Define memory. Explain the types of memory in details.

The memory is the working area of computer. The place where our data and instruction get stored before processing and the result also gets stored before to its output is called memory. Data and instructions are stored in memory in the binary form.

a)   PRIMARY MEMORY

The primary memory of computer is also called main memory of computer. It is fast and less capacity memory of computer. The primary memories of computer are RAM, ROM, Cache memory, registers etc. They are mainly used to store data during a time of processing. The fast and integrated memory of less capacity is termed as primary or main memory. These memories have great effect on processing speed of computer. The memory also has an input-output port through which the data transfer takes place to and from the addressed location. This port is called Memory Buffer Register (MBR) or Memory Data Register (MDR). The main function of main memory is READ, WRITE and CLEAR.

1. RAM (Random Access Memory)

It is called RAM because its contents can be accessed randomly. We can read from RAM as well as write into it. Hence, it is also called ‘read – write’ memory. The main drawback of RAM is that it is volatile memory so the contents of RAM are lost when the computer is switched off.

RAM is mainly used to store the following:

➢ Kernel of running operating system.

➢ Data and instruction currently being processed.

➢ Data and instruction waiting to be processed.

➢ Information waiting to its output.

➢ Currently arrived data and instruction from input system.

 There are 2 types of RAM.

i) DRAM (Dynamic Random Access Memory)

ii) SRAM(Static Random Access Memory)

i) DRAM (Dynamic Random Access Memory)

It is the most common and cheapest type of memory chip. It is made of capacitors which is capable of storing the electric charge. Due to leakage of charges, the capacitors discharge gradually and the memory cells lose their contents. DRAM has to be refreshed periodically by recharging the capacitors to retain its memory contents. It is slower than SRAM but it is dense, consume less electricity, smaller in size and less expensive. Example: - DDR (Dual Data Rate), DDR2 and DDR3 etc.

ii) SRAM (Static Random Access Memory)

It is made of transistors. It is called static because it can remember or retain its memory contents without being refreshed or recharged as long as there is power. SRAM does not need to be refreshed or recharged periodically like DRAM. SRAM is faster than DRAM but more expensive, less in density and bigger in size and consumes more electricity.

 

2. ROM (Read only Memory)

It is called ROM because only read operation can be performed on it. The user can not write anything into ROM. The binary information stored in ROM is written permanently by the manufacture and it can not be altered. The program such as startup or booting instructions which does not need modification are stores in ROM. ROM is necessary to store such software which enables the computer  to boot up. ROM is non-volatile memory because it can retain its contents even after the computer is turned off. It is also made by semiconductor chips.

The program stored permanently in ROM is called Firmware. Hence, firmware is immediately available when a device is powered on to start up the PC or other electronic equipment like mobile, PDA and others.

The types of ROM (READ ONLY MEMORY):

i) PROM (Programmable Read Only Memory)

PROM can be written to or programmed via a special device called a PROM programmer. Typically, this device use high voltages to permanently destroy or create internal links within the chip. Consequently, a PROM can only be programmed once.

ii) EPROM (Erasable Programmable ROM)

EPROM is an erasable PROM. The data stored in EPROM can be erased by exposing it to ultraviolet rays. When an EPROM is exposed to ultraviolet light, the entire data are erased. The user can not erase the content of certain selected memory location. It is used to store programs which are permanent but needs frequent updating. It is usually reprogrammed without removing it from the circuit board. It is far more economical than PROM as it can be reused.

iii)EEPROM (Electrically Erasable Programmable ROM)

It is an electrically erasable PROM. It can be erased and reprogrammed on the byte by byte basis. Either a single byte or the entire chip can be erased in one operation. It requires much shorter time, a few milliseconds for erasing as compared to EPROM. Varying with different manufactures, it has a limit on the number of times for which it can be reprogrammed. It need not be removed from the circuit board for erasing as EPROM.

 

3. Cache Memory

Cache memory pronounced as cash, it a special, high speed and expensive semiconductor memory placed between RAM and CPU. It is used to in CPU, so that CPU doesn’t have to wait for data to be delivered. The most frequently used instructions are kept in the cache memory so that the CPU to run faster, because it doesn’t have to take time to swap instructions in and out of RAM. It is made by the components of static RAM. It is usually 5 to 10 times faster  than main memory. Hence, cache memory is placed between RAM and CPU so that the speed of operation of main memory and cache memory together can meet the speed requirements of the high speed CPU.

There are two processes in Cache:

➢ Cache Hit: At first processor, send address of desired data to cache, if data is available than event is called Cache Hit.

➢    Cache Miss: When processor sent address is not present in cache, event is called Cache Miss. After cache Miss, data of main memory is copied to cache for next cache Hit.

Features of Cache Memory:

● It is temporary memory.

● It is small size memory.

● It is fast memory.

● It increases the speed of computer.

● Its internal cache is inside microprocessor and external caches are outside microprocessor.

L1 and L2 cache

L1 and L2 are levels of cache memory in a computer. If the computer processor can find the data it needs for its next operation in cache memory, it will save time compared to having to get it from RAM. L1 is “Level-1” cache memory usually built onto the microprocessor chip itself. For example, the Intel MMX microprocessor comes with 32,000 bytes of L1.

L2 is “Level-2” cache memory is on a separate chip (possibly on an expansion card) that can be accessed more quickly than the larger “main” memory. A popular L2 cache memory size is 1,024 KB (1MB).

b) Secondary Memory

Secondary memory, also known as secondary storage, is the slower and cheaper form of memory. CPU does not access the secondary memory directly. The content in it must first be copied into the primary storage RAM for CPU to process. Secondary memory devices include hard drives, floppy disks, CDs and CD-ROMs etc. Secondary memory is the place where programs and data are kept on a long-term basis.

Magnetic Disk

Magnetic disk is the most widely used storage medium for computer. A magnetic disk offers high storage capacity, reliability and fast to access data. It access data randomly. Magnetic disk is flat circular plates of metal or plastic, coated on both side with iron oxide. The types of magnetic disk are hard disk, floppy disk, pen drive etc.

Hard Disk

The hard disk drive is the most popularly used secondary memory device. The main reason for this is the access speed and the reliability of the data it can offer. In the case of the tape drives the access speed is much low and the data transfer is comparatively low than the hard disk drive.

The information that is required to boot a computer is stored in the hard disk boot sector. Also the importance of the hard disk drive is to store the backup of the data or any information that is created by the user. The hard disk specification should also match the expectations of the computers; that is the storage capacity and the access speed.

The internal organization of the any hard disk drive consists of four parts primarily. They are PLATTERS, HEAD ARM, CHASIS, and HEAD ACTUATOR. The hard disk drives are also available in two different types that are internal and also the external hard disk drives. The internal hard disk drives are used for the storage of the data in the computer case. They are not portable and usually are inside the case. The external hard disk drives are portable can be connected to other computer systems as well. There is a hard casing over the hard disk.

The platters are rotated at very high speed varying from 3,000 rpm (revolution per minute) to 15,000 rpm. Information is written to, and read from a platter stack, called read –write head. The read –write head is used to detect and modify the magnetization of the material immediately under it.

The hard disk is a non-volatile and random memory of a computer system. The hard disk has very high storage capacity. A hard disk has a storage capacity in terms of Giga bytes such as 80 GB, 320 GB, 500 GB etc.

ADVANTAGES OF HARD DISK

● It can store huge amount of data in terms of MB to GB.

● It is protected from dust, fires etc.

● It stores data safely in comparison to other devices.

● External hard disks are portable.

● Data access time is less in comparison to floppy disk , magnetic tape etc

 FLOPPY DISK

The floppy disk is also called a diskette. Floppy disk is used to transfer data from one computer to another, increase hard disk space by saving information to floppy, backup storage media. The disk is cheap, easy to handle and less storage capacity. Floppy disks have largely been replaced with CD, DVD.

Advantages of Floppy disk

● Direct information encoding onto disk.

● No handling of bulky media.

● Error correction is easier than in cards and paper tapes.

● It can be re-used many times.

Magnetic Tape

Magnetic Tape is a sequential storage device of computer. It is not common storage device now a day. It is mainly used in mainframe computers and supercomputers. It is slow and sequential flow of data. It is very useful for storing huge volume of data economically. A magnetic tape is a continuous trip of plastic. It is coated with magnetic oxide which can be magnetized to record information as a series of magnetized / non-magnetized spots to represents binary 0 and 1. Extracting information from a file is known as reading.

The magnetic tape used for computer is made from the same material used for audio tape and video tape. Today, it is used mainly to provide backup or duplicate storage. At present, it is available in cassette forms, which is called cartridge tape. The storing capacity can be 500 MB, 2 GB, 4 GB or higher. Data density is 120 Kbits per inch. Speed is 200 inch per second and data transfer rate is 240 KB per seconds.

MEMORY CARD (FLASH MEMORY)

A memory card is a small storage medium used to store data such as text, pictures, audio and video, for use on small, portable or remote computing devices. It is non-volatile memory which means that data is stable on the memory card by a loss of power source, and does not need to be periodically refreshed. Because memory cards are solid state media, they have no moving parts, and therefore, are unlikely to suffer mechanical difficulties.

Memory cards offer a number of advantages over the hard disk drives. They are much smaller and lighter, extremely portable, completely silent, allow more immediate access, and are less prone to mechanical damage. The storage capacity is from MB to GB. They are commonly used in many electronic devices, including digital camera, mobile phones, Laptops, MP3 Players, and Video Game Consoles.

 OPTICAL DISK

An optical disk is an electronic data storage medium that can be written to and read using a low powered laser beam. It is mainly used for music, movies and software programs. It is compact, lightweight and durable. Optical disks are CD, DVD etc.

CD-ROM (Compact Disk-Read Only Memory)

CD-ROMs are popularly used to distribute computer software including games and multimedia applications through any data can be stored. The diameter of CD-ROM is 12 cm, thickness is 1 mm and weight is 18 gm. The storage capacity of CD-ROM is 650 MB. It is only used for read only purposed in computer system.

CD-RW (Compact Disk Rewritable)

A CD-RW is a rewritable optical disk. CD-RW Disk required more sensitive laser optics. CD-RWs are composed of a polycarbonate plastic, a thin reflecting metal coating, and a protective outer coating. In a CD-RW the dye is replaced with an alloy that can change back and forth from a crystalline form when exposed to a particular light, through a technology called optical phase change. The pattern created is less distinct than those of others CD formats, requiring more sensitive device for playback.

 

 VCD (Video Compact Disk)

VCD is a compact disk format based on CD-ROM that is specifically designed to fold MPEG-1 videos data and to include interactive capabilities. Each VCD disk holds 72 – 74 minutes of video and has a data transfer rate of 1.44 Mbps. VCD’s can be played on a VCD player connected to a Television set or computers, on a CD player, on same CD-ROM drives, and same DVD player.

DVD (Digital Video Disk)

DVD is an optical disk storage media format. DVD disks offer higher storage capacity. DVD disks are used in DVD video consumer digital video formats and in DVD audio consumer digital audio formats. DVD disks containing other types of information may be referred to as DVD data disks. It can store any types of data such as text, graphics, audio, video etc.