MEMORY

Introduction:
Memory is one of the most important things that are incorporated into digital computers. It is use to store data. There are various computer memory types installed, depending upon the actual need for functioning and specifications of the system, they are semiconductor memory, magnetic memory and optical memory etc. The computer memory relates to the many devices and components that are responsible for storing data and applications on a temporary or a permanent basis. It enables a person to retain the information that is stored on the computer. Without it, the processor would not be able to find a place which is needed to store the calculations and processes. Memory is also known as data storage device.
Purpose of memory:
The main purpose is to store data or information’s. A digital computer represents data using the binary numeral system. Text, numbers, pictures, audio, and nearly any other form of information can be converted into a string of bits, or binary digits, each of which has a value of 1 or 0. A piece of information can be handled by any computer whose storage space is large enough to accommodate the binary representation of the piece of information, or simply data.

Types of memory and their uses:
1. Semiconductor Memory
Semiconductor memory uses semiconductor-based integrated circuits to store information. A semiconductor memory chip may contain millions of tiny transistors or capacitors. Both volatile and non-volatile forms of semiconductor memory exist. In modern computers, primary storage almost exclusively consists of dynamic volatile semiconductor memory or dynamic random access memory. Non-volatile semiconductor memory known as flash memory has steadily gained share as off-line storage for home computers. Non-volatile semiconductor memory is also used for secondary storage in various advanced electronic devices and specialized computers.
Read Only Memory (ROM): It is a kind of permanent non-volatile storage memory. This type of memory is active, regardless of whether the system is turned on or is switched off. The contents in it cannot be changed or modified. It is an integrated circuit which is pre-programmed with important data that should necessarily be present for the computer to carry out its normal functionalities. Basically there are two types of ROM namely PROM, EPROM, EEPROM etc.
a. PROM: A PROM is a programmable ROM. User can write a program, data and other information permanently into PROM but it can use only for once. PROM programmer is used to write program.
b. EPROM: An EPROM is an erasable PROM. Its content can be erasing and it can be used more than once. EPROM is more economical than PROM because they can reuse.
Random Access Memory (RAM): RAM is a volatile memory, which is located within the computer system and is responsible for stacking away data on a temporary basis, so that it can be promptly accessed by the processor. The information stored in RAM is typically loaded from the computer's hard disk, and includes data related to the operating system and certain applications. When the system is switched off, RAM loses all the stored information. The data remains stored and can be retained only when the system is running. When the RAM gets full, the computer system operates at a slow speed. The data can be retrieved in any random order. Generally, there are two types of RAM;
1. Static RAM (SRAM): It uses conventional flip-flop circuit consisting of two cross-coupled inverters to store binary bit 0 and 1. They can store information as long as the power supply is switch on. It’s faster than DRAM but it consumes more power.

2. Dynamic RAM (DRAM): DRAM is temporary storage, they can store information only for few milliseconds, but they consume less power. It’s stores information in the form of charge on the capacitor.

Today DRAM become most important memory, it is again divided into various different categories namely SDRAM, DDR SDRAM, RDRAM etc.

SDRAM (synchronous DRAM): It uses same clock rate as CPU. They run at processor memory bus without imposing wait states.

DDR SDRAM: DDR stands for double data rate. It is a faster version of SDRAM. It is works on both edges of clock signal; as a result their transfer mode is burst transfer mode i.e. it can transfer block of data in a single time. DDR memory operating at 333MHz and is a 2.5 volt technology. Latest versions of DDR SDRAM are DDR2 SDRAM, DDR3 SDRAM etc.

DDR2: DDR2 stands for double data rate twice.

DDR3: DDR3 SDRAM is an improvement over its predecessor, DDR2 SDRAM. DDR2 has memory clock frequency of 100 MHz and gives a maximum transfer rate of 6400 MB/s.
Cache:
It’s a kind of semiconductor memory. Cache is a kind of RAM, which a computer system can access more responsively than it can in regular RAM. The purpose of using cache memory is to increase the speed of overall processing speed of the computer. The central processing unit looks up in the cache memory before searching in the central memory storage area to determine the information it requires. These rules out the need for the system to search for information in larger and bigger memory storage areas, which in turn leads to a faster extraction of data. Most modern desktop and server CPUs have at least three independent caches: an instruction cache to speed up executable instruction fetch, a data cache to speed up data fetch and store, and a translation look aside buffer used to speed up virtual-to-physical address translation for both executable instructions and data

Flash Memory:
This is a non-volatile kind of memory which is intended to contribute to portable storage and a convenient transfer of data from one computer to another. The data in it can be erased and re-programmed as per the user's requirements. It only has a specific number of erase and write cycles that it can withstand, after which it creates a tendency to lose out on the stored information. Memory cards and USB flash drives are some modes of this type of memory storage. These are just the common and main computer memory types which facilitate memory and data storage. However, there are many subtypes which are sorted out according to the memory-related functionalities they perform and the requirements they serve.


Virtual Memory:
Most computers today have limited space of RAM available for the CPU to use. Unfortunately, that amount of RAM is not enough to run all of the programs that most users expect to run at once. With virtual memory, what the computer can do is look at RAM for areas that have not been used recently and copy them onto the hard disk. This frees up space in RAM to load the new application. Because this copying happens automatically, you don't even know it is happening, and it makes your computer feel like is has unlimited RAM space even though it only has 32 megabytes installed. Because hard disk space is so much cheaper than RAM chips, it also has a nice economic benefit.
2. Magnetic Memory

Magnetic storage uses different patterns of magnetization on a magnetically coated surface to store information. Magnetic storage is non-volatile. The information is accessed using one or more read/write heads which may contain one or more recording transducers. A read/write head only covers a part of the surface so that the head or medium or both must be moved relative to another in order to access data.

Hard disk:
Hard disks are on-line storage device. They are secondary storage or external memory, differs from primary storage in that it is not directly accessible by the CPU. The computer usually uses its input/output channels to access secondary storage and transfers the desired data using intermediate area in primary storage. Secondary storage does not lose the data when the device is powered down—it is non-volatile. They store programs, data, operating system, compilers, application programs etc. Hard disk is made up of aluminum base with a thin coating of magnetic material over it. Standard hard disk sizes are: 3.5, 2.5, 2, 1.8 and 1.3 inch diameter. Digital information is stored on the magnetic film by applying the current pulse of suitable polarity to magnetizing coil of the R/W head.
There are two recording technique: Longitudinal and vertical recording. Hard disk is a surface device, its surface is divided into number of concentric tracks and each track is again divided into sectors.



Fig: block diagram of hard disk

Floppy disk:

A floppy disk is a thin circular plastic disk coated with magnetic material on the surface. Floppy disk is also known as diskette. Its used for off-line storage. It is removal disk and it is used as backup memory to store data or other information. Standard size of floppy disk is 3.5 inch diameter. It employs longitudinal and vertical magnetizing technology. Floppy disk is rotates at low speed of 360 r.p.m and its storage capacity is 1.44MB only.



Fig: internal structure a floppy disk.
Magnetic tape:

It is a serial access type mass storage device. They are used for tertiary and off-line storage. Magnetic tapes are larger storage device, a high speed magnetic tapes are used for hard disk backup. They are also used to transfer data from ne place to another. Modern magnetic tapes are comes in cassette.

Other magnetic memories are:

Magnetic bubble memory:
It is also another kind of magnetic memory. It is a non volatile semi random access type memory.

Magnetic core memory:
It is the oldest memory; it was used as primary memory before the development of semiconductor memory.

3. Optical Memory:

Optical storage, the typical Optical disc, stores information in deformities on the surface of a circular disc and reads this information by illuminating the surface with a laser diode and observing the reflection. Optical disc storage is non-volatile. The deformities may be permanent, formed once or reversible media. Various optical memories are use today.

CD, CD-ROM, DVD, BD-ROM: Read only storage, used for mass distribution of digital information (music, video, computer programs)
CD-R, DVD-R, DVD+R BD-R: Write once storage used for tertiary and off-line storage.
CD-RW, DVD-RW, DVD+RW, DVD-RAM, BD-RE: Slow write, fast read storage, used for tertiary and off-line storage. Ultra Density Optical or UDO is similar in capacity to BD-R or BD-RE and is slow write, fast read storage used for tertiary and off-line storage.
Magneto-optical disc storage is optical disc storage where the magnetic state on a ferromagnetic surface stores information. The information is read optically and written by combining magnetic and optical methods. Magneto-optical disc storage is non-volatile, sequential access, slow write, fast read storage used for tertiary and off-line storage.

Conclusion:
In this presentation, we have discussed about the various types of memories and their importance along with their uses. A detail discussion has been mainly carried out on purpose of using memory, different types of semiconductor memory, magnetic memory and optical memory.











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