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73-855: An IBM Redbook named "VSAM PRIMER" (especially when used with the "Virtual Storage Access Method (VSAM) Options for Advanced Applications" manual) explains the concepts needed to make use of VSAM. IBM uses the term data set in official documentation as a synonym for file , and direct-access storage device ( DASD ) for devices with random access to data locations, such as disk drives, as opposed to devices such as tape drives that can only be read sequentially. VSAM records can be of fixed or variable length. They are organised in fixed-size blocks called control intervals (CIs), and then into larger divisions called Control Areas (CAs). Control Interval sizes are measured in bytes – for example 4 kilobytes  – while Control Area sizes are measured in disk tracks or cylinders. Control Intervals are

146-626: A magnetic tape drive , and unit record equipment such as a punched card device. A record on a DASD can be accessed without having to read through intervening records from the current location, whereas reading anything other than the "next" record on tape or deck of cards requires skipping over intervening records, and requires a proportionally long time to access a distant point in a medium. Access methods for DASD include sequential, partitioned , indexed , and direct . The DASD storage class includes both fixed and removable media. IBM mainframes access I/O devices including DASD through channels ,

219-493: A CICS environment is severely restricted. Sharing between CICS regions and batch jobs requires Transactional VSAM , DFSMStvs. This is an optional program that builds on VSAM RLS by adding logging and two-phase commit, using underlying z/OS system services. This permits generalised sharing of VSAM data. VSAM was introduced as a replacement for older access methods and was intended to add function, to be easier to use and to overcome problems of performance and device-dependence. VSAM

292-644: A KSDS instead. The RRDS organization was designed to replace BDAM , the Basic Direct Access Method. In some cases, BDAM data sets contained embedded pointers which prevented them from being moved. However, most BDAM data sets did not and the incentive to move from BDAM to VSAM RRDS was much less compelling than that to move from ISAM to VSAM KSDS. Linear data sets were added later, followed by VSAM RLS and then Transactional VSAM. Direct-access storage device A direct-access storage device ( DASD ) (pronounced / ˈ d æ z d iː / )

365-421: A VMM, guest OS or guest application stack needs three. The difference between paging and segmentation systems is not only about memory division; segmentation is visible to user processes, as part of memory model semantics. Hence, instead of memory that looks like a single large space, it is structured into multiple spaces. This difference has important consequences; a segment is not a page with variable length or

438-499: A contiguous address space or collection of contiguous segments . The operating system manages virtual address spaces and the assignment of real memory to virtual memory. Address translation hardware in the CPU, often referred to as a memory management unit (MMU), automatically translates virtual addresses to physical addresses. Software within the operating system may extend these capabilities, utilizing, e.g., disk storage , to provide

511-425: A file (or a segment from a multi-segment file) is mapped into a segment in the address space, so files are always mapped at a segment boundary. A file's linkage section can contain pointers for which an attempt to load the pointer into a register or make an indirect reference through it causes a trap. The unresolved pointer contains an indication of the name of the segment to which the pointer refers and an offset within

584-489: A generalization of the concept of virtual memory. Virtual memory is an integral part of a modern computer architecture ; implementations usually require hardware support, typically in the form of a memory management unit built into the CPU . While not necessary, emulators and virtual machines can employ hardware support to increase performance of their virtual memory implementations. Older operating systems, such as those for

657-416: 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. Paging is not free from fragmentation — the fragmentation is internal to pages ( internal fragmentation ). If a requested block is smaller than a page, then some space in

730-642: A means to improve performance, rather than to solve the problems involved in multi-programming. The first true virtual memory system was that implemented at the University of Manchester to create a one-level storage system as part of the Atlas Computer . It used a paging mechanism to map the virtual addresses available to the programmer onto the real memory that consisted of 16,384 words of primary core memory with an additional 98,304 words of secondary drum memory . The addition of virtual memory into

803-504: A presentation on virtual memory in the Atlas I computer. Paul King took the ideas back to Burroughs and it was determined that virtual memory should be designed into the core of the B5000. . Burroughs Corporation released the B5000 in 1964 as the first commercial computer with virtual memory. IBM developed the concept of hypervisors in their CP-40 and CP-67 , and in 1972 provided it for

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876-433: A problem called “ thrashing ” can occur, in which the computer spends an unsuitably large amount of time transferring pages to and from a backing store, hence slowing down useful work. A task's working set is the minimum set of pages that should be in memory in order for it to make useful progress. Thrashing occurs when there is insufficient memory available to store the working sets of all active programs. Adding real memory

949-482: 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

1022-475: A simple way to lengthen the address space. Segmentation that can provide a single-level memory model in which there is no differentiation between process memory and file system consists of only a list of segments (files) mapped into the process's potential address space. This is not the same as the mechanisms provided by calls such as mmap and Win32 's MapViewOfFile, because inter-file pointers do not work when mapping files into semi-arbitrary places. In Multics,

1095-414: A strong incentive to switch to virtual memory for all systems. The additional capability of providing virtual address spaces added another level of security and reliability, thus making virtual memory even more attractive to the marketplace. Most modern operating systems that support virtual memory also run each process in its own dedicated address space . Each program thus appears to have sole access to

1168-406: A synonym of disk, flash and optical devices. Modern DASD used in mainframes only very rarely consist of single disk-drives. Most commonly "DASD" means large disk arrays utilizing RAID schemes. Current devices emulate CKD on FBA hardware. Virtual memory In computing , virtual memory , or virtual storage , is a memory management technique that provides an "idealized abstraction of

1241-425: A type of subordinate mini-processor. Channel programs write to, read from, and control the given device. The operating system uses a four byte relative track and record (TTR) for some access methods and for others an eight-byte extent-bin-cylinder-track-record block address, or MBBCCHHR , Channel programs address DASD using a six byte seek address ( BBCCHH ) and a five byte record identifier ( CCHHR ). When

1314-453: A virtual address space that can exceed the capacity of real memory and thus reference more memory than is physically present in the computer. The primary benefits of virtual memory include freeing applications from having to manage a shared memory space, ability to share memory used by libraries between processes, increased security due to memory isolation, and being able to conceptually use more memory than might be physically available, using

1387-420: Is a secondary storage device in which "each physical record has a discrete location and a unique address". The term was coined by IBM to describe devices that allowed random access to data, the main examples being drum memory and hard disk drives . Later, optical disc drives and flash memory units are also classified as DASD. The term DASD contrasts with sequential access storage device such as

1460-404: Is an acronym for Count Key Data , the physical layout of a block on a DASD device, and should not be confused with BBCCH and CCHHR, which are the addresses used by the channel program. CTR in this context may refer to either type of address, depending on the channel command. In 1979 IBM introduced fixed block architecture (FBA) for mainframes. At the programming level, these devices do not use

1533-445: Is an unstructured VSAM dataset with a control interval size of a multiple of 4K. It is used by certain system services. There are four types of access techniques for VSAM data: Sharing of VSAM data between CICS regions can be done by VSAM Record-Level Sharing (RLS). This adds record caching and, more importantly, record locking. Logging and commit processing remain the responsibility of CICS which means that sharing of VSAM data outside

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1606-546: Is exactly the situation in computers with cache memory, one of the earliest commercial examples of which was the IBM System/360 Model 85. In the Model 85 all addresses were real addresses referring to the main core store. A semiconductor cache store, invisible to the user, held the contents of parts of the main store in use by the currently executing program. This is exactly analogous to Güntsch's system, designed as

1679-414: Is normally not visible to the user. There will be a fixed number of control intervals in each control area. A control interval normally contains multiple records. The records are stored within the control interval starting from the low address upwards. Control information is stored at the other end of the control interval, starting from the high address and moving downwards. The space between the records and

1752-539: Is supported by VM/370 and DOS/VSE , but not MVS or successor operating systems in the OS/360 line. Processors with FICON channels can access SCSI drives using Fibre Channel Protocol (FCP). While z/VM and z/VSE fully support FCP, z/OS provides only limited support through IOSFBA. Some programming interface macros and routines are collectively referred to as access methods with names ending in A ccess M ethod. DOS/360 through z/VSE support datasets on DASD with

1825-471: Is the simplest response, but improving application design, scheduling, and memory usage can help. Another solution is to reduce the number of active tasks on the system. This reduces demand on real memory by swapping out the entire working set of one or more processes. A system thrashing is often a result of a sudden spike in page demand from a small number of running programs. Swap-token is a lightweight and dynamic thrashing protection mechanism. The basic idea

1898-420: Is to set a token in the system, which is randomly given to a process that has page faults when thrashing happens. The process that has the token is given a privilege to allocate more physical memory pages to build its working set, which is expected to quickly finish its execution and to release the memory pages to other processes. A time stamp is used to handover the token one by one. The first version of swap-token

1971-516: The Intel 80386 and later IA-32 processors, the segments reside in a 32-bit linear, paged address space. Segments can be moved in and out of that space; pages there can "page" in and out of main memory, providing two levels of virtual memory; few if any operating systems do so, instead using only paging. Early non-hardware-assisted x86 virtualization solutions combined paging and segmentation because x86 paging offers only two protection domains whereas

2044-755: The S/370 as Virtual Machine Facility/370. IBM introduced the Start Interpretive Execution ( SIE ) instruction as part of 370-XA on the 3081, and VM/XA versions of VM to exploit it. Before virtual memory could be implemented in mainstream operating systems, many problems had to be addressed. Dynamic address translation required expensive and difficult-to-build specialized hardware; initial implementations slowed down access to memory slightly. There were worries that new system-wide algorithms utilizing secondary storage would be less effective than previously used application-specific algorithms. By 1969,

2117-585: The SDS 940 , used page registers instead of page tables in memory for address translation. This part of the operating system creates and manages page tables and lists of free page frames. In order to ensure that there will be enough free page frames to quickly resolve page faults, the system may periodically steal allocated page frames, using a page replacement algorithm , e.g., a Least recently used (LRU) algorithm. Stolen page frames that have been modified are written back to auxiliary storage before they are added to

2190-505: The Technische Universität Berlin in 1956 in his doctoral thesis, Logical Design of a Digital Computer with Multiple Asynchronous Rotating Drums and Automatic High Speed Memory Operation does not stand up to careful scrutiny. The computer proposed by Güntsch (but never built) had an address space of 10 words which mapped exactly onto the 10 words of the drums, i.e. the addresses were real addresses and there

2263-466: The mainframes of the 1960s, and those for personal computers of the early to mid-1980s (e.g., DOS ), generally have no virtual memory functionality, though notable exceptions for mainframes of the 1960s include: During the 1960s and early '70s, computer memory was very expensive. The introduction of virtual memory provided an ability for software systems with large memory demands to run on computers with less real memory. The savings from this provided

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2336-550: The 2321 data cell was discontinued in January 1975, the addressing scheme and the device itself was referred to as CHR or CTR for cylinder-track-record, as the bin number was always 0. IBM refers to the data records programmers work with as logical records , and the format on DASD as blocks or physical records . One block might contain several logical (or user) records or, in some schemes, called spanned records , partial logical records. Physical records can have any size up to

2409-447: The Atlas also eliminated a looming programming problem: planning and scheduling data transfers between main and secondary memory and recompiling programs for each change of size of main memory. The first Atlas was commissioned in 1962 but working prototypes of paging had been developed by 1959. As early as 1958, Robert S. Barton , working at Shell Research, suggested that main storage should be allocated automatically rather than have

2482-468: The OS has a special facility for "fast fixing" these short-term fixed data buffers (fixing which is performed without resorting to a time-consuming Supervisor Call instruction ). Multics used the term "wired". OpenVMS and Windows refer to pages temporarily made nonpageable (as for I/O buffers) as "locked", and simply "nonpageable" for those that are never pageable. The Single UNIX Specification also uses

2555-429: The OS writes those pages and segments currently in real memory to swap files. In a swap-in, the OS reads back the data from the swap files but does not automatically read back pages that had been paged out at the time of the swap out operation. IBM's MVS , from OS/VS2 Release 2 through z/OS , provides for marking an address space as unswappable; doing so does not pin any pages in the address space. This can be done for

2628-431: The control information is free space. The control information comprises two types of entry: a control interval descriptor field (CIDF) which is always present, and record descriptor fields (RDF) which are present when there are records within the control interval and describe the length of the associated record. Free space within a CI is always contiguous. When records are inserted into a control interval, they are placed in

2701-426: The correct order relative to other records. This may require records to be moved out of the way inside the control interval. Conversely, when a record is deleted, later records are moved down so that the free space remains contiguous. If there is not enough free space in a control interval for a record to be inserted, the control interval is split. Roughly half the records are stored in the original control interval while

2774-571: The debate over virtual memory for commercial computers was over; an IBM research team led by David Sayre showed that their virtual memory overlay system consistently worked better than the best manually controlled systems. Throughout the 1970s, the IBM 370 series running their virtual-storage based operating systems provided a means for business users to migrate multiple older systems into fewer, more powerful, mainframes that had improved price/performance. The first minicomputer to introduce virtual memory

2847-727: The details: In most cases, there will be an update to the page table, possibly followed by purging the Translation Lookaside Buffer (TLB), and the system restarts the instruction that causes the exception. If the free page frame queue is empty then the paging supervisor must free a page frame using the same page replacement algorithm for page stealing. Operating systems have memory areas that are pinned (never swapped to secondary storage). Other terms used are locked , fixed , or wired pages. For example, interrupt mechanisms rely on an array of pointers to their handlers, such as I/O completion and page fault . If

2920-597: The duration of a job by entering the name of an eligible main program in the Program Properties Table with an unswappable flag. In addition, privileged code can temporarily make an address space unswappable using a SYSEVENT Supervisor Call instruction (SVC); certain changes in the address space properties require that the OS swap it out and then swap it back in, using SYSEVENT TRANSWAP. Swapping does not necessarily require memory management hardware, if, for example, multiple jobs are swapped in and out of

2993-580: The first place, to relieve programmers of such memory considerations. In multi-processing systems, optimal operation of the system depends on the mix of independent processes at any time. Hybrid schemes of segmentation and paging may be used. The Intel 80286 supports a similar segmentation scheme as an option, but it is rarely used. Segmentation and paging can be used together by dividing each segment into pages; systems with this memory structure, such as Multics and IBM System/38 , are usually paging-predominant, segmentation providing memory protection. In

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3066-463: The following access methods: OS/360 through z/OS support datasets on DASD with the following access methods: In MVS , starting with OS/VS2 Release 2 and continuing through z/OS , all of the access methods including EXCP[VR], use the privileged STARTIO macro. IBM in its 1964 first version of the "IBM System/360 System Summary" used the term File to collectively described devices now called DASD. Files provided "random access storage'" At

3139-449: The free queue. On some systems the paging supervisor is also responsible for managing translation registers that are not automatically loaded from page tables. Typically, a page fault that cannot be resolved results in an abnormal termination of the application. However, some systems allow the application to have exception handlers for such errors. The paging supervisor may handle a page fault exception in several different ways, depending on

3212-460: The index component and the data component. These may be stored on separate disk volumes. While a basic KSDS only has one key (the primary key), alternate indices may be defined to permit the use of additional fields as secondary keys. An alternate index (AIX) is itself a KSDS. The data structure used by a KSDS is nowadays known as a B+ tree . An RRDS may have an index defined to it to enable access via keys, by defining an Alternate Index. An LDS

3285-400: The limit of a track, but some devices have a track overflow feature that allows breaking a large block into track-size segments within the same cylinder. The queued access methods, such as QSAM , are responsible for blocking and deblocking logical records as they are written to or read from external media. The basic access methods, such as BSAM , require the user program to do it. CKD

3358-540: The organizations used by other access methods, as follows. A VSAM file is defined as a cluster of VSAM components, e.g., for KSDS a DATA component and an INDEX component. VSAM components consist of fixed length physical blocks grouped into fixed length control intervals (CI) and control areas (CA). The size of the CI and CA is determined by the Access Method Services (AMS), and the way in which they are used

3431-625: The page fault exception to chain with other exceptions without double fault . However, loading segment descriptors was an expensive operation, causing operating system designers to rely strictly on paging rather than a combination of paging and segmentation. Nearly all current implementations of virtual memory divide a virtual address space into pages , blocks of contiguous virtual memory addresses. Pages on contemporary systems are usually at least 4 kilobytes in size; systems with large virtual address ranges or amounts of real memory generally use larger page sizes. Page tables are used to translate

3504-509: The page is stored. When a reference is made to a page by the hardware, if the page table entry for the page indicates that it is not currently in real memory, the hardware raises a page fault exception , invoking the paging supervisor component of the operating system . Systems can have, e.g., one page table for the whole system, separate page tables for each address space or process, separate page tables for each segment; similarly, systems can have, e.g., no segment table, one segment table for

3577-419: The page will be wasted. If a block requires larger than a page, a small area in another page is required resulting in large wasted space. The fragmentation thus becomes a problem passed to programmers who may well distort their program to match certain page sizes. With segmentation, the fragmentation is external to segments ( external fragmentation ) and thus a system problem, which was the aim of virtual memory in

3650-500: The pages containing these pointers or the code that they invoke were pageable, interrupt-handling would become far more complex and time-consuming, particularly in the case of page fault interruptions. Hence, some part of the page table structures is not pageable. Some pages may be pinned for short periods of time, others may be pinned for long periods of time, and still others may need to be permanently pinned. For example: In IBM's operating systems for System/370 and successor systems,

3723-608: 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 Procrustean in nature, a story they would later use for the exact data sizes in the B1000 . In the Burroughs and Unisys systems, each memory segment is described by a master descriptor which is a single absolute descriptor which may be referenced by other relative (copy) descriptors, effecting sharing either within

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3796-474: The priority: the higher the number of swap-out pages of a process, the longer the time stamp for it will be. Some systems, such as the Burroughs B5500, and the current Unisys MCP systems 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

3869-606: The programmer being concerned with overlays from secondary memory, in effect virtual memory. By 1960 Barton was lead architect on the Burroughs B5000 project. From 1959 to 1961, W.R. Lonergan was manager of the Burroughs Product Planning Group which included Barton, Donald Knuth as consultant, and Paul King. In May 1960, UCLA ran a two-week seminar ‘Using and Exploiting Giant Computers’ to which Paul King and two others were sent. Stan Gill gave

3942-881: The remaining records are moved into a new control interval. The new control interval is taken from a pool of free control intervals within the same control area as the original control interval. If there is no remaining free control interval within that control area, the control area itself is split and the control intervals are distributed equally between the old and the new control areas. You can use three types of record-orientated file organization with VSAM (the contents of linear data sets have no record structure): An ESDS may have an index defined to it to enable access via keys, by defining an Alternate Index. Records in ESDS are stored in order in which they are written by address access. Records are loaded irrespective of their contents and their byte addresses cannot be changed. A KSDS has two parts:

4015-529: The same time IBM's product reference manual described such devices as "direct access storage devices " without any acronym. An early public use of the acronym DASD is in IBM's March 1966 manual, "Data File Handbook. " The earliest non-IBM use of the acronym DASD found by the "Google ngram viewer" to refer to storage devices dates from 1968. From then on use of the term grew exponentially until 1990 after which its usage declined substantially. Both drums and data cells have disappeared as products, so DASD remains as

4088-466: 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, 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

4161-618: The segment; the handler for the trap maps the segment into the address space, puts the segment number into the pointer, changes the tag field in the pointer so that it no longer causes a trap, and returns to the code where the trap occurred, re-executing the instruction that caused the trap. This eliminates the need for a linker completely and works when different processes map the same file into different places in their private address spaces. Some operating systems provide for swapping entire address spaces , in addition to whatever facilities they have for paging and segmentation. When this occurs,

4234-401: The storage resources that are actually available on a given machine" which "creates the illusion to users of a very large (main) memory". The computer's operating system , using a combination of hardware and software, maps memory addresses used by a program, called virtual addresses , into physical addresses in computer memory . Main storage , as seen by a process or task, appears as

4307-495: The technique of paging or segmentation. Virtual memory makes application programming easier by hiding fragmentation of physical memory; by delegating to the kernel the burden of managing the memory hierarchy (eliminating the need for the program to handle overlays explicitly); and, when each process is run in its own dedicated address space, by obviating the need to relocate program code or to access memory with relative addressing . Memory virtualization can be considered

4380-613: The term "locked" in the specification for mlock () , as do the mlock () man pages on many Unix-like systems. In OS/VS1 and similar OSes, some parts of systems memory are managed in "virtual-real" mode, called "V=R". In this mode every virtual address corresponds to the same real address. This mode is used for interrupt mechanisms, for the paging supervisor and page tables in older systems, and for application programs using non-standard I/O management. For example, IBM's z/OS has 3 modes (virtual-virtual, virtual-real and virtual-fixed). When paging and page stealing are used,

4453-446: The term is "fixed", and such pages may be long-term fixed, or may be short-term fixed, or may be unfixed (i.e., pageable). System control structures are often long-term fixed (measured in wall-clock time, i.e., time measured in seconds, rather than time measured in fractions of one second) whereas I/O buffers are usually short-term fixed (usually measured in significantly less than wall-clock time, possibly for tens of milliseconds). Indeed,

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4526-611: The traditional CHR addressing, but reference fixed-length blocks by number, much like sectors in mini-computers. More correctly, the application programmer remains unaware of the underlying storage arrangement, which stores the data in fixed physical block lengths of 512, 1024, 2048, or 4096, depending on the device type. As part of the FBA interface IBM introduced new channel commands for asynchronous operation that are very similar to those introduced for ECKD. For some applications, FBA not only offers simplicity, but an increase in throughput. FBA

4599-476: The trap requires that data be read into main memory from secondary memory. The hardware to translate virtual addresses to physical addresses typically requires a significant chip area to implement, and not all chips used in embedded systems include that hardware, which is another reason some of those systems do not use virtual memory. In the 1950s, all larger programs had to contain logic for managing primary and secondary storage, such as overlaying . Virtual memory

4672-911: The units of transfer between disk and computer so a read request will read one complete Control Interval. Control Areas are the units of allocation so, when a VSAM data set is defined, an integral number of Control Areas will be allocated. The Access Method Services utility program IDCAMS is commonly used to manipulate ("delete and define") VSAM data sets. Custom programs can access VSAM datasets through Data Definition (DD) statements in Job Control Language (JCL), via dynamic allocation or in online regions such as in Customer Information Control System (CICS). Both IMS/DB and Db2 are implemented on top of VSAM and use its underlying data structures . The physical organization of VSAM data sets differs considerably from

4745-424: The virtual addresses seen by the application into physical addresses used by the hardware to process instructions; such hardware that handles this specific translation is often known as the memory management unit . Each entry in the page table holds a flag indicating whether the corresponding page is in real memory or not. If it is in real memory, the page table entry will contain the real memory address at which

4818-589: The virtual memory. However, some older operating systems (such as OS/VS1 and OS/VS2 SVS ) and even modern ones (such as IBM i ) are single address space operating systems that run all processes in a single address space composed of virtualized memory. Embedded systems and other special-purpose computer systems that require very fast and/or very consistent response times may opt not to use virtual memory due to decreased determinism ; virtual memory systems trigger unpredictable traps that may produce unwanted and unpredictable delays in response to input, especially if

4891-567: The whole system, separate segment tables for each address space or process, separate segment tables for each region in a tree of region tables for each address space or process. If there is only one page table, different applications running at the same time use different parts of a single range of virtual addresses. If there are multiple page or segment tables, there are multiple virtual address spaces and concurrent applications with separate page tables redirect to different real addresses. Some earlier systems with smaller real memory sizes, such as

4964-555: Was designed to replace ISAM , the Indexed Sequential Access Method. Changes in disk technology had meant that searching for data in ISAM data sets had become very inefficient. It was also difficult to move ISAM data sets as there were embedded pointers to physical disk locations which became invalid if the data set was moved. IBM also provided a compatibility interface to allow programs coded to use ISAM to use

5037-456: Was implemented in Linux 2.6. The second version is called preempt swap-token and is also in Linux 2.6. In this updated swap-token implementation, a priority counter is set for each process to track the number of swap-out pages. The token is always given to the process with a high priority, which has a high number of swap-out pages. The length of the time stamp is not a constant but is determined by

5110-491: Was introduced in the 1970s when IBM announced virtual storage operating systems (DOS/VS, OS/VS1 and OS/VS2) for its new System/370 series, as successors of the DOS/360 and OS/360 operating systems running on its System/360 computer series. While backwards compatibility was maintained, the older access methods suffered from performance problems due to the address translation required for virtual storage. The KSDS organization

5183-500: Was no form of indirect mapping, a key feature of virtual memory. What Güntsch did invent was a form of cache memory , since his high-speed memory was intended to contain a copy of some blocks of code or data taken from the drums. Indeed, he wrote (as quoted in translation ): "The programmer need not respect the existence of the primary memory (he need not even know that it exists), for there is only one sort of addresses ( sic ) by which one can program as if there were only one storage." This

5256-485: Was the Norwegian NORD-1 ; during the 1970s, other minicomputers implemented virtual memory, notably VAX models running VMS . Virtual memory was introduced to the x86 architecture with the protected mode of the Intel 80286 processor, but its segment swapping technique scaled poorly to larger segment sizes. The Intel 80386 introduced paging support underneath the existing segmentation layer, enabling

5329-489: Was therefore introduced not only to extend primary memory, but to make such an extension as easy as possible for programmers to use. To allow for multiprogramming and multitasking , many early systems divided memory between multiple programs without virtual memory, such as early models of the PDP-10 via registers . A claim that the concept of virtual memory was first developed by German physicist Fritz-Rudolf Güntsch at

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