What is Computer Memory?
The memory unit is that component of a computer system, which is used to store the data, instruction, and information before, during and after the processing done by ALU. It is actually a work area within the computer, where the CPU stores the data and instructions. Sometimes unallocated space of the hard disk is used as virtual memory also known as a page file. This type of memory is a temporary memory.
Define Primary and Secondary memory
The are two kinds of computer memory – primary and secondary memory. Primary memory is accessible directly by the processing unit. RAM is an example of primary memory. Primary memory is more expensive than secondary memory. The primary storage section of some computers is designed to store a fixed number of characters in each numbered address location. Such computers are said to be word addressable and they employ a fixed word-length storage approach. In these computers, storage space is always allocated in multiples of word length. In many computers, the primary storage section is also designed in such a way that each numbered address can only store a single character. They employ a variable word length approach.
Secondary memory such as floppy disks, magnetic disk etc is located outside the computer. Primary memory is more expensive than secondary memory. This memory is also called ‘auxiliary memory’.
Difference Between Primary and Secondary Memory:
|1. Primary memory is accessible directly by the processing unit.||1. Secondary memory is an additional memory that is located outside the computer.|
|2. Primary memory is also called main memory.||2. Secondary memory is called auxiliary memory.|
|3. Primary memory is more expensive.||3. Secondary memory is less expensive.|
|4. The size of primary memory is less than of secondary memory.||4. The size of secondary memory is more than that of main memory.|
|5. Primary memory stores program and data which are currently needed by CPU.
|5. Program or information which are not currently used by CPU resides in secondary memory.
Example: Hard disk, floppy disk etc.
|6. This is faster therefore expensive as CPU employs semiconductor technology.||6. This device is cheaper as it employs cables.|
|7. This is known as a semiconductor memory.||7. This is known as magnetic memory.|
|8. It is a volatile memory.||8. It is a nonvolatile memory.|
Difference Between RAM, ROM, PROM, EPROM, and EEPROM
|1. RAM is Random Access Memory.||1. While ROM is, Read Only Memory.|
|2. RAM is volatile and its contents get erased, when the computer is switched off.||2. ROM is non-volatile.|
|3. RAM is used for both read and write.||3. ROM, as the name implies, is used only for reading.|
|4. RAM needs electricity to flow to retain information.||4. While ROM is permanent.|
Difference between PROM, EPROM, and EEROM:
|PROM ( Programmable ROM ) can be programmed only once.||EPROM ( Erasable PROM ) can be programmed and erased with ultraviolet rays.||EEPROM ( Electrically EPROM ) same as EPROM but is erased electrically.|
A pen drive or a USB flash drive is a portable data storage device. Pen drives have replaced the floppy drives of old and have become the most popular data storage devices among consumers.
Characteristics of Pen Drive:
- These are a very fast device for data transfer.
- These are plug and play USB devices. It means pen devices need not be installed.
- Pen drive has a high capacity of data storage. Its capacity range from 128 MB data to 8 GB and increasing day by data as the technology changes.
- This is very portable and small in size with large capacity.
- This is easy erasable and rewritable device. These are advantageous over CDs as these are rewritable and erasable.
Direct and Sequential Access Memory:
Several different devices can be used as a secondary storage, but the one selected for a particular application mainly depends upon how the stored information needs to be accessed. There are two types of accessing information ways – Sequential or Serial Access and Direct or Random Access. A sequential access device is one in which the arrival at the location desired may be preceded by sequencing through other locations so that access time varies according to location. In other words, information on a serial device can only be retrieved in the same sequence in which it is stored. For Example, It is suitable for
- Preparation of monthly pay slips.
- Monthly electrical bills etc.
Magnetic Tape and Punched Paper Media are typical sequential access storage devices.
In many applications, we need to access information in a more direct manner than serial devices allows. These direct access devices are also called Random Access Devices because the information is literally available at random i.e. it is available in any order. For example, in a computerized bank at any instant, it is required to determine the exact balance in a savings account of a particular customer.
Magnetic disk and Magnetic Drum are typical direct access storage devices.
Magnetic tape is the oldest storage device available for microcomputers. It is generally used to store a large volume of data that is needed to be sequentially accessed and processed. The tape is made up of a plastic ribbon coated with an iron-oxide material, which can be magnetized. The data stored on tape can be read as well as erased and written again.
Magnetic tape is a sequential access storage device, hence it is not possible to read the data randomly or directly. Therefore, magnetic tapes are suitable only for storing data for backup and batch mode applications and not for on-line applications. On the other hand, magnetic disks (floppy and hard disks) are considered best storage devices for online applications.
Compact Disk-Read Only Memory ( CD-ROM ):
CD-ROM means compact disk – read only memory. It is the necessity to store a large volume of data that has led to the development of optical disk storage medium. Like a floppy disk, a CD-ROM only starts spinning when requested and it has to spin up to correct speed each time it is accessed. It is much faster to access than a floppy but it is currently slower than a hard disk.
Compact Disk (CD) is the latest storage device, used to store data, information, and software, which can be read only and not be changed or erased. However, the information is stored on CDs by using an expensive drive, called CD-ROM drive.
Nowadays compact disks are very popular storage devices for microcomputers because a large number of software including multimedia, audio, and graphics software are available only on these disks. Compact Disks can store a large volume of data (up to 680 MB), which is almost the same as a storage capacity of a 640 MB Hard Disk.
WORM (Write Once Read Many) is a type of compact disk which can be recorded only once and not erased. It can store more data than a CD-ROM, generally measured in gigabytes.
Its Advantages :
- Storage cost is very low.
- CD-ROM’s hold large quantities of data.
- Access time is relatively fast.
The capacity of Primary Storage:
The storage capacity of large computer systems is normally more than small systems. This capacity is defined in terms of bytes or words. A symbol commonly used to denote the storage capacity is the letter K (kilo) which is equal to 210 or 1024. Thus a 32KB memory is capable of storing 32 x 1024 = 32,768 bytes. Memory size range from a few K bytes or words in small machines to several thousand K bytes or words in large machines. If memory capacity is stated in terms of words, it is necessary to know the word size in bits or bytes in order to determine the actual storage capacity of the computer.
Thus, while specifying the memory capacity in terms of words, it is customary to specify the total number of bits per word along with the total number of words. So a memory with 4096 locations each with a different address and with each location storing 16 bits is called a “16-bit 4096-word memory”.
The primary storage section of some computers is designed to store a fixed number of characters in each numbered address location. Such computers
are said to be word addressable and they employ a fixed word-length storage approach. In these computers, storage space is always allocated in multiples of word length.
In many computers, the primary storage section is also designed in such a way that each numbered address can only store a single character (A, B, 1, 2, +, -, etc). Computers designed in this manner are said to be character addressable and they employ a variable word length storage approach.
Optical Disks are becoming a very popular among the users. Optical disks technology is still emerging and has yet to stabilize, however at present there are three different types of optical disks in common use.
1. Recordable disk: Write Once, Read Many (WORM). As with the CD, once data have been attached to the surface of a worm disk they cannot be changed. WORM is an is an ideal medium for making a permanent record of data, it can be a feasible alternative to magnetic tape for archival storage.
Example: Many Banks use.WORM disk to store a record of each day’s transactions. The transactions are written to an optical disk and become a permanent record that can be read but never altered.
2. Erasable Disk: Magnetic – Optical disk:
A hybrid disk, called magnetic – optical (MO), combines the best features of magnetic and optical disk technologies. A magnetic optical disk has the high volume capacity of an optical but can be written over as a magnetic disk. The disk surface is coated with plastic and embedded with magnetically sensitive metallic crystals. To write data, a laser beam melts a tiny spot on the plastic surface and a magnet aligns the crystal before the plastic coils. The crystals are placed is such a way that some reflect light and others do not. Later on, when the data is read by a laser beam, only the crystals that reflect light are picked up.
- Magnetic-optical disk are too expensive and do not alter.
- The access time in the magneto-optical disk is relatively slow. About the same as a low end, Winchester disk.
In WORM: The user can store permanent data, information, nd files for maintaining record. Once data have been attached into the surface of a WORM disk, they cannot be changed.
In Erasable Disk or CD-ROM: It is a read / write optical disk memory, information can be written to and read from the disk. The disk contents can be erased and new data can be re-written, so it can serve as a secondary memory of a computer.
Data Storage Devices:
Data storage devices can be divided into two main categories:
- Magnetic Media which stores the binary data on a disk or tape coated with a material that can be magnetized differently, depending on whether 0 or 1 is stored.
- Optical Media which stores the binary data in a surface that reflects laser light differently, depending on whether a 0 or 1 is stored.
1. Floppy Disk: An increasing popular direct access secondary storage medium for micro and mini computer systems is the floppy disk. Floppy disks are also referred to as diskettes. A floppy disk is made of flexible plastic which is coated with magnetic oxide. The flexible disk is enclosed within a square plastic. Floppy disks are typically 3, 5.25 or 8 inches in diameter. They come in either single or double density versions and record on one or both surfaces of a diskette.
Its Advantages :
- Floppy disks are very cheap as compared to other storage devices.
- They greatly enhance the online storage capacity of small systems at affordable price.
- They are also a convenient offline storage medium for the small system.
2. Magnetic Disk: A magnetic disk is a thin, circular metal plate coated on both sides with a magnetic material. It is very similar to LP gramophone record. A disk pack consists of a number of these disks, three or more, mounted about half an inch apart from each other on a central shaft which rotates at speeds of 2,400 or more revolutions per minute. Magnetic disks are the most popular medium for direct access secondary storage. In a disk pack, information is stored on both the surfaces of each disk plate except the upper surface of the top plate arid the lower surface of the bottom plate which are not used. Each disk consists of a number of invisible concentric circles called tracks. A set of corresponding tracks in all the surfaces is called a cylinder. Information is recorded on the tracks of a disk surface in the form of invisible tiny magnetic spots.
- The magnetic disk has the flexibility of being used as a sequential as well as a direct access storage device.
- Magnetic disks are less vulnerable to damage from dust or careless handling.
3. Magnetic Tape: Magnetic tape is one of the most popular storage media for large data that are sequentially accessed and processed. Information is recorded on the tape in the form of tiny invisible magnetized and nonmagnetized spots on the iron-oxide side of the tape. The tape is divided into vertical columns called frames and horizontal rows called tracks.
- The storage capacity of a magnetic tape is virtually unlimited because we can use as many tapes as required for recording our data.
- A reel of tape is also a convenient way of carrying information from one place to another.
4. Winchester Disk: In this unit, disks are permanently housed in sealed, contamination free containers. The disks are coated with a special lubricant which reduces friction when the read/ write heads land on the disk surface. The container is usually not removed from the disk drive. High capacity systems using these sealed housing are said to employ Winchester technology.
Its Advantages :
- Winchester disks are fast and reliable.
- Less expensive when compared with hard disk devices.
- Winchester disks are used in all but the smallest computer systems.