Unit Pengolah Pusat

Unit Pengolah Pusat (UPP) (bahasa Inggris: CPU, singkatan dari Central Processing Unit), merujuk kepada perangkat keras komputer yang memahami dan melaksanakan perintah dan data dari perangkat lunak. Istilah lain, prosesor (pengolah data), sering digunakan untuk menyebut CPU. Adapun mikroprosesor adalah CPU yang diproduksi dalam sirkuit terpadu, seringkali dalam sebuah paket sirkuit terpadu-tunggal. Sejak pertengahan tahun 1970-an, mikroprosesor sirkuit terpadu-tunggal ini telah umum digunakan dan menjadi aspek penting dalam penerapan CPU.

Pin mikroprosesor Intel 80486DX2.

Daftar isi [sembunyikan] 1 Komponen CPU 2 Cara Kerja CPU 2.1 Fungsi CPU 2.2 Percabangan instruksi 2.3 Bilangan yang dapat ditangani 3 Referensi 4 Pranala luar 4.1 Perancang CPU 4.2 Informasi lain

[sunting] Komponen CPU

Diagram blok sederhana sebuah CPU.

Komponen CPU terbagi menjadi beberapa macam, yaitu sebagai berikut.

• Unit kontrol yang mampu mengatur jalannya program. Komponen ini sudah pasti terdapat dalam semua CPU.CPU bertugas mengontrol komputer sehingga terjadi sinkronisasi kerja antar komponen dalam menjalankan fungsi-fungsi operasinya. termasuk dalam tanggung jawab unit kontrol adalah mengambil intruksi-intruksi dari memori utama dan menentukan jenis instruksi tersebut. Bila ada instruksi untuk perhitungan aritmatika atau perbandingan logika, maka unit kendali akan mengirim instruksi tersebut ke ALU. Hasil dari pengolahan data dibawa oleh unit kendali ke memori utama lagi untuk disimpan, dan pada saatnya akan disajikan ke alat output. Dengan demikian tugas dari unit kendali ini adalah:
  • Mengatur dan mengendalikan alat-alat input dan output.
  • Mengambil instruksi-instruksi dari memori utama.
  • Mengambil data dari memori utama (jika diperlukan) untuk diproses.
  • Mengirim instruksi ke ALU bila ada perhitungan aritmatika atau perbandingan logika serta mengawasi kerja dari ALU.
  • Menyimpan hasil proses ke memori utama.

• Register merupakan alat penyimpanan kecil yang mempunyai kecepatan akses cukup tinggi, yang digunakan untuk menyimpan data dan/atau instruksi yang sedang diproses. Memori ini bersifat sementara, biasanya di gunakan untuk menyimpan data saat di olah ataupun data untuk pengolahan selanjutnya. Secara analogi, register ini dapat diibaratkan sebagai ingatan di otak bila kita melakukan pengolahan data secara manual, sehingga otak dapat diibaratkan sebagai CPU, yang berisi ingatan-ingatan, satuan kendali yang mengatur seluruh kegiatan tubuh dan mempunyai tempat untuk melakukan perhitungan dan perbandingan logika.
• ALU unit yang bertugas untuk melakukan operasi aritmetika dan operasi logika berdasar instruksi yang ditentukan. ALU sering di sebut mesin bahasa karena bagian ini ALU terdiri dari dua bagian, yaitu unit arithmetika dan unit logika boolean yang masing-masing memiliki spesifikasi tugas tersendiri. Tugas utama dari ALU adalah melakukan semua perhitungan aritmatika (matematika) yang terjadi sesuai dengan instruksi program. ALU melakukan semua operasi aritmatika dengan dasar penjumlahan sehingga sirkuit elektronik yang digunakan disebut adder.

Tugas lain dari ALU adalah melakukan keputusan dari suatu operasi logika sesuai dengan instruksi program. Operasi logika meliputi perbandingan dua operand dengan menggunakan operator logika tertentu, yaitu sama dengan (=), tidak sama dengan (¹ ), kurang dari (<), kurang atau sama dengan (£ ), lebih besar dari (>), dan lebih besar atau sama dengan (³ ).

• CPU Interconnections adalah sistem koneksi dan bus yang menghubungkan komponen internal CPU, yaitu ALU, unit kontrol dan register-register dan juga dengan bus-bus eksternal CPU yang menghubungkan dengan sistem lainnya, seperti memori utama, piranti masukan /keluaran.

[sunting] Cara Kerja CPU

Saat data dan/atau instruksi dimasukkan ke processing-devices, pertama sekali diletakkan di RAM (melalui Input-storage); apabila berbentuk instruksi ditampung oleh Control Unit di Program-storage, namun apabila berbentuk data ditampung di Working-storage). Jika register siap untuk menerima pengerjaan eksekusi, maka Control Unit akan mengambil instruksi dari Program-storage untuk ditampungkan ke Instruction Register, sedangkan alamat memori yang berisikan instruksi tersebut ditampung di Program Counter. Sedangkan data diambil oleh Control Unit dari Working-storage untuk ditampung di General-purpose register (dalam hal ini di Operand-register). Jika berdasar instruksi pengerjaan yang dilakukan adalah arithmatika dan logika, maka ALU akan mengambil alih operasi untuk mengerjakan berdasar instruksi yang ditetapkan. Hasilnya ditampung di Accumulator. Apabila hasil pengolahan telah selesai, maka Control Unit akan mengambil hasil pengolahan di Accumulator untuk ditampung kembali ke Working-storage. Jika pengerjaan keseluruhan telah selesai, maka Control Unit akan menjemput hasil pengolahan dari Working-storage untuk ditampung ke Output-storage. Lalu selanjutnya dari Output-storage, hasil pengolahan akan ditampilkan ke output-devices.

[sunting] Fungsi CPU

CPU berfungsi seperti kalkulator, hanya saja CPU jauh lebih kuat daya pemrosesannya. Fungsi utama dari CPU adalah melakukan operasi aritmatika dan logika terhadap data yang diambil dari memori atau dari informasi yang dimasukkan melalui beberapa perangkat keras, seperti papan ketik, pemindai, tuas kontrol, maupun tetikus. CPU dikontrol menggunakan sekumpulan instruksi perangkat lunak komputer. Perangkat lunak tersebut dapat dijalankan oleh CPU dengan membacanya dari media penyimpan, seperti cakram keras, disket, cakram padat, maupun pita perekam. Instruksi-instruksi tersebut kemudian disimpan terlebih dahulu pada memori fisik (RAM), yang mana setiap instruksi akan diberi alamat unik yang disebut alamat memori. Selanjutnya, CPU dapat mengakses data-data pada RAM dengan menentukan alamat data yang dikehendaki.
Saat sebuah program dieksekusi, data mengalir dari RAM ke sebuah unit yang disebut dengan bus, yang menghubungkan antara CPU dengan RAM. Data kemudian didekode dengan menggunakan unit proses yang disebut sebagai pendekoder instruksi yang sanggup menerjemahkan instruksi. Data kemudian berjalan ke unit aritmatika dan logika (ALU) yang melakukan kalkulasi dan perbandingan. Data bisa jadi disimpan sementara oleh ALU dalam sebuah lokasi memori yang disebut dengan register supaya dapat diambil kembali dengan cepat untuk diolah. ALU dapat melakukan operasi-operasi tertentu, meliputi penjumlahan, perkalian, pengurangan, pengujian kondisi terhadap data dalam register, hingga mengirimkan hasil pemrosesannya kembali ke memori fisik, media penyimpan, atau register apabila akan mengolah hasil pemrosesan lagi. Selama proses ini terjadi, sebuah unit dalam CPU yang disebut dengan penghitung program akan memantau instruksi yang sukses dijalankan supaya instruksi tersebut dapat dieksekusi dengan urutan yang benar dan sesuai.

[sunting] Percabangan instruksi

Pemrosesan instruksi dalam CPU dibagi atas dua tahap, Tahap-I disebut Instruction Fetch, sedangkan Tahap-II disebut Instruction Execute. Tahap-I berisikan pemrosesan CPU dimana Control Unit mengambil data dan/atau instruksi dari main-memory ke register, sedangkan Tahap-II berisikan pemrosesan CPU dimana Control Unit menghantarkan data dan/atau instruksi dari register ke main-memory untuk ditampung di RAM, setelah Instruction Fetch dilakukan. Waktu pada tahap-I ditambah dengan waktu pada tahap-II disebut waktu siklus mesin (machine cycles time).
Penghitung program dalam CPU umumnya bergerak secara berurutan. Walaupun demikian, beberapa instruksi dalam CPU, yang disebut dengan instruksi lompatan, mengizinkan CPU mengakses instruksi yang terletak bukan pada urutannya. Hal ini disebut juga percabangan instruksi (branching instruction). Cabang-cabang instruksi tersebut dapat berupa cabang yang bersifat kondisional (memiliki syarat tertentu) atau non-kondisional. Sebuah cabang yang bersifat non-kondisional selalu berpindah ke sebuah instruksi baru yang berada di luar aliran instruksi, sementara sebuah cabang yang bersifat kondisional akan menguji terlebih dahulu hasil dari operasi sebelumnya untuk melihat apakah cabang instruksi tersebut akan dieksekusi atau tidak. Data yang diuji untuk percabangan instruksi disimpan pada lokasi yang disebut dengan flag.

[sunting] Bilangan yang dapat ditangani

Kebanyakan CPU dapat menangani dua jenis bilangan, yaitu fixed-point dan floating-point. Bilangan fixed-point memiliki nilai digit spesifik pada salah satu titik desimalnya. Hal ini memang membatasi jangkauan nilai yang mungkin untuk angka-angka tersebut, tetapi hal ini justru dapat dihitung oleh CPU secara lebih cepat. Sementara itu, bilangan floating-point merupakan bilangan yang diekspresikan dalam notasi ilmiah, di mana sebuah angka direpresentasikan sebagai angka desimal yang dikalikan dengan pangkat 10 (seperti 3,14 x 10⁵⁷). Notasi ilmiah seperti ini merupakan cara yang singkat untuk mengekspresikan bilangan yang sangat besar atau bilangan yang sangat kecil, dan juga mengizinkan jangkauan nilai yang sangat jauh sebelum dan sesudah titik desimalnya. Bilangan ini umumnya digunakan dalam merepresentasikan grafik dan kerja ilmiah, tetapi proses aritmatika terhadap bilangan floating-point jauh lebih rumit dan dapat diselesaikan dalam waktu yang lebih lama oleh CPU karena mungkin dapat menggunakan beberapa siklus detak CPU. Beberapa komputer menggunakan sebuah prosesor sendiri untuk menghitung bilangan floating-point yang disebut dengan FPU (disebut juga math co-processor) yang dapat bekerja secara paralel dengan CPU untuk mempercepat penghitungan bilangan floating-point. FPU saat ini menjadi standar dalam banyak komputer karena kebanyakan aplikasi saat ini banyak beroperasi menggunakan bilangan floating-point.

[sunting] Referensi

• Architecture of the IBM System
• First Draft of a Report on the EDVAC
• The MIPS32® Instruction Set
• Ir. SNMP Simamora, MT,"Course Work: Mikroprosesor dan Antar-muka", Pangalengge Educations, 2006.

[sunting] Pranala luar

[sunting] Perancang CPU

• Advanced Micro Devices (AMD)
• ARM Ltd.
• Freescale Semiconductor (Motorola)
• IBM Microelectronics (IBM)
• Intel Corporation (Intel)
• MIPS Technologies (MIPS)
• Texas Instruments

[sunting] Informasi lain

• Cara kerja mikroprosesor
• Desain prosesor

[sembunyikan] l • b • s Komponen dasar komputer Peranti masukan Papan ketik · Light pen · Tetikus · Mikrofon · Webcam Peranti keluaran Tampilan komputer  · Speaker  · Printer Penyimpanan data Removable Disket · Compact Disc ReWritable/DVD Drive · USB flash drive · Kartu memori Kesing komputer Unit Pengolah Pusat · RAM · Video card · Sound card · Motherboard · Power supply · Hard drive Port data Serial port · Parallel port · Universal Serial Bus (USB)

Artikel bertopik perangkat keras ini adalah sebuah rintisan. Anda dapat membantu Wikipedia dengan mengembangkannya.

Basis data

Database

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This article may be confusing or unclear to readers. Please help clarify the article; suggestions may be found on the talk page. (May 2011)

It has been suggested that Database administrator be merged into this article or section. (Discuss)

A database is a system intended to organize, store, and retrieve large amounts of data easily. It consists of an organized collection of data for one or more uses, typically in digital form. One way of classifying databases involves the type of their contents, for example: bibliographic, document-text, statistical. Digital databases are managed using database management systems, which store database contents, allowing data creation and maintenance, search and other access.

Contents [hide] 1 Architecture 2 Database management systems 2.1 Components of DBMS 2.1.1 RDBMS components 2.1.2 ODBMS components 3 Types 3.1 Analytical database 3.2 Data warehouse 3.3 Distributed database 3.4 End-user database 3.5 External database 3.6 Hypermedia databases 3.7 Operational database 4 Models 4.1 Object database models 4.2 Post-relational database models 5 Storage structures 6 Indexing 7 Transactions 7.1 The ACID rules 7.2 Concurrency control and locking 7.2.1 Isolation 7.2.2 Lock types 7.2.3 Lock granularity 7.2.4 Deadlocks 8 Replication 9 Security 9.1 Confidentiality 10 See also 11 References 12 Further reading 13 External links

[edit] Architecture

Database architecture consists of three levels: external, conceptual and internal. Clearly separating the three levels was a major feature of the relational database model that dominates 21st century databases.^[1]
The external level defines how users understand the organization of the data. A single database can have any number of views at the external level. The internal level defines how the data is physically stored and processed by the computing system. Internal architecture is concerned with cost, performance, scalability and other operational matters. The conceptual is a level of indirection between internal and external. It provides a common view of the database that is uncomplicated by details of how the data is stored or managed, and that can unify the various external views into a coherent whole.^[1]

[edit] Database management systems

Main article: Database management system

A database management system (DBMS) consists of software that operates databases, providing storage, access, security, backup and other facilities. Database management systems can be categorized according to the database model that they support, such as relational or XML, the type(s) of computer they support, such as a server cluster or a mobile phone, the query language(s) that access the database, such as SQL or XQuery, performance trade-offs, such as maximum scale or maximum speed or others. Some DBMS cover more than one entry in these categories, e.g., supporting multiple query languages. Examples of some commonly used DBMS are MySQL, PostgreSQL, Microsoft Access, SQL Server, FileMaker,Oracle,Sybase, dBASE, Clipper,FoxPro etc. Almost every database software comes with an Open Database Connectivity (ODBC) driver that allows the database to integrate with other databases.

[edit] Components of DBMS

Most DBMS as of 2009^[update] implement a relational model.^[2] Other DBMS systems, such as Object DBMS, offer specific features for more specialized requirements. Their components are similar, but not identical.

[edit] RDBMS components

• Sublanguages— Relational DBMS (RDBMS) include Data Definition Language (DDL) for defining the structure of the database, Data Control Language (DCL) for defining security/access controls, and Data Manipulation Language (DML) for querying and updating data.
• Interface drivers—These drivers are code libraries that provide methods to prepare statements, execute statements, fetch results, etc. Examples include ODBC, JDBC, MySQL/PHP, FireBird/Python.
• SQL engine—This component interprets and executes the DDL, DCL, and DML statements. It includes three major components (compiler, optimizer, and executor).
• Transaction engine—Ensures that multiple SQL statements either succeed or fail as a group, according to application dictates.
• Relational engine—Relational objects such as Table, Index, and Referential integrity constraints are implemented in this component.
• Storage engine—This component stores and retrieves data from secondary storage, as well as managing transaction commit and rollback, backup and recovery, etc.

[edit] ODBMS components

Object DBMS (ODBMS) has transaction and storage components that are analogous to those in an RDBMS. Some DBMS handle DDL, DML and update tasks differently. Instead of using sublanguages, they provide APIs for these purposes. They typically include a sublanguage and accompanying engine for processing queries with interpretive statements analogous to but not the same as SQL. Example object query languages are OQL, LINQ, JDOQL, JPAQL and others. The query engine returns collections of objects instead of relational rows.

[edit] Types

This section does not cite any references or sources. Please help improve this section by adding citations to reliable sources. Unsourced material may be challenged and removed. (January 2011)

[edit] Analytical database

Analysts may do their work directly against a data warehouse or create a separate analytic database for Online Analytical Processing (OLAP). For example, a company might extract sales records for analyzing the effectiveness of advertising and other sales promotions at an aggregate level.

[edit] Data warehouse

Data warehouses archive modern data from operational databases and often from external sources such as market research firms. Often operational data undergoes transformation on its way into the warehouse, getting summarized, anonymized, reclassified, etc. The warehouse becomes the central source of data for use by managers and other end-users who may not have access to operational data. For example, sales data might be aggregated to weekly totals and converted from internal product codes to use UPCs so that it can be compared with ACNielsen data. Some basic and essential components of data warehousing include retrieving and analyzing data, transforming,loading and managing data so as to make it available for further use.
Operations in a data warehouse are typically concerned with bulk data manipulation, and as such, it is unusual and inefficient to target individual rows for update, insert or delete. Bulk native loaders for input data and bulk SQL passes for aggregation are the norm.

[edit] Distributed database

These are databases of local work-groups and departments at regional offices, branch offices, manufacturing plants and other work sites. These databases can include segments of both common operational and common user databases, as well as data generated and used only at a user’s own site.

[edit] End-user database

These databases consist of data developed by individual end-users. Examples of these are collections of documents in spreadsheets, word processing and downloaded files, even managing their personal baseball card collection.

[edit] External database

These databases contain data collected for use across multiple organizations, either freely or via subscription. The Internet Movie Database is one example.

[edit] Hypermedia databases

The World Wide Web can be thought of as a database, albeit one spread across millions of independent computing systems. Web browsers "process" this data one page at a time, while Web crawlers and other software provide the equivalent of database indexes to support search and other activities.

[edit] Operational database

These databases store detailed data about the operations of an organization. They are typically organized by subject matter, process relatively high volumes of updates using transactions. Essentially every major organization on earth uses such databases. Examples include customer databases that record contact, credit, and demographic information about a business' customers, personnel databases that hold information such as salary, benefits, skills data about employees, Enterprise resource planning that record details about product components, parts inventory, and financial databases that keep track of the organization's money, accounting and financial dealings.

[edit] Models

Main article: Database model

[edit] Object database models

Main article: Object database

In recent years^[update], the object-oriented paradigm has been applied in areas such as engineering and spatial databases, telecommunications and in various scientific domains. The conglomeration of object oriented programming and database technology led to this new kind of database. These databases attempt to bring the database world and the application-programming world closer together, in particular by ensuring that the database uses the same type system as the application program. This aims to avoid the overhead (sometimes referred to as the impedance mismatch) of converting information between its representation in the database (for example as rows in tables) and its representation in the application program (typically as objects). At the same time, object databases attempt to introduce key ideas of object programming, such as encapsulation and polymorphism, into the world of databases.
A variety of these ways have been tried^{[by whom?]} for storing objects in a database. Some products have approached the problem from the application-programming side, by making the objects manipulated by the program persistent. This also typically requires the addition of some kind of query language, since conventional programming languages do not provide language-level functionality for finding objects based on their information content. Others^[which?] have attacked the problem from the database end, by defining an object-oriented data model for the database, and defining a database programming language that allows full programming capabilities as well as traditional query facilities..

[edit] Post-relational database models

Products offering a more general data model than the relational model are sometimes classified as post-relational.^[3] Alternate terms include "hybrid database", "Object-enhanced RDBMS" and others. The data model in such products incorporates relations but is not constrained by E.F. Codd's Information Principle, which requires that

all information in the database must be cast explicitly in terms of values in relations and in no other way^[4]

Some of these extensions to the relational model integrate concepts from technologies that pre-date the relational model. For example, they allow representation of a directed graph with trees on the nodes. The German company sones implements this concept in its GraphDB.
Some post-relational products extend relational systems with non-relational features. Others arrived in much the same place by adding relational features to pre-relational systems. Paradoxically, this allows products that are historically pre-relational, such as PICK and MUMPS, to make a plausible claim to be post-relational.

[edit] Storage structures

Main article: Database storage structures

Databases may store relational tables/indexes in memory or on hard disk in one of many forms:

• ordered/unordered flat files
• ISAM
• heaps
• hash buckets
• logically-blocked files
• Fractal Tree indexes
• B+ trees

The most commonly used^{[citation needed]} are B+ trees and ISAM.
Object databases use a range of storage mechanisms. Some use virtual memory-mapped files to make the native language (C++, Java etc.) objects persistent. This can be highly efficient but it can make multi-language access more difficult. Others disassemble objects into fixed- and varying-length components that are then clustered in fixed sized blocks on disk and reassembled into the appropriate format on either the client or server address space. Another popular technique involves storing the objects in tuples (much like a relational database), which the database server then reassembles into objects for the client.^{[citation needed]}
Other techniques include clustering by category (such as grouping data by month, or location), storing pre-computed query results, known as materialized views, partitioning data by range (e.g., a data range) or by hash.
Memory management and storage topology can be important design choices for database designers as well. Just as normalization is used to reduce storage requirements and improve database designs, conversely denormalization is often used to reduce join complexity and reduce query execution time.^[5]

[edit] Indexing

Main article: Index (database)

Indexing is a technique for improving database performance. The many types of indexes share the common property that they eliminate the need to examine every entry when running a query. In large databases, this can reduce query time/cost by orders of magnitude. The simplest form of index is a sorted list of values that can be searched using a binary search with an adjacent reference to the location of the entry, analogous to the index in the back of a book. The same data can have multiple indexes (an employee database could be indexed by last name and hire date.)
Indexes affect performance, but not results. Database designers can add or remove indexes without changing application logic, reducing maintenance costs as the database grows and database usage evolves.
Given a particular query, the DBMS' query optimizer is responsible for devising the most efficient strategy for finding matching data. The optimizer decides which index or indexes to use, how to combine data from different parts of the database, how to provide data in the order requested, etc.
Indexes can speed up data access, but they consume space in the database, and must be updated each time the data is altered. Indexes therefore can speed data access but slow data maintenance. These two properties determine whether a given index is worth the cost.

[edit] Transactions

Main article: Database transaction

This section may stray from the topic of the article into the topic of another article, Database management system. Please help improve this section or discuss this issue on the talk page. (November 2010)

As every software system, a DBMS operates in a faulty computing environment and prone to failures of many kinds. A failure can corrupt the respective database unless special measures are taken to prevent this. A DBMS achieves certain levels of fault tolerance by encapsulating in database transactions units of work (executed programs) performed upon the respective database.

[edit] The ACID rules

Main article: ACID

Most DBMS provide some form of support for transactions, which allow multiple data items to be updated in a consistent fashion, such that updates that are part of a transaction succeed or fail in unison. The so-called ACID rules, summarized here, characterize this behavior:

• Atomicity: Either all the data changes in a transaction must happen, or none of them. The transaction must be completed, or else it must be undone (rolled back).
• Consistency: Every transaction must preserve the declared consistency rules for the database.
• Isolation: Two concurrent transactions cannot interfere with one another. Intermediate results within one transaction must remain invisible to other transactions. The most extreme form of isolation is serializability, meaning that transactions that take place concurrently could instead be performed in some series, without affecting the ultimate result.
• Durability: Completed transactions cannot be aborted later or their results discarded. They must persist through (for instance) DBMS restarts.

In practice, many DBMSs allow the selective relaxation of these rules to balance perfect behavior with optimum performance.

[edit] Concurrency control and locking

Main article: Concurrency control

Concurrency control is essential for the correctness of transactions executed concurrently in a DBMS, which is the common execution mode for performance reasons. The main concern and goal of concurrency control is isolation.

[edit] Isolation

Isolation refers to the ability of one transaction to see the results of other transactions. Greater isolation typically reduces performance and/or concurrency, leading DBMSs to provide administrative options to reduce isolation. For example, in a database that analyzes trends rather than looking at low-level detail, increased performance might justify allowing readers to see uncommitted changes ("dirty reads".)
A common way to achieve isolation is by locking. When a transaction modifies a resource, the DBMS stops other transactions from also modifying it, typically by locking it. Locks also provide one method of ensuring that data does not change while a transaction is reading it or even that it does not change until a transaction that once read it has completed.

[edit] Lock types

Locks can be shared^[6] or exclusive, and can lock out readers and/or writers. Locks can be created implicitly by the DBMS when a transaction performs an operation, or explicitly at the transaction's request.
Shared locks allow multiple transactions to lock the same resource. The lock persists until all such transactions complete. Exclusive locks are held by a single transaction and prevent other transactions from locking the same resource.
Read locks are usually shared, and prevent other transactions from modifying the resource. Write locks are exclusive, and prevent other transactions from modifying the resource. On some systems, write locks also prevent other transactions from reading the resource.
The DBMS implicitly locks data when it is updated, and may also do so when it is read. Transactions explicitly lock data to ensure that they can complete without complications. Explicit locks may be useful for some administrative tasks.^[7][8]
Locking can significantly affect database performance, especially with large and complex transactions in highly concurrent environments.

[edit] Lock granularity

Locks can be coarse, covering an entire database, fine-grained, covering a single data item, or intermediate covering a collection of data such as all the rows in a RDBMS table.

[edit] Deadlocks

Deadlocks occur when two transactions each require data that the other has already locked exclusively. Deadlock detection is performed by the DBMS, which then aborts one of the transactions and allows the other to complete.

[edit] Replication

Main article: Database replication

Database replication involves maintaining multiple copies of a database on different computers, to allow more users to access it, or to allow a secondary site to immediately take over, if the primary site stops working. Some DBMS piggyback replication on top of their transaction logging facility, applying the primary's log to the secondary in near real-time. Database clustering is a related concept for handling larger databases and user communities by employing a cluster of multiple computers to host a single database that can use replication as part of its approach.^[9][10]

[edit] Security

Main article: Database security

Database security denotes the system, processes, and procedures that protect a database from unauthorized activity.
DBMSs usually enforce security through access control, auditing, and encryption:

• Access control manages who can connect to the database via authentication and what they can do via authorization.
• Auditing records information about database activity: who, what, when, and possibly where.
• Encryption protects data at the lowest possible level by storing and possibly transmitting data in an unreadable form. The DBMS encrypts data when it is added to the database and decrypts it when returning query results. This process can occur on the client side of a network connection to prevent unauthorized access at the point of use.

[edit] Confidentiality

Law and regulation governs the release of information from some databases, protecting medical history, driving records, telephone logs, etc.
In the United Kingdom, database privacy regulation falls under the Office of the Information Commissioner. Organizations based in the United Kingdom and holding personal data in digital format such as databases must register with the Office.^[11]

[edit] See also

• Comparison of relational database management systems
• Comparison of database tools
• Data hierarchy
• Data store
• Database design
• Database theory
• Database-centric architecture
• Datastructure
• Document-oriented database
• Government database
• Graph database
• In-memory database
• Real time database

[edit] References

1. ^ ^{a b} Date 1990, pp. 31–32
2. ^ "Design of Main Memory Database System/Overview of DBMS". En.wikibooks.org. http://en.wikibooks.org/wiki/Design_of_Main_Memory_Database_System/Overview_of_DBMS. Retrieved 2010-08-29. 
3. ^ Introducing databases by Stephen Chu, in Conrick, M. (2006) Health informatics: transforming healthcare with technology, Thomson, ISBN 0-17-012731-1, p. 69.
4. ^ Date, C. J. (June 1, 1999). "When's an extension not an extension?". Intelligent Enterprise 2 (8). http://intelligent-enterprise.informationweek.com/db_area/archives/1999/990106/online1.jhtml;jsessionid=Y2UNK1QFKXMBTQE1GHRSKH4ATMY32JVN. 
5. ^ Lightstone, Teorey & Nadeau 2007
6. ^ "Information on Shared Locks". Methodsandtools.com. http://www.methodsandtools.com/archive/archive.php?id=83. Retrieved 2010-08-29. 
7. ^ ""Locking tables and databases" (section in some documentation / explanation from IBM)". Publib.boulder.ibm.com. http://publib.boulder.ibm.com/infocenter/rbhelp/v6r3/index.jsp?topic=/com.ibm.redbrick.doc6.3/wag/wag80.htm. Retrieved 2010-08-29. 
8. ^ "Routine Database Maintenance". Postgresql.org. http://www.postgresql.org/docs/8.3/static/routine-vacuuming.html. Retrieved 2010-08-29. 
9. ^ "MySQL Cluster". Mysql.com. 2010-08-25. http://www.mysql.com/products/database/cluster/. Retrieved 2010-08-29. 
10. ^ "Oracle Real Application Cluster (RAC)". Oracle.com. 2010-03-23. http://www.oracle.com/lang/de/database/rac_home.html. Retrieved 2010-08-29. 
11. ^ "Information Commissioner's Office". ICO. http://www.ico.gov.uk/. Retrieved 2010-08-29. 

[edit] Further reading

• Ling Liu and Tamer M. Özsu (Eds.) (2009). "Encyclopedia of Database Systems, 4100 p. 60 illus. ISBN 978-0-387-49616-0. Table of Content available at http://refworks.springer.com/mrw/index.php?id=1217
• Beynon-Davies, P. (2004). Database Systems. 3rd Edition. Palgrave, Houndmills, Basingstoke.
• Connolly, Thomas and Carolyn Begg. Database Systems. New York: Harlow, 2002.
• Date, C. J. An Introduction to Database Systems, Eighth Edition, Addison Wesley, 2003.
• Date, C. J. (2003). An Introduction to Database Systems, Fifth Edition. Addison Wesley. ISBN 0-201-51381-1. 
• Galindo, J.; Urrutia, A.; Piattini, M. Fuzzy Databases: Modeling, Design and Implementation (FSQL guide). Idea Group Publishing Hershey, USA, 2006.
• Galindo, J., Ed. Handbook on Fuzzy Information Processing in Databases. Hershey, PA: Information Science Reference (an imprint of Idea Group Inc.), 2008.
• Gray, J. and Reuter, A. Transaction Processing: Concepts and Techniques, 1st edition, Morgan Kaufmann Publishers, 1992.
• Kroenke, David M. Database Processing: Fundamentals, Design, and Implementation (1997), Prentice-Hall, Inc., pages 130–144.
• Kroenke, David M. and David J. Auer. Database Concepts. 3rd ed. New York: Prentice, 2007.
• Lightstone, S.; Teorey, T.; Nadeau, T. (2007). Physical Database Design: the database professional's guide to exploiting indexes, views, storage, and more. Morgan Kaufmann Press. ISBN 0-12-369389-6. 
• O'Brien, James. "Management Information Systems". New York 1999
• Shih, J. "Why Synchronous Parallel Transaction Replication is Hard, But Inevitable?", white paper, 2007.
• Teorey, T.; Lightstone, S. and Nadeau, T. Database Modeling & Design: Logical Design, 4th edition, Morgan Kaufmann Press, 2005. ISBN 0-12-685352-5
• Tukey, John W. Exploratory Data Analysis. Reading, MA: Addison Wesley, 1977.
• Manovich, Lev.Database as a Symbolic Form, Cambridge: MIT press, 2001.

visual basic

Visual Basic (VB) is the third-generation event-driven programming language and integrated development environment (IDE) from Microsoft for its COM programming model. Visual Basic is relatively easy to learn and use.[1][2]

Visual Basic was derived from BASIC and enables the rapid application development (RAD) of graphical user interface (GUI) applications, access to databases using Data Access Objects, Remote Data Objects, or ActiveX Data Objects, and creation of ActiveX controls and objects. Scripting languages such as VBA and VBScript are syntactically similar to Visual Basic, but perform differently.[3]

A programmer can put together an application using the components provided with Visual Basic itself. Programs written in Visual Basic can also use the Windows API, but doing so requires external function declarations.

The final release was version 6 in 1998. Microsoft's extended support ended in March 2008 and the designated successor was Visual Basic .NET (now known simply as Visual Basic).Contents [hide]
1 Language features
2 Characteristics
3 History
3.1 Timeline
4 Derivative languages
5 Performance and other issues
6 Legacy development and support
7 Example code
8 See also
9 References
10 External links

[edit]
Language features

Like the BASIC programming language, Visual Basic was designed to be easily learned and used by beginner programmers. The language not only allows programmers to create simple GUI applications, but can also develop complex applications. Programming in VB is a combination of visually arranging components or controls on a form, specifying attributes and actions of those components, and writing additional lines of code for more functionality. Since default attributes and actions are defined for the components, a simple program can be created without the programmer having to write many lines of code. Performance problems were experienced by earlier versions, but with faster computers and native code compilation this has become less of an issue.

Although programs can be compiled into native code executables from version 5 onwards, they still require the presence of runtime libraries of approximately 1 MB in size. This runtime is included by default in Windows 2000 and later, but for earlier versions of Windows like 95/98/NT it must be distributed together with the executable.

Forms are created using drag-and-drop techniques. A tool is used to place controls (e.g., text boxes, buttons, etc.) on the form (window). Controls have attributes and event handlers associated with them. Default values are provided when the control is created, but may be changed by the programmer. Many attribute values can be modified during run time based on user actions or changes in the environment, providing a dynamic application. For example, code can be inserted into the form resize event handler to reposition a control so that it remains centered on the form, expands to fill up the form, etc. By inserting code into the event handler for a keypress in a text box, the program can automatically translate the case of the text being entered, or even prevent certain characters from being inserted.

Visual Basic can create executables (EXE files), ActiveX controls, or DLL files, but is primarily used to develop Windows applications and to interface database systems. Dialog boxes with less functionality can be used to provide pop-up capabilities. Controls provide the basic functionality of the application, while programmers can insert additional logic within the appropriate event handlers. For example, a drop-down combination box will automatically display its list and allow the user to select any element. An event handler is called when an item is selected, which can then execute additional code created by the programmer to perform some action based on which element was selected, such as populating a related list.

Alternatively, a Visual Basic component can have no user interface, and instead provide ActiveX objects to other programs via Component Object Model (COM). This allows for server-side processing or an add-in module.

The language is garbage collected using reference counting, has a large library of utility objects, and has basic object oriented support. Since the more common components are included in the default project template, the programmer seldom needs to specify additional libraries. Unlike many other programming languages, Visual Basic is generally not case sensitive, although it will transform keywords into a standard case configuration and force the case of variable names to conform to the case of the entry within the symbol table. String comparisons are case sensitive by default, but can be made case insensitive if so desired.

The Visual Basic compiler is shared with other Visual Studio languages (C, C++), but restrictions in the IDE do not allow the creation of some targets (Windows model DLLs) and threading models.
[edit]
Characteristics

Visual Basic has the following traits which differ from C-derived languages:
Multiple assignment available in C language is not possible. A = B = C does not imply that the values of A, B and C are equal. The boolean result of "Is B = C?" is stored in A. The result stored in A would therefore be either false or true.
Boolean constant True has numeric value −1.[4] This is because the Boolean data type is stored as a 16-bit signed integer. In this construct −1 evaluates to 16 binary 1s (the Boolean value True), and 0 as 16 0s (the Boolean value False). This is apparent when performing a Not operation on a 16 bit signed integer value 0 which will return the integer value −1, in other words True = Not False. This inherent functionality becomes especially useful when performing logical operations on the individual bits of an integer such as And, Or, Xor and Not.[5] This definition of True is also consistent with BASIC since the early 1970s Microsoft BASIC implementation and is also related to the characteristics of CPU instructions at the time.
Logical and bitwise operators are unified. This is unlike some C-derived languages (such as Perl), which have separate logical and bitwise operators. This again is a traditional feature of BASIC.
Variable array base. Arrays are declared by specifying the upper and lower bounds in a way similar to Pascal and Fortran. It is also possible to use the Option Base statement to set the default lower bound. Use of the Option Base statement can lead to confusion when reading Visual Basic code and is best avoided by always explicitly specifying the lower bound of the array. This lower bound is not limited to 0 or 1, because it can also be set by declaration. In this way, both the lower and upper bounds are programmable. In more subscript-limited languages, the lower bound of the array is not variable. This uncommon trait does exist in Visual Basic .NET but not in VBScript.
OPTION BASE was introduced by ANSI, with the standard for ANSI Minimal BASIC in the late 1970s.
Relatively strong integration with the Windows operating system and the Component Object Model. The native types for strings and arrays are the dedicated COM types, BSTR and SAFEARRAY.
Banker's rounding as the default behavior when converting real numbers to integers with the Round function.[6] ? Round(2.5, 0) gives 2, ? Round(3.5, 0) gives 4.
Integers are automatically promoted to reals in expressions involving the normal division operator (/) so that division of one integer by another produces the intuitively correct result. There is a specific integer divide operator (\) which does truncate.
By default, if a variable has not been declared or if no type declaration character is specified, the variable is of type Variant. However this can be changed with Deftype statements such as DefInt, DefBool, DefVar, DefObj, DefStr. There are 12 Deftype statements in total offered by Visual Basic 6.0. The default type may be overridden for a specific declaration by using a special suffix character on the variable name (# for Double, ! for Single, & for Long, % for Integer, $ for String, and @ for Currency) or using the key phrase As (type). VB can also be set in a mode that only explicitly declared variables can be used with the command Option Explicit.
[edit]
History

VB 1.0 was introduced in 1991. The drag and drop design for creating the user interface is derived from a prototype form generator developed by Alan Cooper and his company called Tripod. Microsoft contracted with Cooper and his associates to develop Tripod into a programmable form system for Windows 3.0, under the code name Ruby (no relation to the Ruby programming language).

Tripod did not include a programming language at all. Microsoft decided to combine Ruby with the Basic language to create Visual Basic.

The Ruby interface generator provided the "visual" part of Visual Basic and this was combined with the "EB" Embedded BASIC engine designed for Microsoft's abandoned "Omega" database system. Ruby also provided the ability to load dynamic link libraries containing additional controls (then called "gizmos"), which later became the VBX interface.[7]
[edit]
Timeline
Project 'Thunder' was initiated
Visual Basic 1.0 (May 1991) was released for Windows at the Comdex/Windows World trade show in Atlanta, Georgia.
Visual Basic 1.0 for DOS was released in September 1992. The language itself was not quite compatible with Visual Basic for Windows, as it was actually the next version of Microsoft's DOS-based BASIC compilers, QuickBASIC and BASIC Professional Development System. The interface used a Text user interface, using extended ASCII characters to simulate the appearance of a GUI.

 Visual Basic for MS-DOS
Visual Basic 2.0 was released in November 1992. The programming environment was easier to use, and its speed was improved. Notably, forms became instantiable objects, thus laying the foundational concepts of class modules as were later offered in VB4.
Visual Basic 3.0 was released in the summer of 1993 and came in Standard and Professional versions. VB3 included version 1.1 of the Microsoft Jet Database Engine that could read and write Jet (or Access) 1.x databases.
Visual Basic 4.0 (August 1995) was the first version that could create 32-bit as well as 16-bit Windows programs. It also introduced the ability to write non-GUI classes in Visual Basic. Incompatibilities between different releases of VB4 caused installation and operation problems. While previous versions of Visual Basic had used VBX controls, Visual Basic now used OLE controls (with files names ending in .OCX) instead. These were later to be named ActiveX controls.
With version 5.0 (February 1997), Microsoft released Visual Basic exclusively for 32-bit versions of Windows. Programmers who preferred to write 16-bit programs were able to import programs written in Visual Basic 4.0 to Visual Basic 5.0, and Visual Basic 5.0 programs can easily be converted with Visual Basic 4.0. Visual Basic 5.0 also introduced the ability to create custom user controls, as well as the ability to compile to native Windows executable code, speeding up calculation-intensive code execution. A free, downloadable Control Creation Edition was also released for creation of ActiveX controls. It was also used as an introductory form of Visual Basic: a regular .exe project could be created and run in the IDE, but not compiled.
Visual Basic 6.0 (Mid 1998) improved in a number of areas [8] including the ability to create web-based applications. VB6 has entered Microsoft's "non-supported phase" as of March 2008. Although the Visual Basic 6.0 development environment is no longer supported, the runtime is supported on Windows Vista, Windows Server 2008 and Windows 7.[9]
Mainstream Support for Microsoft Visual Basic 6.0 ended on March 31, 2005. Extended support ended in March 2008.[10] In response, the Visual Basic user community expressed its grave concern and lobbied users to sign a petition to keep the product alive.[11] Microsoft has so far refused to change their position on the matter. (but see [12]) Ironically, around this time (2005), it was exposed that Microsoft's new anti-spyware offering, Microsoft AntiSpyware (part of the GIANT Company Software purchase), was coded in Visual Basic 6.0.[13] Its 
 VB DOS Logo
replacement, Windows Defender, was rewritten as C++ code. 
 VB DOS Logo[14]
[edit]
Derivative languages

Microsoft has developed derivatives of Visual Basic for use in scripting. Visual Basic itself is derived heavily from BASIC, and subsequently has been replaced with a .NET platform version.

Some of the derived languages are:
Visual Basic for Applications (VBA) is included in many Microsoft applications (Microsoft Office), and also in many third-party products like SolidWorks, AutoCAD, WordPerfect Office 2002, ArcGIS, Sage Accpac ERP, and Business Objects Desktop Intelligence. There are small inconsistencies in the way VBA is implemented in different applications, but it is largely the same language as VB6 and uses the same runtime library.
VBScript is the default language for Active Server Pages. It can be used in Windows scripting and client-side web page scripting. Although it resembles VB in syntax, it is a separate language and it is executed by vbscript.dll as opposed to the VB runtime. ASP and VBScript should not be confused with ASP.NET which uses the .NET Framework for compiled web pages.
Visual Basic .NET is Microsoft's designated successor to Visual Basic 6.0, and is part of Microsoft's .NET platform. Visual Basic.Net compiles and runs using the .NET Framework. It is not backwards compatible with VB6. An automated conversion tool exists, but fully automated conversion for most projects is impossible.[15]
StarOffice Basic is a Visual Basic compatible interpreter included in StarOffice suite, developed by Sun Microsystems.
Gambas is a Visual Basic inspired free software programming language. It is not a clone of Visual Basic, but it does have the ability to convert Visual Basic programs to Gambas.
[edit]
Performance and other issues

Earlier counterparts of Visual Basic (prior to version 5) compiled the code to P-Code only. The P-Code is interpreted by the language runtime, also known as a virtual machine. The benefits of P-Code include portability and smaller binary file sizes, but it usually slows down the execution, since having a runtime adds an additional layer of interpretation. However, small amounts of code and algorithms can be constructed to run faster than compiled native code.

Visual Basic applications require Microsoft Visual Basic runtime MSVBVMxx.DLL, where xx is the relevant version number, either 50 or 60. MSVBVM60.dll comes as standard with Windows in all editions after Windows 98 while MSVBVM50.dll comes with all editions after Windows 95. A Windows 95 machine would however require inclusion with the installer of whichever dll was needed by the program.

Visual Basic 5 and 6 can compile code to either native or P-Code.

Criticisms levelled at Visual Basic editions prior to VB.NET include:[16]
Versioning problems associated with various runtime DLLs, known as DLL hell
Poor support for object-oriented programming[17]
Inability to create multi-threaded applications, without resorting to Windows API calls
Inability to create Windows services
Variant types have a greater performance and storage overhead than strongly typed programming languages
Dependency on complex and fragile COM Registry entries[18]
The development environment is no longer supported by Microsoft.
[edit]
Legacy development and support

All versions of the Visual Basic development environment from 1.0 to 6.0 have been retired and are now unsupported by Microsoft. The associated runtime environments are unsupported too, with the exception of the Visual Basic 6 core runtime environment, which will be officially supported by Microsoft for the lifetime of Windows 7.[19] Third party components that shipped with Visual Studio 6.0 are not included in this support statement. Some legacy Visual Basic components may still work on newer platforms, despite being unsupported by Microsoft and other vendors.

Development and maintenance development for Visual Basic 6 is possible on legacy Windows XP, Windows Vista and Windows 2003 using Visual Studio 6.0 platforms, but is unsupported. Documentation for Visual Basic 6.0, its application programming interface and tools is best covered in the last MSDN release before Visual Studio.NET 2002. Later releases of MSDN focused on .NET development and had significant parts of the Visual Basic 6.0 programming documentation removed. The Visual Basic IDE can be installed and used on Windows Vista, where it exhibits some minor incompatibilities which do not hinder normal software development and maintenance. As of August 2008, both Visual Studio 6.0 and the MSDN documentation mentioned above are available for download by MSDN subscribers.
[edit]
Example code

Here is an example of the language. The following code snippet displays a message box saying "Hello, World!" as the window loads:
Private Sub Form_Load()
    ' Execute a simple message box that will say "Hello, World!"
    MsgBox "Hello, World!"
End Sub

Later in VB6 you can use more complex coding. This snippet makes a counter that moves up 1 every second. You need to have the timer control added on to the form for this to work
Option Explicit
Dim Count As Integer
Private Sub Form_Load()
    Count = 0
    Timer1.Interval = 1000 ' units of milliseconds
End Sub
Private Sub Timer1_Timer()
    Count = Count + 1
    lblCount.Caption = Count
End Sub