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These type of Memories were used in the computers built during 1960’s. The basic storage device in these type of memories consist of a small ring-shaped piece of magnetic material called magnetic core. Each magnetic core in a memory is capable of storing one binary digit of digital information. Depending on the direction of the current through the input winding, the core will become magnetized in either clockwise or counter-clockwise direction. The retentivity of the core material is such that when the magnetizing force is removed; the core remains magnetized, retaining a large part of its flux. These devices are now obsolete and have been taken over by digital memories which consume very less power.
What is a Primary Memory, It just is a part of CPU (Central Processing Unit) whereas the secondary memory is external to the CPU. The Secondary Memory stores the data to keep it even when the electricity to the PC (Personal Computer) is cut off whereas the Primary Memory losses its contents immediately. Primary Memory is of two types from the technological point of view and these are analog memory and digital memory. Each of Primary memories are discussed below.
Semi-conductory or digital memories are static, lighter, and cheaper than magnetic core memories. These are volatile and data stored in them is lost when power supply to the machine is switched off. Digital memories can be divided into the following two types and these are described in the following subsections:
Random Access Memory (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 retains the stored information as long as it is supplied with power. When power supply of computer is switched off or interrupted the stored information in the RAM is lost. RAMs of various capacities are available for example, 1K, 4K, 16K, 64K, 1M, 4M and so on.
There are two important types of RAMs that are Static RAM and Dynamic RAM. Static RAMs retain stored information as long as power supply is on . But a Dynamic RAM loses its stored information in a very short time even though the power supply is on. Therefore, Dynamic RAMs have to be refreshed periodically, generally every 2 millisecond. The Dynamic RAMs are cheaper and have high packing density and moderate speed. They consume less power. They are used where large capacity memories are needed. Static RAMs are costlier and consume more power. They do not need refreshing circuits. They have higher speed than Dynamic RAMs. Static RAM and Dynamic RAM are also written as 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 of computer is switched off. The user cannot write into a ROM. Its contents are written at manufacturing time. ROMs store permanent programs and other types of information which are needed by the computer to execute user programs. ROMs store function such as sine, cosine, logarithm, square root, exponential and code conversion tables, etc. An example of a ROM is Toshiba Mask ROM, TCS 534000, 512 KX8 bits again. There are two subsections of ROM such as – PROM and CD-ROM which are described below:
PROMs (PROGRAMMABLE ROMs)
Programmable ROMs called PROMs are also available. Further, different types of PROMs such as Erasable PROM called EPROM, electrically erasable PROM called E2PROM are available. Users can write information in PROMs, EPROMs and E2PROMs. Recorded information into an EPROM or E2PROM can be erased to record different information, whereas in a PROM the information once written by the user can not be changes. Such information is required while executing user programs. An example of PROM is 74S287.
PROM (PROGRAMMABLE READ ONLY MEMORY)
A variation of ROM chip is programmable read only memory. ROM chip are supplied by the computer system manufacture and it is not possible for a customer to modify the programs stored inside the ROM chip. However, it is possible for a user to customize a system by converting his own programs to micro programs and storing them in a PROM chip. Once the users programs are stored in a PROM chip, they can usually be executed in a fraction of the time previously required. PROMs are programmed to record information using a special facility known as a pro-programmer. However, once the chip has been programmed, the recorded information can not be changed, i.e. the PROM becomes a ROM and it is only possible to read the stored information. PROM is also non-volatile storage, i.e. the stored information remains intact even if power supply of computer is switched off.
EPORM (ERASABLE PROGRAMMABLE READ ONLY MEMORY)
Once information is stored in a ROM or a PROM chip it can not be changed or altered. However, there is another type of memory chip called erasable programmable read only memory that overcomes this problem. As the name implies, it is possible to erase information stored in an EPROM chip. The chip can be reprogrammed to store new information using a special prom-programmer facility. Information stored in an EPROM chip is erased by exposing the chip for some time to ultraviolet light. When an EPROM is in use, information can only be read and the information remains on the chip until it is erased. EPROMs are mainly used by R & D personnel because they frequently change the micro programs to test the efficiency of the computer system with new programs.
E2PROM (ELECTRONICALLY ERASABLE PROGRAMMABLE ROM )
It is also called flash BIOS, in this type of memory high voltage electric pulses are used to erase information content in the ROM. New information can be recorded in ROM by special software program although there is a limit to the number of times that can be programmed. E2PROM chip allows user to upgrade their BIOS.
CD-ROM (COMPACT DISK ROM)
CD-ROM stands for Compact Disk Read Only Memory. It is an optical ROM.
All computers except very small computer system contain both semiconductor as well as magnetic memory. The semiconductor memory is employed as the main memory or primary memory of the computer. It stores programs and data which are currently needed by the CPU. The magnetic memory is used as the secondary or auxiliary memory. The 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 package, large data files, etc. The secondary memory is non-volatile in nature.The magnetic memory retains the information stored in it. The magnetic memories such as hard disks and floppy disk are the most common secondary memories used in computer system. Back up memory use to store the copy of the important programs of a computer such as operating system, compilers etc. Floppy disks and magnetic tape can be employed as back up storage. These programs are generally available in the secondary memory but they are copies are also kept in the back up memory so that they can be loaded into the secondary memory in case the original program stored in the secondary memory are lost accidentally or due to any other reason.
TYPES OF SECONDARY MEMORY
A floppy disk is made of flexible plastic that is coated with iron oxide. The flexible disk is placed within a square cardboard jacket, or the jacket is often referred as cartridge. The cardboard jacket gives protection to the outer disk surface. Moreover, it has a liner which gives a wiping action to remove dust particles from the disk surface because it is harmful to disk surface as well as to the read/write head. The disk is loaded in the disk drive along with its jacket. Information is read or written through a small hole in the jacket. The jacket can be easily loaded into and unloaded from a disk drive whenever necessary. The read/write head of a floppy disk make direct contact with the disk surface during processing and that’s why floppy disk gets worn because of its constant use. Floppy disks are generally found in 3, 5.25, 8 inches in diameter. Floppy disks can be coated with iron oxide either one side or both side of the disk. One can record anything on each side of the disk. Floppy disks are cheaper than other types of storage devices. The capacity of disk surface of floppy disk varies depending on their diameter and mode of data transfer. The capacity of a single-sided-single-density 5.25 inch disk surface may only be a little over 100 kilobyte, while the capacity of double-sided-double-density 8 inch disk surface may be almost 2 megabytes.
MAGNETIC DISK OR HARD DISK
This storage device has very much higher storage capacity than floppy disk or other types of storage devices. The principle of operation of hard disk or magnetic disk is similar to that of a gramophone recorder. A magnetic disk is a circular disk coated with magnetic oxide for easy magnetization. A large number of disks are stacked firmly on a common spindle. A motor rotates the spindle at a rapid rate. To access each surface of every disk a read/write head is there that is attached to a common arm. The read/write head can move in between inner most track to outer most track along with the arm. The heads do not touch the disk but float on air cushion at a few microns apart. Arrangement of data in each surface of hard disk can be divided in several tracks and each track can be further divided into several sections called sectors. Each sector has fixed size but the number of sectors in each track may vary. Each sector has specific address for data stored in these sectors. A track in a given sector near the circumference is longer than a track near the center of the disk. If bits are recorded with equal density, some tracks will contain more recorded bit than other.
In order to access information from a disk, the disk address of the desired data has to be specified. The disk address is specified in terms of the track number, the surface number, and the sector number. Information is always written from the beginning of a sector and can be read only from the track beginning. As soon as a read/write command is received by the disk unit, the read/write heads are first positioned on to the specified track number by moving the arm assembly in the proper direction. Once the heads are positioned on the desired track, the head on the specified surface is activated, and hence it transfers the data to the primary memory.
A magnetic tape transport consists of the electrical, mechanical, and electronic components to provide the parts and control mechanism for a magnetic-tape unit. The tape itself is a strip of plastic coated with a magnetic recording medium. Bits are recorded as magnetic spots on the tape along several tracks. Usually, seven or nine bits are recorded simultaneously to form a character together with a parity bit. Read/write heads are mounted one in each track so that data can be recorded and read as a sequence of characters.
Magnetic tape unit can be stopped, stated, to move forward or in reverse, or can be rewound. However, they cannot be started or stopped fast enough between individual characters. For this reason, information is recorded in blocks referred to as records. Gaps of unrecorded tape are inserted between records where the tape can be stopped. The tape starts moving while in a gap and attains its constant speed by the time it reaches the next record. Each record on tape has an identification bit pattern at the beginning and end. By reading the bit pattern at the end of the record, the control recognizes the beginning of the gap. A tape unit is addressed by specifying the record number and the number of characters in the record. Records may be of fixed or variable length.
OPTICAL DISK (CD-ROM)
A CD-ROM is called compact disk read only memory. It consists of a rotating disk which is coated with a thin metal that is highly reflective. An intense laser light is focused on reflective surface of the rotating disk. Data recording is accomplished by turning the laser beam. The laser beam is turned on and off at a varying rate, because of tiny holes called PITS is burnt into the metal coating at the disk along its tracks. A tiny PIT represent 1 and a surface without a PIT is known as land represented by 0. In order to record to read the recorded data a less powerful laser beam is focused on the disk surface. In case of write enable CD-ROM separate laser beam are implied. The storage density of CD-ROM is enormous, the storage cost is extremely low, and the access time is relatively fast. It is used as good economical backup storage for any short of data and information. Since optical disk system can store as much as much as hundred times the contents of current magnetic tape drives. One small inexpensive disk will be able to replace 25 to 30 reels of magnetic tape and any data on the disk can be accessed within a few milliseconds.
PUNCHED PAPER TAPE
This type of storage device was used during 1980’s. It was a sequential-access secondary memory. In this type of storage device data is coded on paper tape in the form of punched hole combinations. It was 1 inch wide and its length was of any size even 100th feet long. It was found in the form of rolls. There was a line of sprocket holes in the middle of the tape. The use of sprocket hole was to feed the tape through the tape punching and reading device. Information is recorded as holes punched in rows across the width of the tape. The one row of a punched paper tape represented a character. The maximum number of holes per row was referred to as the number of channel on the tape. An eight channel tape was commonly used. The number of characters that can be stored in a given physical space was low for paper storage. Moreover, the paper tape media is easily torn and mutilated that is why the use of paper media as a secondary storage device was vanished day by day.
Like the magnetic disk, there is another storage device which is called magnetic drum is a direct access storage device that can be used for both sequential and random processing. It is made of a cylinder whose outer surface is coated magnetic material. There is a motor which rotates the cylinder on its own axis of rotation at a constant and high-speed. The surface of the cylinder is divided in several tracks. Data is stored on those tracks as magnetic spots similarly as in the case of a magnetic disk. The recorded data on the tracks is read through a set of stationary read/write heads which are placed little distance apart from the surface of the drum. Whenever new data is recorded on the surface of the drum previously stored data gets erased. The data recorded on the surface of the drum will remain indefinitely until they are erased. The drum can rotate to a speed of few hundred to several thousand rotations per minute to produce a fast access time. The size and storage capacity of magnetic drums are very high. A small size drum having 20 to 30 tracks can store 5000 bytes. A drum having 500 to 1000 tracks can store up to 100 megabytes. Larger size drums rotate about its axis slower than that of smaller size drums, because of that the access time of larger drums decreases as the drum speed increases. Magnetic drums have fast data transfer but are more limited in capacity than magnetic disk that is why they are not used nowadays.
Mass storage systems are the storage systems that provide access to the hundreds of billions of bytes of stored data. They combine the advantage of both tape and disk technology. The storage medium is essentially a length of flexible plastic material upon which short strips of magnetic tape are mounted. These strips are then placed in cartridges, and the cartridges are loaded into a storage device that is online to the CPU. The same read/write technique is used as used with magnetic tape.
Flash Memory (sometimes called “flash RAM”) is a type of constantly – powered non-volatile memory that can be erased and reprogrammed in units of memory called blocks. It is a variation of electrically erasable programmable read only memory (EEPROM) which, unlike flash memory, is erased and rewritten at the byte level, which is slower than flash memory up dating. Flash memory is often used to hold control code such as the basic input/output system (BIOS) in a personal computer. when BIOS needs to be changed (rewritten), the flash memory can be written to in block (rather than byte) sizes making it easy to update on the other hand, flash memory is not useful as random access memory (RAM) because RAM needs to be addressable at the byte (not the block) level. Flash memory gets its name because the microchip is organised so that a section of memory cells are erased in a single action or flash. The erasure is caused by Fowler-Nordheim tunneling in which electrons pierce through a thin dielectric material to remove an electronic charge from a floating gate associated with each memory cell. Intel offers a form of flash memory that holds two bits (rather than one) in each memory cell thus doubling the capacity of the memory without a corresponding increase in price. Flash memory is used in digital cellular phones, digital cameras, LAN switch, PC cards for notebook computers, digital set up boxes, embedded controllers and other devices.
DIFFERENCE BETWEEN MEMORIES
DIFFERENCE BETWEEN PRIMARY MEMORY AND SECONDARY MEMORY
- Primary memory of a computer is purely made of electronic devices either by transistor or capacitor.
- Primary memory can directly communicate with CPU (Central Processing Unit) hence it is called main memory.
- Primary memory is relatively faster than secondary memory.
- Primary memory is needed basically by the system itself for processing functions.
- Primary memory is relatively much costly than secondary memory.
- Primary memory has less memory space as compared to the secondary memory.
- Secondary memory is not purely made of electronic devices.
- Secondary memory cannot directly communicate with CPU hence it is called auxiliary memory.
- Secondary memory is comparatively slower than primary memory.
- Secondary memory is needed basically by users for storing of data and information permanently.
- Secondary memory is comparatively much cheaper than primary memory.
- Secondary memory has a huge memory space as compared to primary memory.
DIFFERENCE BETWEEN RAM AND ROM
RAM (RANDOM ACCESS MEMORY)
- RAM is both read and write enable means users can write into and read from a RAM.
- RAM is volatile in nature means its content gets erased when power supply to the computer is switched off.
- A user can write information in RAM temporarily.
- RAM is necessary for holding of data at the time of processing.
ROM (READ ONLY MEMORY)
- ROM is only read enable.
- ROM is very much permanent like such as secondary memory.
- Normally users cannot write anything into ROM other than E2PROM.
- ROM is necessary only at the time of starting up (Booting) of the computer.
- ROM’s contents are written at the time of manufacturing.
- ROM contains functions such as sine, cosine, square root, logarithm, code conversion table, exponential tables, etc.
DIFFERENCE BETWEEN STATIC RAM AND DYNAMIC RAM
SRAM (STATIC RAM)
- SRAM is called static RAM because each bit of Static RAM remains stored until power supply to the computer goes off.
- For Static RAM each bit is stored in flip-flop.
- Static RAM does not need any extra circuit for maintaining bit value (logic 1 or 0) in flip-flops.
- With respect to same size of Dynamic RAM, Static RAM can store fewer amounts of data.
- Static RAM cells are large with respect to Dynamic RAM cells.
- Static RAM is faster than Dynamic RAM because Static RAM does not need any refreshing circuit before accessing the bits of Static RAM.
- This memory is made of transistors.
- Static RAM is very costly as compared to Dynamic RAM.
- Static RAM has less storage density.
DRAM (DYNAMIC RAM)
- DRAM is also called Dynamic RAM, because each bit of Dynamic RAM needs to be refreshed periodically.
- For Dynamic RAM each bit is stored in capacitors.
- Dynamic RAM needs extra circuits for maintaining each bit value.
- With respect to same size of Static RAM, Dynamic RAM can store more amounts of data.
- Dynamic RAM cells are smaller with respect to Static RAM cells.
- Dynamic RAM is slower because it needs refreshing periodically before accessing bits in the cell.
- This memory is made of capacitors.
- Dynamic RAM is relatively much cheaper.
- Dynamic RAM has higher storage density.