II. Data Storage
Introduction
Data storage represents the storage of data within a computer. Types of data include text, numeric values, images, audio, and video.
Bits and Their Storage
Data are encoded as patterns of 0s and 1s called bits (short for binary digits).
Boolean operations manipulate true/false values. 0 represents the value false and 1 represents the value true.
The input of a Boolean operation into a device which produces an output is called a gate.
Hexadecimal Notation
A long string of bits is called a stream.
To simplify such long bit patterns hexadecimal notation is used. With it, bit patterns in multiples of four are grouped and encoded together as single symbols. As such, a string consisting of twelve bits can be rewritten with only three symbols.
Main Memory
Data are stored in the computer’s main memory which contains a large collection of circuits each capable of storing a single bit.
A computer’s main memory contain units of cells.
A string of 8 bits contained within a cell is called a byte.
The high-order end of a byte is at its left end with greatest significance placed on the last bit (most significant bit). Comparatively, the low-order end is at its right end with least significance placed on the last bit (least significant bit).
Each cell is assigned a unique name called its address which helps to locate and identify individual cells in a computer’s main memory.
Because cells in a computer’s main memory are accessed in any order, they are often called random access memory (RAM).
Dynamic memory (DRAM) utilizes a technology which increases response time by storing bits as tiny electric charges which dissipates quickly.
Synchronous DRAM (SDRAM) is also used to add additional techniques in reference to DRAM to create even faster response time.
Measuring Memory Capacity
The term kilobyte (KB) is used to refer to 1,024 bytes. (Example: 4,096 bytes = 4KB)
The term megabyte (MB) is used to refer to 1,048,576 bytes. (Example 3,145,728 bytes = 3 MB)
The term gigabyte (GB) is used to refer to 1,048,576,000 bytes (Example 20,971,520,000 bytes = 20GB)
Mass Storage
Most computers have additional memory devices called mass storage (or secondary storage) systems which enables higher capacity data storage.
Magnetic Systems
A mass storage system known as the magnetic disk uses a thin spinning disk with magnetic coating to hold data.
As the disk spins, the read/write head(s) traverses a circle, called a track, above or below the disk’s surface.
Several adjacent tracks are collectively known as zones.
Information is recorded as a continuous string of bits within divisions of small sectors in each track.
The space on the outer edge of the disk is used more efficiently via zone-bit recording which takes into account that, in a high capacity disk storage system, each zone has more sectors per track than the zone inside of it.
A diskette is a lower capacity system consisting of a single plastic disk or, in the case in which the disk is flexible, a floppy disk.
Seek time measures the time required to move the read/write heads from one track to another.
Rotation delay or latency delay measures the time required for the disk to make a complete rotation.
Access time measures the sum of seek time and rotation delay.
A magnetic technology known as magnetic tape records information on a thin plastic tape that is wound on a reel for storage.
Information on a magnetic tape is accessed using a device called a tape drive which can typically read, write, and rewind the tape.
Optical Systems
Information is recorded on a compact disk (CD), approximately five inches in diameter, by creating variations and distinctions in its reflective surface. The retrieval of information is monitored by means of a laser beam which scans for irregularities on the surface of the spinning CD.
Flash Drives
As opposed to receiving information through physical motion, electronic signals are sent directly to the storage medium by means of a newer mass storage technology known as flash memory which diminishes the disadvantage of physical motion, that of spinning disks, moving read/write heads, and aiming laser beams.
Devices that take advantage of flash memory technology are known as flash drives; with capacities of up to a few GB and are known for its wide array of general purpose usage, off-line storage and small form factor of approximately only three inches long.
File Storage and Retrieval
Information of similar application is grouped into large units called files. Typical file types include text documents, photographs, programs, and music recordings.
A file is stored in a storage device in the form of a physical record which conforms to the characteristic of the storage device being used; as in the case of a mass storage, a file would typically incorporate many physical records within manageable sectors each consisting of small, multiple byte units.
Determined by the information being represented, natural divisions are often given to files by separating its components into naturally occurring blocks of data called logical records.
Logical records are composed of smaller units called fields.
Key fields help give fields their unique identity whose value is held in its key.
