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Basic Indexed Sequential Access Method
Indexed Sequential Access Method (ISAM) is a method for creating, maintaining, and manipulating computer files of data so that records can be retrieved sequentially or randomly by one or more keys. Indexes of key fields are maintained to achieve fast retrieval of required file records in indexed files. IBM originally developed ISAM for mainframe computers, but implementations are available for most computer systems. The term ''ISAM'' is used for several related concepts: *The IBM ISAM product and the algorithm it employs. *A database system where an application developer directly uses an application programming interface to search indexes in order to locate records in data files. In contrast, a relational database uses a query optimizer which automatically selects indexes. *An indexing algorithm that allows both sequential and keyed access to data. Most databases use some variation of the B-tree for this purpose, although the original IBM ISAM and VSAM implementations did not d ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Computer File
A computer file is a System resource, resource for recording Data (computing), data on a Computer data storage, computer storage device, primarily identified by its filename. Just as words can be written on paper, so too can data be written to a computer file. Files can be shared with and transferred between computers and Mobile device, mobile devices via removable media, Computer networks, networks, or the Internet. Different File format, types of computer files are designed for different purposes. A file may be designed to store a written message, a document, a spreadsheet, an Digital image, image, a Digital video, video, a computer program, program, or any wide variety of other kinds of data. Certain files can store multiple data types at once. By using computer programs, a person can open, read, change, save, and close a computer file. Computer files may be reopened, modified, and file copying, copied an arbitrary number of times. Files are typically organized in a file syst ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Client–server Model
The client–server model is a distributed application structure that partitions tasks or workloads between the providers of a resource or service, called servers, and service requesters, called clients. Often clients and servers communicate over a computer network on separate hardware, but both client and server may be on the same device. A server host runs one or more server programs, which share their resources with clients. A client usually does not share its computing resources, but it requests content or service from a server and may share its own content as part of the request. Clients, therefore, initiate communication sessions with servers, which await incoming requests. Examples of computer applications that use the client–server model are email, network printing, and the World Wide Web. Client and server role The server component provides a function or service to one or many clients, which initiate requests for such services. Servers are classified by the servic ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Self-modifying Code
In computer science, self-modifying code (SMC or SMoC) is source code, code that alters its own instruction (computer science), instructions while it is execution (computing), executing – usually to reduce the instruction path length and improve computer performance, performance or simply to reduce otherwise duplicate code, repetitively similar code, thus simplifying software maintenance, maintenance. The term is usually only applied to code where the self-modification is intentional, not in situations where code accidentally modifies itself due to an error such as a buffer overflow. Self-modifying code can involve overwriting existing instructions or generating new code at run time and transferring control to that code. Self-modification can be used as an alternative to the method of "flag setting" and conditional program branching, used primarily to reduce the number of times a condition needs to be tested. The method is frequently used for conditionally invoking test/deb ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Cylinder-head-sector
Cylinder-head-sector (CHS) is an early method for giving addresses to each physical block of data on a hard disk drive. It is a 3D-coordinate system made out of a vertical coordinate ''head'', a horizontal (or radial) coordinate ''cylinder'', and an angular coordinate ''sector''. Head selects a circular surface: a platter in the disk (and one of its two sides). Cylinder is a Cylinder, cylindrical intersection through the stack of platters in a disk, centered around the disk's spindle. Combined, cylinder and head intersect to a circular line, or more precisely: a circular strip of physical data blocks called ''track''. Sector finally selects which data block in this track is to be addressed, as the track is subdivided into several equally-sized portions, each of which is an arc of (360/n) degrees, where n is the number of sectors in the track. CHS addresses were exposed, instead of simple linear addresses (going from ''0'' to the ''total block count on disk - 1''), because early h ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Track (disk Drive)
A disk drive track is a circular path on the surface of a disk or diskette on which information is magnetically recorded and from which recorded information is read. A track is a physical division of data in a disk drive, as used in the Cylinder-Head-Record (CCHHR) addressing mode of a CKD disk. The concept is concentric, through the physical platters, being a data circle per each cylinder of the whole disk drive. In other words, the number of tracks on a single surface in the drive exactly equals the number of cylinders of the drive. Tracks are subdivided into blocks (or sectors, pages) (see: Storage block and Virtual page). The term track is sometimes prefaced with the word logical (i.e. "3390-9 has 3 logical tracks per physical track") to emphasize that it is used as an abstract concept, not a track in the physical sense. See also * Hard disk drive * Disk sector In computer disk storage, a sector is a subdivision of a track on a magnetic disk or optical disc. ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Channel I/O
In computing, channel I/O is a high-performance input/output (I/O) architecture that is implemented in various forms on a number of computer architectures, especially on mainframe computers. In the past, channels were generally implemented with custom devices, variously named channel, I/O processor, I/O controller, I/O synchronizer, or '' DMA controller''. Overview Many I/O tasks can be complex and require logic to be applied to the data to convert formats and other similar duties. In these situations, the simplest solution is to ask the CPU to handle the logic, but because I/O devices are relatively slow, a CPU could waste time waiting for the data from the device. This situation is called 'I/O bound'. Channel architecture avoids this problem by processing some or all of the I/O task without the aid of the CPU by offloading the work to dedicated logic. Channels are logically self-contained, with sufficient logic and working storage to handle I/O tasks. Some are powerful or fle ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Computer Memory
Computer memory stores information, such as data and programs, for immediate use in the computer. The term ''memory'' is often synonymous with the terms ''RAM,'' ''main memory,'' or ''primary storage.'' Archaic synonyms for main memory include ''core'' (for magnetic core memory) and ''store''. Main memory operates at a high speed compared to mass storage which is slower but less expensive per bit and higher in capacity. Besides storing opened programs and data being actively processed, computer memory serves as a Page cache, mass storage cache and write buffer to improve both reading and writing performance. Operating systems borrow RAM capacity for caching so long as it is not needed by running software. If needed, contents of the computer memory can be transferred to storage; a common way of doing this is through a memory management technique called ''virtual memory''. Modern computer memory is implemented as semiconductor memory, where data is stored within memory cell (com ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Record Management Services
Record Management Services (RMS) are procedures in the VMS, RSTS/E, RT-11 and RSX-11M operating systems that programs may call to process files and records within files. Its file formats and procedures are similar to of those in some IBM access methods for several of its mainframe computer A mainframe computer, informally called a mainframe or big iron, is a computer used primarily by large organizations for critical applications like bulk data processing for tasks such as censuses, industry and consumer statistics, enterprise ... operating systems and by other vendors for file and record management. VMS RMS is an integral part of the system software; its procedures run in executive mode. (RMS was not initially integrated into RT-11 and RSTS/E, but was available as an added charge "layered product". It was later added to RSTS/E and was a part of its standard documentation set.) RMS supports four record access methods: * Sequential Access * Relative Record Number Acces ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Files-11
Files-11 is the file system used in the RSX-11 and OpenVMS operating systems from Digital Equipment Corporation. It supports record-oriented I/O, remote network access, and file versioning. The original ODS-1 layer is a flat file system; the ODS-2 version is a hierarchical file system, with support for access control lists,. Files-11 is similar to, but significantly more advanced than, the file systems used in previous Digital Equipment Corporation operating systems such as TOPS-20 and RSTS/E. History The native OpenVMS file system is descended from older DEC operating systems and is similar in many ways, both having been designed by Dave Cutler. A major difference is the layout of directories. These file systems all provided some form of rudimentary non-hierarchical directory structure, typically based on assigning one directory per user account. Under RSTS/E, each user account was represented by two numbers, a 'project'',''programmer''/code> pair, and had one associ ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
OpenVMS
OpenVMS, often referred to as just VMS, is a multi-user, multiprocessing and virtual memory-based operating system. It is designed to support time-sharing, batch processing, transaction processing and workstation applications. Customers using OpenVMS include banks and financial services, hospitals and healthcare, telecommunications operators, network information services, and industrial manufacturers. During the 1990s and 2000s, there were approximately half a million VMS systems in operation worldwide. It was first announced by Digital Equipment Corporation (DEC) as VAX/VMS (''Virtual Address eXtension/Virtual Memory System'') alongside the VAX-11/780 minicomputer in 1977. OpenVMS has subsequently been ported to run on DEC Alpha systems, the Itanium-based HPE Integrity Servers, and select x86-64 hardware and hypervisors. Since 2014, OpenVMS is developed and supported by VMS Software Inc. (VSI). OpenVMS offers high availability through computer cluster, clustering—the ability t ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
IBM SQL/DS
SQL/DS (Structured Query Language/Data System), released in 1981, was IBM's first commercial relational-database management system. It implemented the SQL database-query language. SQL/DS ran on the DOS/VSE and VM/CMS operating systems. A little later, IBM also introduced Db2, another SQL-based DBMS, this one for the MVS operating system. The two products have coexisted since then; however, SQL/DS was rebranded as "DB2 for VM and VSE" in the late 1990s. Third party software Software AG's ''Natural'' 4GL A fourth-generation programming language (4GL) is a high-level computer programming language that belongs to a class of languages envisioned as an advancement upon third-generation programming languages (3GL). Each of the programming language g ... was an early third-party software product that facilitated using ''SQL/DS''. ''Software AG'' used the name ''Natural 2/SQL-DS'' and, later on, for a related offering, ''Natural 2/DB2''. References External links SQL Reunion ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
