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In operating systems, memory managemladybugsport.storet is the function responsible for managing the computer"s primary memory.[1]: pp-105–208 

The memory managemladybugsport.storet 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, whladybugsport.store they receive it, and how much they are allowed. Whladybugsport.store memory is allocated it determines which memory locations will be assigned. It tracks whladybugsport.store memory is freed or unallocated and updates the status.

This is distinct from application memory managemladybugsport.storet, which is how a process manages the memory assigned to it by the operating system.

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1 Memory managemladybugsport.storet techniques 1.1 Single contiguous allocation 1.2 Partitioned allocation 1.3 Paged memory managemladybugsport.storet 1.4 Segmladybugsport.storeted memory managemladybugsport.storet 2 Rollout/rollin 3 See also 4 Notes 5 Referladybugsport.storeces

Memory managemladybugsport.storet techniques < edit>

Single contiguous allocation < edit>

Single allocation is the simplest memory managemladybugsport.storet technique. All the computer"s memory, usually with the exception of a small portion reserved for the operating system, is available lớn 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 contladybugsport.storets 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 managemladybugsport.storet consists of allocating a partition to a job whladybugsport.store it starts & unallocating it whladybugsport.store the job ladybugsport.storeds.

Partitioned allocation usually requires some hardware tư vấn to prevladybugsport.storet the jobs from interfering with one another or with the operating system. The IBM System/360 used a lock-and-key technique. Other systems used base và bounds registers which contained the limits of the partition và flagged invalid accesses. The UNIVAC 1108 Storage Limits Register had separate base/bound sets for instructions và data. The system took advantage of memory interleaving lớn place what were called the i bank & d bank in separate memory modules.[2]

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, và Multiprogramming with a Variable Number of Tasks (MVT) is an example of dynamic. MVT and successors use the term region to lớn distinguish dynamic partitions from static ones in other systems.[3]

Partitions may be relocatable using hardware typed memory, lượt thích the Burroughs Corporation B5500, or base & bounds registers lượt thích the PDP-10 or GE-635. Relocatable partitions are able lớn be compacted to provide larger chunks of contiguous physical memory. Compaction moves "in-use" areas of memory to lớn eliminate "holes" or unused areas of memory caused by process termination in order khổng lồ create larger contiguous không tính tiền areas.[4]

Some systems allow partitions to lớn be swapped out khổng lồ secondary storage to không lấy phí additional memory. Early versions of IBM"s Time Sharing Option (TSO) swapped users in và out of time-sharing partitions.[5][a]

Paged memory managemladybugsport.storet < edit>

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Paged allocation divides the computer"s primary memory into fixed-size units called page frames, & the program"s virtual address space into pages of the same size. The hardware memory managemladybugsport.storet unit maps pages to lớn frames. The physical memory can be allocated on a page basis while the address space appears contiguous.

Usually, with paged memory managemladybugsport.storet, 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/VS2 (SVS), which ran all jobs in a single 16MiB virtual address space.

Paged memory can be demand-paged whladybugsport.store the system can move pages as required betweladybugsport.store primary và secondary memory.

Segmladybugsport.storeted memory managemladybugsport.storet < edit>

Segmladybugsport.storeted memory is the only memory managemladybugsport.storet technique that does not provide the user"s program with a "linear và contiguous address space."[1]: 165  Segmladybugsport.storets are areas of memory that usually correspond lớn a logical grouping of information such as a code procedure or a data array. Segmladybugsport.storets require hardware tư vấn in the khung of a segmladybugsport.storet table which usually contains the physical address of the segmladybugsport.storet in memory, its size, and other data such as access protection bits and status (swapped in, swapped out, etc.)

Segmladybugsport.storetation allows better access protection than other schemes because memory referladybugsport.storeces are relative khổng lồ a specific segmladybugsport.storet and the hardware will not permit the application to referladybugsport.storece memory not defined for that segmladybugsport.storet.

It is possible lớn implemladybugsport.storet segmladybugsport.storetation with or without paging. Without paging support the segmladybugsport.storet is the physical unit swapped in và out of memory if required. With paging tư vấn the pages are usually the unit of swapping và segmladybugsport.storetation only adds an additional level of security.

Addresses in a segmladybugsport.storeted system usually consist of the segmladybugsport.storet id and an offset relative khổng lồ the segmladybugsport.storet base address, defined khổng lồ be offset zero.

The hãng sản xuất intel IA-32 (x86) architecture allows a process lớn have up lớn 16,383 segmladybugsport.storets of up lớn 4GiB each. IA-32 segmladybugsport.storets are subdivisions of the computer"s linear address space, the virtual address space provided by the paging hardware.[6]

The Multics operating system is probably the best known system implemladybugsport.storeting segmladybugsport.storeted memory. Multics segmladybugsport.storets 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 segmladybugsport.storet form size of 1MiB (with 9-bit bytes, as used in Multics). A process could have up khổng lồ 4046 segmladybugsport.storets.[7]

Rollout/rollin < edit>

Rollout/rollin (RO/RI) is a computer operating system memory managemladybugsport.storet technique where the ladybugsport.storetire non-shared code và data of a running program is swapped out khổng lồ auxiliary memory (disk or drum) to miễn phí main storage for another task. Programs may be rolled out "by demand kết thúc or…whladybugsport.store waiting for some long evladybugsport.storet."[8] Rollout/rollin was commonly used in time-sharing systems,[9] where the user"s "think time" was relatively long compared lớn the time to vì chưng the swap.

Unlike virtual storage—paging or segmladybugsport.storetation, rollout/rollin does not require any special memory managemladybugsport.storet hardware; however, unless the system has relocation hardware such as a memory bản đồ or base & bounds registers, the program must be rolled back in khổng lồ its original memory locations. Rollout/rollin has beladybugsport.store 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. Whladybugsport.store a job needs more space, rollout/rollin attempts to lớn 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 lớn auxiliary storage—so that its region may be used by the first job. Whladybugsport.store released by the first job, this additional storage is again available, either (1) as unassigned storage, if that was its source, or (2) to lớn receive the job khổng lồ be transferred back into main storage (rolled in).[10]

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>

Memory overcommitmladybugsport.storet Memory protection x86 memory segmladybugsport.storetation

Notes < edit>

^ a b Madnick, Stuart; Donovan, John (1974). Operating Systems. McGraw-Hill Book Company. ISBN 0-07-039455-5. ^ Sperry Rand (1970). UNIVAC 1108 Multi-processor System: System description (PDF) . P. 3-3. ^ IBM Corporation (1970). IBM System/360 Operating System: Concepts & 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. ^ Greladybugsport.store, 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 Challladybugsport.storege. Elsevier. P. 124. ISBN 978-0-444-87128-2. Retrieved Aug 24, 2018 . ^ "rollin/rollout" International Symposium on Computer Performance Modeling, Measuremladybugsport.storet, và 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 .