Computer data storage

Computer data storage is a technology consisting of computer components and recording media that are used to retain digital data. It is a core function and fundamental component of computers.[1]: 15–16 

The central processing unit (CPU) of a computer is what manipulates data by performing computations. In practice, almost all computers use a storage hierarchy,[1]: 468–473  which puts fast but expensive and small storage options close to the CPU and slower but less expensive and larger options further away. Generally the fast volatile technologies (which lose data when off power) are referred to as “memory”, while slower persistent technologies are referred to as “storage”.

Even the first computer designs, Charles Babbage’s Analytical Engine and Percy Ludgate’s Analytical Machine, clearly distinguished between processing and memory (Babbage stored numbers as rotations of gears, while Ludgate stored numbers as displacements of rods in shuttles). This distinction was extended in the Von Neumann architecture, where the CPU consists of two main parts: The control unit and the arithmetic logic unit (ALU). The former controls the flow of data between the CPU and memory, while the latter performs arithmetic and logical operations on data.

In the 1950s memory technology sometimes also called computer storage, was primarily electromechanical with relays and drum-type rotating magnetic media. In the 1960s and 1970s, semiconductor memory was developed as a way to reduce cost and increase reliability of computers. During the same period magnetic tape storage was being superseded by optical recording methods for most applications, while magnetic disks were being introduced. Some early computers implemented a hybrid of these technologies, storing data in disk form, then loading it into memory on demand.

Most modern computers are based on integrated circuits (ICs), which are memory on a chip. Integrated circuits replaced the costly and slow semiconductor memory with custom built integrated circuits that are smaller, faster, have lower power consumption, are less susceptible to damage from transient electrical noise, are sometimes error-correcting, and are extremely reliable. Integrated circuits have replaced most tape drives for consumer computers now that they are much cheaper. Some now say that optical discs may take over for enterprise computing in the future.[citation needed]

Contents [hide] 1 General functions of computer memory 1.1 Storage 1.2 Retrieval 1.3 Caching 1.4 Organization and access

2 Types of memory 2.1 Main types 2.2 Further types

3 Computer data storage systems 3.1 Magnetic storage devices 3.2 Optical disc storage 3.3 Flash memory and the future of data storage 3.4 Storage as a service (cloud computing)

General functions of computer memory

Storage Computer data storage is routinely the permanent memory, and often main memory, of a computer. Storage is so important to modern computers that it is typically not referenced as “memory”, even though it serves the same abstraction to enable access to frequently needed information quickly.[1]: 89–90  The capacity of storage devices is typically measured as the number of bits that can be stored, which has traditionally been expressed as some multiple of the unit byte (8 bits). The term “memory” is now usually used to refer to computer storage of data, e.g., CPU cache, virtual memory or RAM.

Retrieval In modern computers the CPU contains a CPU cache from which it can directly retrieve bytes of data from memory as needed.[2] In contrast, the CPU cache is typically much smaller than main memory and is volatile (data is lost when power is turned off).[1]: 90–91  Main memory is also typically referred to as “secondary storage”. If computer data is retrieved from non-volatile storage, e.g., flash disk or hard drive, the CPU cache is typically much smaller than main memory and is volatile (data is lost when power is turned off).[1]: 92–93 

Caching Main memory can be used to temporarily store recently used data for fast retrieval in the absence of an efficient CPU cache. This technique allows data that cannot fit in main memory to be stored temporarily in a high-speed less expensive storage medium. The transfer of data from the main memory into the cache will speed up the computer’s operation, especially when retrieving frequently used data items. If the cache is small, however, this technique may result in erratic results. Caching is often used to store frequently accessed data in an inexpensive storage medium, which can be readily replenished if necessary.

Organization and access Memory organization is a general term referring to how data are stored within a computer system. It may be described as a hierarchy of storage devices. The most basic form is a linear hierarchy, where all memory addresses are aligned with a particular byte size. An example of this is a 32-bit processor with 4 GB of main memory and 16 GB of disk storage. In this case, the address space is 3 bytes long, and the address above 1 GB has 4 bits set to zero. When data are conceptually stored in memory, it is frequently for ease of access. Thus one might store an item at memory location 2000 to reflect that it is 2000 bytes (2 kB) into the address space.

Computer data storage systems

Main types

Computer data can be stored on various media, each of which have advantages and disadvantages. The choice of whether or not to use different types of storage rests with the designer of the computer. For example, in the 1980s many systems often came with both a hard disk drive and an optical disc drive.

Magnetic storage devices Most modern computers use rotating magnetic media to store data on spinning platters. On these devices bytes are laid out in specific patterns, called sectors, which allow efficient reading of data from the disk surface by the computer’s CPU. Magnetic storage devices are also widely used in the mass production of consumer electronics, especially computer hard drives contained within desktop computers and laptops.

Optical disc storage Optical discs are similar to magnetic tape, but use lasers instead of magnetism to store data. Optical discs are highly durable compared with conventional hard disks, due to the optical nature of the medium. However, optical discs are relatively slow compared with modern solid-state mass storage media (mainly flash memory) due to the time required for the read/write head to traverse across the disc, as well as from physical defects in optics which can reduce data density or jitter.

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