib with 9 bit bytes as used in multics a process could have up to 4046 segments 6 rollout rollin edit not to be confused with roll on roll off rollout rollin ro ri is a computer operating system memory management technique where the entire non shared code and data of a running program is swapped out to auxiliary memory disk or drum to free main storage for another task programs may be rolled out by demand end or when waiting for some long event 7 rollout rollin was commonly used in time sharing systems 8 where the user s think time was relatively long compared to the time to do the swap unlike virtual storage paging or segmentation rollout rollin does not require any special memory management hardware however unless the system has relocation hardware such as a memory map or base and bounds registers the program must be rolled back in to its original memory locations rollout rollin has been largely superseded by virtual memory rollout rollin was an optional feature of os 360 multiprogramming with a variable number of tasks mvt rollout rollin allows the temporary dynamic expansion of a particular job beyond its originally specified region when a job needs more space rollout rollin attempts to obtain unassigned storage for the job s use if there is no such unassigned storage another job is rolled out i e is transferred to auxiliary storage so that its region may be used by the first job when released by the first job this additional storage is again available either 1 as unassigned storage if that was its source or 2 to receive the job to be transferred back into main storage rolled in 9 in os 360 rollout rollin was used only for batch jobs and rollin does not occur until the jobstep borrowing the region terminates see also edit base and bounds memory overcommitment memory protection region based memory management x86 memory segmentation notes edit known as tso regions references edit 1 2 madnick stuart donovan john 1974 operating systems mcgraw hill book company isbn 0 07 039455 5 ibm corporation 1970 ibm system 360 operating system concepts and facilities pdf p 73 samanta d 2004 classic data structures phi learning pvt ltd p 94 isbn 8120318749 ibm corporation 1972 ibm system 360 operating system time sharing option guide pdf p 10 gc28 6698 5 intel corporation ia 32 intel architecture software developer s manual volume 1 basic architecture green paul multics virtual memory tutorial and reflections archived from the original on 2001 03 05 retrieved may 9 2012 walraet bob 2014 programming the impossible challenge elsevier p 124 isbn 978 0 444 87128 2 retrieved aug 24 2018 rollin rollout international symposium on computer performance modeling measurement and evaluation association for computing machinery march 29 31 1976 p 137 retrieved aug 24 2018 ibm corporation june 1970 ibm system 360 operating system concepts and facilities pdf p 55 retrieved aug 24 2018 v t e memory management memory management as a function of an operating system hardware memory management unit mmu translation lookaside buffer tlb input output memory management unit iommu virtual memory demand paging memory paging page table virtual memory compression segmentation protected mode real mode virtual 8086 mode x86 memory segmentation allocator dlmalloc hoard jemalloc libumem mimalloc ptmalloc manual means static memory allocation c dynamic memory allocation new and delete c garbage collection automatic reference counting boehm garbage collector cheney s algorithm concurrent mark sweep collector finalizer garbage garbage first collector mark compact algorithm reference counting tracing garbage collection strong reference weak reference safety buffer overflow buffer over read dangling pointer stack overflow issues fragmentation memory leak unreachable memory other automatic variable constructors destructors region based memory management memory pool memory management virtual memory automatic memory management memory management algorithms memory management software v t e operating systems general comparison forensic engineering history list timeline usage share user features comparison variants disk operating system distributed operating system embedded operating system hobbyist operating system just enough operating system mobile operating system network operating system object oriented operating system real time operating system supercomputer operating system kernel architectures exokernel hybrid microkernel monolithic multikernel vkernel rump kernel unikernel components device driver loadable kernel module user space and kernel space process management concepts computer multitasking cooperative preemptive context switch interrupt ipc process process control block real time thread time sharing scheduling algorithms fixed priority preemptive multilevel feedback queue round robin shortest job next memory management resource protection bus error general protection fault memory paging memory protection protection ring segmentation fault virtual memory storage access file systems boot loader defragmentation device file file attribute inode journal partition virtual file system virtual tape library supporting concepts api computer network hal live cd live usb shell cli user interface pxe retrieved from https en wikipedia org w index php title memory_management_ operating_systems oldid 1329564368 categories memory management operating systems hidden categories articles with short description short description is different from wikidata this page was last edited on 26 december 2025 at 20 25 utc page was rendered with parsoid text is available under the creative commons attribution sharealike 4 0 license additional terms may apply by using this site you agree to the terms of use and privacy policy wikipedia is a registered trademark of the wikimedia foundation inc a non profit organization privacy policy about wikipedia disclaimers contact wikipedia legal safety contacts code of conduct developers statistics cookie statement mobile view search search toggle the table of contents memory management operating systems 4 languages add topic