Primary Storage dev ices has two limitations

1. Limited Capacity

2. Volatile

Secondary storage devices

1. To overcome this limitation all computers use additional memory called auxiliary memory or secondary memory

2. They are in volatile

3. Less expensive

4. Slow Speed

Sequential and Direct Access Devices

The secondary devices are divided in to two groups depending on the method they access data

1. Sequential Storage

2. Direct Access

Sequential Storage

· A sequential access storage deice is one in which arrival at a desired storage is preceded by sequencing through other locations . So access time varies according to location

· We retrieve stored information in same sequence in which it is stored .

· We can not access data , at last few memory location , before we have traversed all proceeding locations

· Its is similar to Audio Cassettes

· Example Magnetic Tape

Direct Access Device

· In this method we can reach any location with almost equal access time

· Magnetic Disk is a Example

Magnetic Tape

· Used for back up of data

· Made of plastic Ribbon

· Usually ½ inch or ¼ inch wide

· 50 to 2400 feet long

· Surface is coated such that it can be magnetized

· Data is saved in form of small magnetic spots (zero one)

Storage Information

· Consists of vertical columns called frames

· Horizontal rows are called channels or tracks

· Older tapes0 had 7 tracks and used 6 bit BCD

· First six bits represented the code for the data where as seventh is parity bit

· Modern tapes have 9 tracks and uses EBCDIC (Extended Binary Coded Decimal Interchange Code )
magnetic Tapes record data continuously

Parity Check

· A parity bit, or check bit is a bit added to the end of a string of binary code that indicates whether the number of bits in the string with the value one is even or odd.

· Parity bits are used as the simplest form of error detecting code.

Storage Capacity

Storage capacity of a tape = Data recording density x length

· It is the amount of data that can be stored

· Measured in bytes per inches

· Varies from 800 bpi to 77000 bpi

Data Transfer Rate

· It is number of characters that are transmitted per second from tape to primary storage

· Measured in bps (bytes per second)

· Depends on data density and speed f data movement

Tape Drive

· Used for storage or data retrieval

· Have read write heads

· As tape ribbon passes under the heads , the tape heads can either read or write

Advantages

· Storage is virtually unlimited

· Low coast

· High data rate

· Portable and easy to handle

Disadvantage

· Not suitable for accessing random data

· Must be stored in dust free environment

· Needs to be labeled

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