Computer Memory

 Computer Memory (Primary and Secondary)

            Primary memory

Primary storage (or main memory or internal memory), often referred to simply as memory, is the only one directly accessible to the CPU. The CPU continuously reads instructions stored there and executes them as required. Any data actively operated on is also stored there in uniform manner.

 Types of Primary Memory

RAM (Random Access Memory/Read-Write Memory)

Random-access memory (RAM) is a form of computer data storage. A random-access device allows stored data to be accessed quickly in any random order. In contrast, other data storage media such as hard disks, CDs, DVDs and magnetic tape, as well as early primary memory types such as drum memory, read and write data only in a predetermined order, consecutively, because of mechanical design limitations. Therefore the time to access a given data location varies significantly depending on its physical location

 

ROM (Read-only-memory)

Read-only memory (ROM) is a class of storage medium used in computers and other electronic devices. Data stored in ROM cannot be modified, or can be modified only slowly or with difficulty, so it is mainly used to distribute firmware (software that is very closely tied to specific hardware, and unlikely to need frequent updates).

 

Computer memory

Secondary Memory

storage device or secondary memory
Secondary Memory is the external memory of a computer. Secondary Memory is also called Auxiliary memory or device. Since the RAM is not permanent, acomputer system requires secondary memory to hold data and programs permanently. The CPU can not directly access data and programs of secondarymemory. They first need to transfer into RAM before being used. Most widely used secondary storage devices are magnetic tapes, magnetic disk (Hard disk, floppy disk), optical disk (CD, DVD) and flash drive. These devices store data, information and programs permanently in the form of binary digits till we don’t erase them.

Hard Disk

Hard Disk is present inside the system unit. Hard disk is the main secondary storage device that stores large amount of data permanently. Hard disk is also called fixed disk. Hard disk stores operating system software of a computer, data and other applications software. Hard disk consists of many metal disks or platters coated with magnetic material that store data in tracks, which is further divided into sectors. The various capacities of Hard disks are available in the market. Nowadays hard disk of 1TB (Tera Byte) is available in the market.

Floppy Disk

A floppy disk is a portable storage device that stores data permanently. The floppy disk drive is used to read and write data on a floppy disk. Floppy disk may be mini floppy disk or micro floppy disk. The storage capacity of a Micro Floppy disk is 1.44 MB (Mega Byte) and of a Mini floppy disk is 1.2 MB. Floppy disk gets damaged easily. So, nowadays it is not used. Floppy disk is a thin plastic disk coated with magnetic material for storage of data or information. Floppy disk has a hard outer covering. Floppy disk is inserted into the disk drive while saving information into it or copying files into it. Thus, the file can be transferred into other computer.

Compact Disk/DVD

Compact Disk/Digital Video Disk stores huge amount of data permanently. The Compact Disk Drive (CD-ROM) is required to read data from a compact disk. A CD-RW drive is used to write data on blank CDs and delete, modify or add data on an already written CD-RW (CD-Rewritable). DVD Drive (DVD-ROM) is required to read the data from DVD (Digital Versatile Disk). DVD Writer allows you to read as well as write data on DVDs. The capacity of a compact disk is 700MB and DVD is 7 GB. CD/DVD is portable disk that can be used for transferring data and programs from one computer to another computer. SinceCDs and DVDs are reliable storage devices they are also used for keepingbackup copy of data and programs.

Pen Drive

Pen drive is also called Flash Drive. Pen drive is a portable device used for transferring data and programs from a computer to another computer. You can use a pen drive to copy data, songs, games and programs from one computer to another computer. Different storage capacities of pen drives (256 MB to 16 GB) are available in the market.

Primary memory vs Secondary memory

  • Processor access the primary memory in a random fashion. Unlike primary memory, secondary memory is not directly accessed through CPU. The accessing of the primary memory through CPU is done by making use of address and data buses, whereas input/ output channels are used to access the secondary memory.
  • The primary memory is embedded with two types of memory technologies; they are the RAM (Random Access Memory) and ROM (Read Only Memory). The secondary memory is accessible in the form of Mass storage devices such as hard disk, memory chips, Pen drive, floppy disk storage media, CD and DVD.
  • Primary memory is volatile in nature, while  secondary memory is non volatile. The information that is stored in the primary memory cannot be retained when the power is turned off. In case of secondary memory, the information can be retrieved even if the power is turned off because the data will not be destructed until and unless the user erases it.
  • When the data processing speed is compared between the primary and secondary memory, the primary memory is much faster than the secondary memory.
  • In the cost perspective, the primary memory is costlier than the secondary memory devices. Because of this reasons most of the computer users install smaller primary memory and larger secondary memory.
  • As the secondary memory is permanent, all the files and programs are stored in the secondary memory most and as the primary memory interacts very fast with the micro processor, when the computer needs to access the files that are stored in the secondary memory, then such files are first loaded into the primary memory and then accessed by the computer.

 

Primary memory Secondary memory
The memory devices used for primary memory are semiconductor memories The secondary memory devices are magnetic and optical memories.
The primary memory is categorized as volatile and non volatile memories, RAM is the volatile memory and ROM is the non volatile memory The secondary memory is always non volatile
The primary memory is composed of programs and data that are presently being used by the  micro processor The secondary memory is enough capable to store huge amount of information
The primary memories are more effective and fast to interact with the micro processor The secondary memories are somewhat slow in interacting with the micro processor, when compared with the primary memory.
Primary memory is known as main memory Secondary memory is known as additional memory or back memory
These memories are also called as internal memory These memories are also called as external memory
Primary memory is temporary The secondary memory is permanent
Commonly used primary memory (main memory) available in the range of 512 MB to 8 GB RAMs. Generally secondary memories range between 80 GB to 4 TB Hard Disc Drives
The primary memory devices are connected to the computer through “slots” The secondary memory devices are connected to the computer through Cables

 

 

Cache Memory

Cache memory is a very high speed semiconductor memory, which can speed up CPU. It acts as a buffer between the CPU and main memory.

It is used to hold those parts of data and program which are most frequently used by CPU. The parts of data and programs are transferred from disk to cache memory by operating system, from where CPU can access them.

Advantage

  • Cache memory is faster than main memory.
  • It consumes less access time as compared to main memory.
  • It stores the program that can be executed within a short period of time.
  • It stores data for temporary use.
  • Cache memory has limited capacity.
  • It is very expensive.

Disadvantage:

  • Cache memory has limited capacity.
  • It is very expensive.

 

 

References

http://www.ask.com/question/difference-between-primary-and-secondary-storage

http://www.tutorialspoint.com/computer_fundamentals/computer_memory.htm

http://en.wikipedia.org/wiki/Computer_data_storage

http://www.teach-ict.com/as_as_computing/ocr/H447/F453/3_3_1/memory%20management/miniweb/pg2.htm

http://readanddigest.com/what-are-the-differences-between-primary-and-secondary-memory/

http://searchstorage.techtarget.com/definition/primary-storage

 

 

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