Memory-Types

                               Memory

Introduction:

• Storage space in computer.
• Used to process data and instructions required for processing are stored. 
• Divided into large number of small parts called cells. 
• Each location or cell has a unique address which varies from 0 to memory size minus one.

                 Types Of Memory

Primary Memory	secondary Memory
Volatile in nature	Non-volatile in nature
Used for Processing	Used for Storage
Small memory	Large memory
Examples are Static RAM,Dynamic RAM	Examples are PROM,EPROM,EEPROM

I.PRIMARY MEMORY

  1.Random Access Memory (RAM)

Read/write memory. Volatile -When power supply of computer is switched off or interrupted the stored information in the RAM is lost.

                                                     There are two types of RAM.

	Static RAM	Dynamic RAM
Retain Stored Information	Yes	Yes
Need to be refreshed	No	Yes
Speed	High	Moderate
Power Consumption	More	Less

2.Read Only Memory (ROM)

Read-only memory. Its contents are not lost when power supply of computer is switched off. Contents are written at manufacturing time. Store permanent programs and information needed by the computer to execute user programs.

PROMs (PROGRAMMABLE ROMs)

• Users can write information in PROMs, EPROMs and E2PROMs                                              

	PROM	EPROM	EEPROM
Can be programmed	Yes	Yes	Yes
Can be Erased/Modified	No	Yes	Yes
Probability to be programmed	Only one time	Multiple	Multiple
How can be erased	Cannot be erased	With UV Light	With Electrical energy
Retain the old data	No	No	Yes
Speed	Fast	Fast	Slow

     

             

 II. SECONDARY MEMORY

Floppy Disk

• Soft magnetic disk often called floppies or diskettes.
• Portable.
• Slower to access.
• Less storage capacity.

      

Magnetic Disk or Hard Disk

• Higher storage capacity.
• Coated with magnetic oxide.
• Information stored with the help of magnetization.
• Read/write head is used to read or write the data on disk.
• Disk is divided into tracks,sectors and clusters.
• Address of a disk is required to read or write data.
• Address is specified in terms of surface number,track number and sector number.
• Once the address is found the header transfers the data to primary memory.                    

Magnetic Tape

• It consists of electrical,mechanical and electronic components.
• Bits are recorded as magnetic spots on the tape.
• Read/write heads are mounted one in each track to record and read sequence of characters.
• Information is recorded in blocks called records.
• Each record has identification bit pattern.
• Addressed by specifying record number and number of charectors.

     

Optical Disk(CD-ROM)

• Compact disk-Read Only memory.
• Coated with highly reflective metal.
• Data recording is accomplished by turning the laser beam on.
• Tiny holes called PITS are burnt into metal coating on disk to represent 1 or 0.
• Storage density is enormous.
• Fast access and Low cost.
• High storage capacity than magnetic tapes.

                                 

             

Magnetic Drum

• Direct access storage device for sequential and random processing.
• Contains cylinder coated with magnetic material.
• Cylinder surface is divided into tracks on which data is stored in the form of magnetic spots.
• Data is read through read/write heads.
• Whenever new data is recorded old data gets erased. 

                                             

Flash Memory

• Non-Volatile.
• Can be erased and reprogrammed very fast.
• Its a variation of EEPROM which takes much time to update.
• Often used to hold controls code i.e., input/output system code in a computer.