In contrast to this division into physical records, a file often has natural divisions determined by the information represented. For example, a file containing information regarding a company’s employees would consist of multiple units, each consisting of the information about one employee. Or, a file containing a text document would consist of paragraph or pages. These naturally occurring blocks of data are called logical records.

Logical records often consist of smaller units called fields. For example, a logical record containing information about an employee would probably consist of fields such as name, address, employee identification number, etc. Sometimes each logical record within a file is uniquely indentified by means of a particular field within a record. Such as identifying fields is called a key field. The value held in a key field is called a key.

Logical record sizes rarely match the physical record size dictated by a mass storage device. In turn, one may find several logical records residing within a single physical record or perhaps a logical record split between two or more physical records. The result is that a certain amount of unscrambling is associated within a retrieving data from mass storage systems. A common solution to this problem is to set aside an area of main memory that is large enough to hold several physical records and to use this memory space as a regrouping area. That is, a blocks of data compatible with physical records can be transferred between this main memory area and the mass storage system, while the data residing in the main memory area can be referenced in terms of logical records.

An area of memory used in this manner is called a buffer. In general, a buffer is a storage area used to hold data on temporary basis, usually during the process of being transferred from one device to another. For example, modern printers contain memory circuitry of their own, a large part of which is used as a buffer for holding portions of a document that have been received by the printer but not yet printed.

Further Reading:
1. Wikipedia : Information Retrieval, File System, Database
2. Information Retrieval Wiki

Related posts:

1. Mass Storage
2. Flash Drives
3. Magnetic Systems

Although data stored in flash memory systems can be accessed in small byte-size units as in RAM applications, current technology dictates that stored data be erased in large blocks. Moreover, repeated erasing slowly damages the silicon dioxide chambers, meaning that current flash memory technology is not suitable for general main memory applications where its content might be altered many times a second. However, in those applications in which alterations can be controlled to a reasonable level, such as in digital cameras, cellular phones, and handheld PDAs, flash memory has become the mass storage technology of choice. Indeed, since flash memory is not sensitive to physical shock (unlike magnetic and optic systems) its potential in portable application is enticing.

Flash memory devices called flash drive, with capacities of up to a few GB are available for general mass storage applications. These units are usually packaged in small plastic cases. The high capacity of these portable units as well as the fact that they are easily connected to and disconnected from a computer make them ideal for off-line data storage. However, the vulnerability of their tiny storage chambers dictates that they are not reliable as optical disks for truly long term applications.

Related posts:

1. File Storage and Retrieval
2. Mass Storage
3. Magnetic Systems

CD technology was originally applied to audio recordings using a recording format known as CD-DA (compact disk-digital audio), and the CDs used today for computer data storage use essentially the same format. In particular, information on these CDs is stored on a single track that spirals around the CD like a groove on an old fashioned record, however, unlike old-fashioned records, the track on a CD spirals from the inside out. This track is divided into units called sectors, each with its own identifying markings and a capacity of a 2KB of data, which equates to 1/75 of a second of music in the case of audio recordings..

Note that the distance around the spiralled track is greater toward the outer-edge of the disk than at the inner portion. To maximize the capacity of a CD, information is stored at a uniform linear density over the entire spiralled track, which means that more information is stored in a loop around the outer portion of the spiral than in a loop around the inner portion. In turn, more sectors will be read in a single revolution of the disk when the laser beam is scanning the outer  portion of the spiralled track than when the beam is scanning the inner portion of the track. Thus, to obtain a uniform rate of data transfer, CD-DRA players are designed to vary the rotation speed depending on the location of the laser beam. However, most CD system used for computer data storage spin at a faster, constant speed and thus most accommodate variations in data transfer rates.

As a consequence of such design decisions, CD storage systems perform best when dealing with long, continuous strings of data, as when reproducing music. In contrast, when an application requires access to items of data in a random manner, the approach used in magnetic dish storage (individual, concentric tracks divided into individually accessible sectors) outperforms the spiral approach used in CDs.

Traditional CDs have capacities in the range of the 600 to 700MB. However, newer DVDs (Digital Versatile Disks), which are constructed from multiple, semitransparent layers that serve as distinct surfaces when viewed by a precisely focused laser, provide storage capacities of several GB. Such disks are capable of storing lengthy multimedia presentations, including  entire motion pictures.

Related posts:

1. Magnetic Systems
2. Flash Drives
3. File Storage and Retrieval

A disk storage system

Since a track can contain more information than we would normally want to manipulate at any time, each track is divided into small arcs called sectors on which information is recorded as a continuous string of bits. All sectors on a disk contain the same number if bits (typical capacities are in the range of 512 bytes to a few KB), an in the simplest disk storage systems each track contains the same number of sectors. Thus, the bits within a sector on a track near the outer edge if the disk are less compactly stored than those on the tracks near the center, since the outer tracks are longer than the inner ones. In fact, in high capacity disk storage systems, the track near the outer edge are capable of containing significantly more sectors than those near the center, and this capability is often utilized by applying a technique called zoned-bit recording. Using zoned-bit recording, several adjacent tracks are collectively known as zones, with a typical disk containing approximately ten zones. All tracks within a zone have the same number of sectors, but each zone has more more sectors per track than the zone inside of it. In this manner, the storage space near the outer edge of the disk is used more efficiently than in a traditional disk system. Regardless of the details, a disk storage system consists of many individual sectors, each of which can be accessed as an independent string of bits.

The location of tracks and sectors is not a permanently part of a disk’s physical structure. Instead, they are marked magnetically through a process called formatting (or initializing) the disk. This process is usually performed by the disk’s manufacturer, resulting in what are known as formatted disks. Most computer systems can also perform this task. Thus, if the format information in a disk is damaged, the disk can be reformatted, although this process destroys all the information that was previously recorder or stored on the disk.

The capacity of a disk storage system depends on the number of a disks used and the density in which the tracks and sectors are placed. Lower-capacity systems consist of a single plastic disk known as a diskette or, in those cases in which the disk is flexible, by the less prestigious title floppy disk. Diskettes are easily inserted and removed form their corresponding read/write units and are easily stored. As a consequence, diskettes have been popular for off-line storage information. However, since the generic 3.5 inch diskettes have a capacity of only 1.44MB, their use has largely been replaced by other technologies.

High capacity disk systems, capable of holding many gigabytes, consist if perhaps five to ten rigid disk mounted on a common spindle. The fact that the disks used in these systems are rigid leads them to be known as hard-disk systems, in contrast to their floppy counterparts. To allow for faster rotation speeds, the read/write heads in these systems do not touch the disk but instead “float” just off the surface. The spacing is so close that even a single particle of dust could become jammed between the head and disk surface, destroying both (a phenomenon known as a head crash). Thus hard-disk systems are housed in cases that are sealed at the factory.

Several measurements are used to evaluate a disk system’s performance:

1. Seek Time (The Time required to move the read/write heads from one track to other)
2. Rotation delay or latency time (half the time required for the disk to make a complete rotation, which is the average amount of time required for the desired data to rotate around to the read/write head once the head has been positioned over the desired track)
3. Access time (The sum of seek time and rotation delay)
4. Transfer rate (the rate at which data can be transferred to or from the disk)

Hard-disk system generally have significantly better characteristics than floppy systems. Since the read/write heads do not touch the disk surface in a hard-disk system, one find rotation speeds of several thousand revolutions per-minute, whereas disks in floppy disk systems rotate on the order of 300 revolutions per-minute. Consequently, transfer rates for hard-disk systems, usually measured in MB per second, are much greater than those associated with floppy-disk systems, which tend to be measured in KB per second.

Since disk systems require physical motion for their operation, both hard and floppy systems suffer when compared to speed within electronic circuitry. Delay times within an electronic circuit are measured in units of nanoseconds or less, whereas seek times, latency times, and access times of a disk systems are measured in milliseconds. Thus the time required to retrieve information from a disk system can seem like an eternity to an electronic circuit awaiting result.

Disk storage systems are not the only mass storage devices that apply magnetic technology. An older form of mass storage using magnetic technology is magnetic tape. In these systems, information is recorded on the magnetic coating of a thin plastic tape that is wound on a reel of storage. To access the data, the tape is mounted in a device called a tape drive that typically can read, write, and rewind the tape under control of the computer. Tape drives range in size from small cartridge units, called a streaming tape units, which use tape similar in appearance to that in stereo systems to older, large reel-to-reel units. Although the capacity if those devices depends on the format used, most can hold many GB.

A major disadvantage of magnetic tape is that moving between different position on a tape can be very time-consuming owing to the significant amount of a tape that must be moved between the reels. Thus tape systems have much longer data access times than magnetic disk systems in which different sectors can be accessed by short movement of the read/write head. In turn, tape systems are not popular for on-line data storage. Instead, magnetic tape technology is reserved for off-line archival data storage application where its high capacity, reliability, and cost efficiency are beneficial, although advances in alternatives, such as DVDs and flash drives, are rapidly challenging this last vestige of magnetic tape.

IBM Magnetic Tape Unit

Related posts:

1. Optical Systems
2. Flash Drives
3. Mass Storage

The term on-line and off-line are often used to describe devices that can be either attached to or detached from a machine. On-line means that the device or information is connected and readily available to the machine without human intervention. Off-line means that human intervention is required before the device or information can be accessed by the machine-perhaps because the device must be turned on, or the medium holding the information must be inserted into some mechanism.

A major disadvantage of mass storage systems is that they typically require mechanical motion and therefore require significantly more time to store and retrieve data than a machine’s main memory, where all activities are performed electronically.

Related posts:

1. Flash Drives
2. File Storage and Retrieval
3. Memory Organization

Unfortunately, this application of prefixes represents a misuse of terminology because these prefixes are already used in other fields in reference to units that are powers of ten. For example, when measuring distance, kilometer refers to 1000 meters, and when measuring radio frequencies, megahertz refers to 1,000,000 hertz. To make matters even worse, some manufactures of computer equipment have mixed the two sets of terminology by using KB to refer to 1024 bytes but using MB to mean an even 1000KB (which is 1,024,000). Needless to say, these discrepancies have led to confusion and misunderstanding over the years.

To clarify matters, a proposal has been made to reserve the prefixes kilo, mega, and giga for units that are powers of ten, and to introduce the new prefixes kibi (short for kilobinary and abbreviated (Ki) in reference to the corresponding units that are powers of two. Under the system, the term kibibyte (KiB) would refer to 1024 bytes, whereas kilobyte (KB) would refer to 1000 bytes. Whether these prefixes become a part of the popular vernacular remains to be seen. For now, the traditional, “misuse” of the prefixes kilo, mega, and giga remains ingrained in the computing community when referring to main memory, and thus we will follow this tradition in our study when referring to data storage. However, the proposal prefixes kibi, megi, and gibi do represent an attempt to solve a growing problem, and one would be wise to interpret terms such as kilobyte and megabyte with caution in the future.

Recommend Reading:

Wikipedia : Binary Prefix
Coding Horror : Dude, Where’s My 4 Gigabytes of RAM?

Related posts:

1. Memory Organization
2. Flash Drives
3. Magnetic Systems

To identify individual cells in a computer’s main memory, each cell is assigned a unique “name”, called its address. The system is analogous to the technique of identifying houses in a city by addresses. In the case of memory cells however, the addresses used are entirely numeric. To be more precise, we envision all the cells being placed in a single row and numbered in this order starting with the value zero. Such an addressing system not only gives us a way of uniquely identifying each cell but also associates order to the cells giving us phrases such as “the next cell” or “the previous cell”.

An important consequence of assigning an order to both the cells in main meory and the bits within each cell is that the entire collection of bits within a computer’s main memory is essentially ordered in one long row. Pieces of this long row can therefore be used to store bit patterns that may be longer than the length of a single cell. In particular, we can still store a string of 16 bits merely by using to consecutive memory cells.

To complete the main memory of a computer, the circuitry that actually holds the bits is combined with the circuitry to allow other circuit to store and retrieve data from the memory cells. In this way, other circuits can get data from the memory cells. In this way, other circuits can get data from the memory by electronically asking for the contents of a certain address (read operation), or they can record information in the memory by requesting that a certain bit pattern be placed in the cell at a particular address (write operation).

Because a computer’s main memory is organized as individual, addressable cells, the cells can be accessed independently as required. To reflect the ability to cells, the cells can be accessed independently as required. To reflect the ability to access cells in any order, a computer’s main memory is often called random access memory (RAM). This random access feature of main memory is in stark contrast to the mass storage systems that will be discussed on the next article, in which long strings of bits are manipulated as amalgamated blocks.

Memory cells arranged by address

Although flip-flops that introduced earlier can be used as a means of storing bits, the RAM in most modern computers is constructed using other technologies that provide greater miniaturization and faster response time. Many of these technologies store bits as tiny electric charges that dissipate quickly. Thus these devices require additional circuitry, known as refresh circuit, that repeatedly replenishes the charges many times a second. In recognition of this volatility, computer memory must be constructed from such technology called dynamic memory, leading to term DRAM (Dynamic RAM). Or, at times the term SDRAM (Synchronous DRAM) is used in reference to DRAM that applies additional technique to decrease the time needed to retrieve the contents from its memory cells.

Related posts:

1. Measuring Memory Capacity
2. Flash Drives
3. Magnetic Systems

The left column displays all possible bit pattern of length four; the right column shows the symbol used in hexadecimal notation to represent the bit pattern to its left. Using this system, the bit pattern 10110101 is represented as B5. This is obtained by dividing the bit pattern into substrings of length four and then representing each substring by its hexadecimal equivalent – 1011 is represented by B, and 0101 is represented by 5. In this manner, the 16-bit pattern 1010010011001000 can be reduced to the more palatable form A4C8

Related posts:

1. Bits
2. Memory Organization
3. Gates and Flip-flops

Boolean Operations AND, OR, XOR (exclusive or)

Note that the AND, OR, XOR, and NOT gates are represented by distinctively shaped symbols, with the input values entering on one side and the output exiting on the other.

Gates provide the building blocks from which computers are constructed. One important step in this direction is depicted in the circuit. This is a particular example from a collection of circuits causes it to shift to other value. In other words, the output will flop or flop between two values under control of external stimuli. As long as both input in the circuit remain 0, the output (whether 0 or 1) will not be change. However, temporarily placing a 1 on the upper input will force the output to be 1, whereas temporarily placing a 1 on the lower input will force the output to be 0.

Related posts:

1. Boolean Operation
2. Memory Organization
3. Bits

The Boolean operation AND is designed to reflect the truth or falseness of a statement formed by combining two smaller, or simpler, statements with conjunction and. Such statement have the generic form

P AND Q

where P represents one statement and Q represents another; for example,

Kermit is a frog AND Miss Piggy is an actress

The inputs to the AND operation represent the truth or falseness of the compound statement’s components; the output represents the truth or falseness of the compound statement itself. Since a statement of the form P AND Q is true only when both of its components are true, we conclude that 1 AND 1 should be 1, whereas all other cases should produce an output of 0.

In a similar manner, the OR operation is based on compound statements of the form

P OR Q

where, again, P represents one statement and Q represents another. Such statements are true when at least one of their components is true, which agrees with OR operation.

There is not a single conjunction in the English language that captures the meaning of the XOR operation. XOR produces an output 1 (true) when one of its input is 1 (true) and other is 0 (false). For example, a statement of the form P XOR Q means “either P OR Q but not both”.

The operation NOT is another Boolean operation. It differs from AND, OR, and XOR because it has only one input. Its output is the opposite of that input; if the input of the operation NOT is true, then output is false, and vice versa. This, if the input of NOT operation is the truth or falseness of the statement

Fozzie is a bear

then the output would represent the truth or falseness of the statement

Fozzie is not a bear

Related posts:

1. Gates and Flip-flops
2. Bits
3. The Role of Algorithms