Secondary storage means external storage. This storage
device includes a magnetic disk, optical disk, magnetic tape
and others. The most widely used external storage is as
Logical Data Elements
If there is no systematic way to store and retrieve data, it is too difficult to get any information from an information system. Therefore, data resource should be organized in some logical manner. Data are logically organized into characters, fields, records, files, and database.
- Character: A character is the most elementary logical data element, whereas bit and byte are basic physical storage elements. A character consists of a single alphabetic letter, numeric digit, or special symbol. The character is equivalent to a byte.
- Field: The next higher level of data is the field. A field consists of a grouping of related characters. For example, a set of characters in a customer's name makes a name field. A field represents an attribute of an entity.
- File: A set of related records makes a file. For example, a customer file consists of many records of customers.
- Database: A database is an integrated collection of logically related files. A database combines several records previously stored in separated files.
Floppy disks are removable, direct access storage media inserted into disk drives. Floppy disks are flat, circular pieces of Mylar plastic that rotate within a jacket. These are also called flexible disks, floppies, Diskettes or simply disks.
Data and instructions are stored as forms of bits and bytes using the ASCII or EBCDIC data coding schemes. They are stored as electromagnetic charges on a disk surface. The two most common disks are 5 1/4-inch (5.25") and 3 1/2-inch (3.5") in their diameter. More efficient size, high storage capacity and sturdier design of a 3.5"-disk make microcomputers now use the 3.5" disks more.
Hard / Soft Sectored Disks and Formatting
- Track: The disk surface is divided into several concentric circles called tracks. The thinner the tracks, the more storage capacity of the disk. Data are recorded as tiny spots on these tracks. These tracks are closed concentric circles, not a single spiral. Each track has the same number of bits although the outer tracks are longer than the inner ones.
- Sector: The circular tracks are further divided into wedge-shaped sections known as sectors. The fields of data within a particular record are organized according to tracks and sectors on a disk. - graph
There are two kinds of sectors for a disk. A hard sectored disk is one and a soft sectored disk is the other. The hard sectored disk identifies sectors with some physical marks on the disk, whereas the soft sectored disk identifies sectors with sector identification contained in the recording tracks.Capacity
A format program places the sector identification on the disk. When a user buys a disk manufactured without tracks and sectors in place (this kind of disk does not have "Formatted" label on it), he or she must put the tracks and sectors using a format program.
- DS/DD (2S/2D) Disk: DS/DD means Double Sided Double Density. This is a floppy disk recorded on both of its sides (DS) and has twice the capacity of the prior format (DD). For the IBM and IBM compatible PCs, DD/DS disk usually has 40 tracks on each side and each side contains 9 sectors of 512 bytes each.
DS/DD: 720 KB (PCs), 800 KB for Mac formats.
- DS/HD Disk: DS/HD means Double Sided High Density. An HD disk has increased storage capacity usually with more tracks per square inch. For IBM and IBM compatible PCs, an HD disk is usually formatted to have 80 tracks on each side. Each side then contains 18 sectors of 512 bytes each - double capacity of a DD disk.
DS/HD: 1.44 MB (PCs), 1.44 MB for Mac formats.
A floppy drive grabs a disk at its center and spins it inside its plastic jacket. The floppy drive obtains stored data and instructions from a floppy disk and stores them onto the disk. The drive is made up of a box with a slot into which a user inserts a disk. The slot has a drive gate. This drive rotates the disk with a motor inside the drive. Electronic read/write heads "read" data from the disk and "write" data to it while the disk rotates.Working Process of Floppy Drive
A microcomputer usually has internal floppy drives inside the computer cabinet, but it sometimes has external floppy drive, a separate component outside the cabinet.
- Inserting disk: Inserting a disk to let it work.
- Closing gate: After inserting, the drive gate should be closed. This places the disk around a spindle and holds it.
- Revolving: Spinning the disk inside the jacket.
- Reading/ Writing: Read- write heads on an access arm transfer data signals from disk to computer or computer to disk.
- Seek: Seek operation is to let the access arm move the read-write head to the requested track on the disk.
- Search: Search operation is to let the drive rotate the disk to the proper position.
Hard Disks (Hard Drives)
A hard disk is a magnetic disk made of metal and
covered with a magnetic recording surface. Hard
disks come in removable and fixed varieties that
hold from several hundreds of megabytes to several
gigabytes. They are tightly sealed to
prevent any foreign matter from getting inside which causes head crash.
SCSI (Small Computer System Interface): SCSI is pronounced scuzzy. This is an 8-bit-bus peripheral interface for up to seven peripherals. The SCSI bus allows any two devices to communicate at one time (host to peripheral, peripheral to peripheral). SCSI provides high-speed (4MB/sec.), parallel data transfer and multiple peripheral connections while taking only one expansion slot.
More advanced technology created a new disk storage forms. Optical disk is a disk written and read by laser beam. This optical disk has a great impact on today's storage technology. Optical disk does not spin, does not need to move access arms and read/write heads, because a laser beam can be moved electronically. The capacity of the storage is considerably greater than their magnetic disk counterparts, and optical disk storage may eventually replace all magnetic tape and disk storage.
Then how do they work? To write data, a laser beam burns tiny cavities into the surface of a disk to mark bits for data. To read the data, a laser beam scans these areas. There are three forms of optical disks available:
- CD-ROM:CD-ROM (compact disk read only memory) is an optical disk storage that contains text, graphics and hi-fi stereo sound. CD-ROM is a 4.75-inch optical disk storage that can store around 650 MB of data. CD-ROM disk is almost the same as the music CD, but uses different forms of track for data. A CD- ROM drive can read music CD, but a CD player cannot read CD-ROM. CD-ROM is a read-only disk that cannot be written on or erased by the user. In CD- ROM standard, data (text or pictures) cannot be viewed with audio play simultaneously. CD-ROM XA standard can do.
- WORM: A WORM (write once, read many) disk is an optical disk that written on just once by the user's environment and then cannot be overwritten. A WORM disk is ideal for use as archive because it can be read many times, but the data cannot be erased. The storage capacity of WORM disk ranges from 400 MB to 6.4 GB.
- Erasable Optical Disks: This is an optical disk that can be erased and written on repeatedly. An erasable optical disk has a great deal of data capacity. It can store up to 4.6 GB. An erasable optical disk functions like a magnetic disk and has huge capacity, so it will replace the magnetic disk in the future.
A magnetic tape is a tape coated with a magnetic material on which data can be stored. This is a sequential storage device that is usually used for a backup purpose. A magnetic tape is slower than direct access storage such as disk, because it is sequential access storage. The biggest advantage of magnetic tape is the cost. It is much less expensive than magnetic disk and optical disk. An advanced tape backup technology (DAT) is available these days.
A cache is a specially designed buffer storage used to improve computer performance by reducing access time. It holds instructions and data that are likely to be needed for next operation by the processor. The cache copies frequently accessed data and instructions from primary storage (main memory) or secondary storage (disks).
- Disk Cache: A disk cache is in a reserved segment of primary memory or in an extra memory on the disk controller card. It contains a large block of frequently accessed data copied from a disk. The data in a disk cache can be used to fulfill the following data requests from a processor in a high speed. The disk cache lets the processor avoid a slow disk access.
- Memory Cache: A memory cache is a high-speed memory storage between memory and the CPU. It is smaller and much faster than main memory (primary storage). The memory cache copies blocks of instructions and data from the main memory so that execution and data updating are performed in the higher-speed memory bank.