A memory is just like a Human brain . A Computer needs memory to store data & instructions .
It also needs memory to perform arithmetical operations and storing programs and pictures.
The memory is divided into large number of small parts called cells. Each location or cell has a unique address, which varies from zero to memory size minus one.
For example, if the computer has 64 k words, then this memory unit has 64 * 1024 = 65536 memory locations. The address of these locations varies from 0 to 65535.
Computer memory is of two types:
a) Primary Memory
c) Cache Memory
Primary Memory is a part of the CPU whereas the Secondary memory is external to the C.P.U. The Secondary Memory Store the data and keep it even when the electricity to the PC is cut off whereas the primary memory looses its contents immediately .
Main Memory (Primary) is made up of electronic circuits and keeps data as long as voltage supply is ON .
But when the power is cut off the data store in primary memory is lost.
Primary Memory classified into two types :
RAM (Random Access Memory )
RAM are further divided into two parts :
a) Static Ram
b) Dynamic Ram
Whats is RAM?????
RAM stands for Random Access Memory and it is a read / write memory. Information can be written into and read from a RAM .It is a volatile memory .It retained the stored information as long as it is supplied with power. When power supply is switched off or interrupted the stored information in the RAM is lost.
RAMs of various capacities are available for example 512 MB, 4 MB and 8 GB RAMs are available for PCs.
Static & Dynamic RAM :
Static RAM retained stored information only as long as power supply is “ON” but a Dynamic RAM loses its information in a very short time even though the power supply is “ON”.
Dynamic RAMs have to be refreshed periodically generally every milliseconds. The dynamic RAMs are cheaper and have high packing density and moderate speed. They also consume less power. They are used where large capacity of main memory is needed.
Static RAMs are costlier and consume more power .They don’t need refreshing circuitry. They have higher speeds than Dynamic RAMs .
Static RAM and Dynamic RAM are also written as SRAM & DRAM.
A short note on difference of SRAM and DRAM:
ROM (Read Only Memory)
ROM stands for Read Only Memory and it is a permanent type memory. Its contents are not lost when power supply is switched off. The user cannot write into a ROM . Its contents are written at the time of manufacturing.
ROM stored permanent programs and other types of information which are needed by the computer to execute the programs. ROMs store functions, such as sine , cosine , logarithm, square root & code conversion tables etc.
Types of ROMs:
Programmable ROMs called PROMs are also available. Further , different types of PROM such as erasable PROM called EPROM, electrically erasable PROM called EEPROM are also available.
Users can write information into a EPROM or EEPROM can be erased to record different information into it whereas in a PROM the information once written by the user can not be changed.
- Secondary memory refers to storage devices, such as hard drives and solid state drives. It may also refer to removable storage media, such as USB flash drives, CDs, and DVDs.
- Secondary Memory is employed for bulk storage of programs, data and other information. It has much larger capacity than the main memory .
- It stores system software , assemblers, compilers, useful packages, large data files etc..
- It is non volatile memory in nature .The magnetic memories ,such as hard disks and floppy disks are the most common secondary memories used in computers.
Random access devices are direct access devices . In such devices the information is available at random , i.e. it is available in any order.
Magnetic disks and magnetic drums are typical direct access storage devices.
Characteristics of Secondary Memory
- It is non-volatile, i.e. it retains data when power is switched off
- It is large capacities to the tune of terabytes
- It is cheaper as compared to primary memory.
Hard Disk Drive
Hard disk drive is made up of a series of circular disks called platters arranged one over the other almost ½ inches apart around a spindle. Disks are made of non-magnetic material like aluminum alloy and coated with 10-20 nm of magnetic material.
Standard diameter of these disks is 14 inches and they rotate with speeds varying from 4200 rpm (rotations per minute) for personal computers to 15000 rpm for servers. Data is stored by magnetizing or demagnetizing the magnetic coating. A magnetic reader arm is used to read data from and write data to the disks. A typical modern HDD has capacity in terabytes (TB).
CD stands for Compact Disk. CDs are circular disks that use optical rays, usually lasers, to read and write data. They are very cheap as you can get 700 MB of storage space for less than a dollar. CDs are inserted in CD drives built into CPU cabinet. They are portable as you can eject the drive, remove the CD and carry it with you. There are three types of CDs −
CD-ROM (Compact Disk – Read Only Memory) − The data on these CDs are recorded by the manufacturer. Proprietary Software, audio or video are released on CD-ROMs.
CD-R (Compact Disk – Recordable) − Data can be written by the user once on the CD-R. It cannot be deleted or modified later.
CD-RW (Compact Disk – Rewritable) − Data can be written and deleted on these optical disks again and again.
DVD stands for Digital Video Display. DVD are optical devices that can store 15 times the data held by CDs. They are usually used to store rich multimedia files that need high storage capacity. DVDs also come in three varieties – read only, recordable and rewritable.
Pen drive is a portable memory device that uses solid state memory rather than magnetic fields or lasers to record data. It uses a technology similar to RAM, except that it is nonvolatile. It is also called USB drive, key drive or flash memory.
Blu Ray Disk
Blu Ray Disk (BD) is an optical storage media used to store high definition (HD) video and other multimedia filed. BD uses shorter wavelength laser as compared to CD/DVD. This enables writing arm to focus more tightly on the disk and hence pack in more data. BDs can store up to 128 GB data.