Introduction To Computer Memory

The function of memories is to store program, data and results. There are two kinds of memory according of 

use of computer.

 

1. Primary memories

2. Secondary memories.

Primary Memories: the main memory of the computer is a fast memory. It is used to store programs and data during computer operation. The memory location of main memory can be accessed directly by the CPU.

Semiconductor memories are used as Primary memories or main memories in computers. Semiconductor memories are faster, smaller, and lighter and consume less power. Magnetic memories are used as secondary memories of a computer for bulk storage of data. Magnetic memories are slow but cheaper than semiconductor memories.

RAM (Random access memory): The read and write memory of a computer is known as RAM. It is also called as R/W memory. Information can be read from and written into it during simple operation. It has random (direct access) property. It retains stored information as long as power supply is on. Its contents are lost when power supply is switched off. Hence it is volatile in nature. There are two kinds of RAM: Static RAM and Dynamic RAM. In a static RAM the stores information are retained init as long as power supply is on. But in dynamic RAM losses its contents in a very short time even though the power supply is on.

ROM (Read Only Memory): a ROM is a read only memory. It is used for permanent storage. It has also a random access property. It is non volatile in nature. The contents of ROM are decided by the manufacturer. These components are permanently stored in a ROM at the time of manufacturing. The user cannot write into ROM. ROMs are simple and cheap.

PROM (Programmable Read Only Memory): A PROM is a programmable ROM. The contents of a PROM are decided by the user. The user can write permanent program into PROM. A Special equipment for PROM programming is called PROM programmer.

EPROM (Erasable PROM): An EPROM is an erasable PROM. The contents are erased by exposing EPROM to a high intensity short wave ultraviolet for 10 to 20 minutes. There should not be overexposure. The process of changing is not convenient. The unit has to removed from the board for exposure to ultraviolet source. The user can not erase the content of single memory location. The entire contents are erased.

EEPROM (Electrically Erasable PROM): EEPROM are electrically erasable PROM. They need not be removed from a microcomputer board for erasing. The change in content is made in milliseconds which is much less than erasing time of EPROM. A single byte of data or entire device can be erased in about 10ms. If required single bit can also be erased.

Secondary Memories: it is much larger in capacity but slower than main memory. It stores system program, large data files, assembler, compiler etc. the information stored in secondary memory first transferred to main memory, then processed by the CPU. Final result again placed in the secondary memory. Magnetic disks are used as secondary memory. The cost of these devices per bit is less than main memory. Secondary memories are connected to a computer as IO devices.

Magnetic memories are non volatile. They are slower than semiconductor memories. They used in computer for bulk storage.

Magnetic Tape

Magnetic tapes are used for bulk storage. Tapes are cheaper than other magnetic storage devices. The tape is usually made of Mylar with thin coating of magnetic iron oxide. Its width varies from ¼ inch to ½ inch. Tape is a surface device. Storage on magnetic tape is nonvolatile. It is slow being serial access type memory. Sometime locating a file takes several seconds. Magnetic tapes are used for backup storage.

Floppy Disk

floppy disk is Mylar disk coated with magnetic material iron oxide. 0 and 1 are stored in the magnetic material in magnetic directions. Floppy disks have shorter access time than magnetic tapes. They have semi random property. They are widely used in microcomputer system. The disk is permanently enclosed in a low friction envelop. The thickness of the disk is only a few thousand of an inch. It is called as floppy because it is very thin. The disk is rotating at a constant speed of 360rpm. The floppy is a surface device. The surface is divided into numbers of tracks. Each track future divided into number of sectors. There is a hole at the center for driving the disk. A head slot is used for reading and writing operations. The slot allows a small surface area of disk to come in contact with the disk drive. A write protect notch is provided to protect stored information. The disk contains an index hole which is used as the reference to count the sectors. 5¼ inch floppy disks are commonly used. They are called mini floppy. 3½ inch floppy disks are also available. They are called microfloppy. The capacity of 5½ inch double side density is 360kb. The capacity of a 5¼ inch double sided high density is 1.2mb. The capacity of 3½ inch floppy high density is 1.44mb. The average access time for a floppy disk is about 150 to 250ms. Once a sector or file is found, data can be transferred at about 250kb/second. A floppy disk may be damaged by heat, dirt or magnetic field.

Hard Disk

A hard disk uses a rigid alumininum base with a thin coating of magnetic material. The disk is enclosed in a sealed container. In most cases a hard disk is fastened with the drive unit where as floppy disk is removable. To increase the storage capacity per drive several disks can be attached to a common spindle to form a disk pack. An individual read/write head for each disk is used. A hard disk is highly precise and reliable. As the disk is a rigid one it is more stable and hence it contains more tracks per inch and more bit per track than a floppy disk. The hard disk is expensive as it requires sophisticated control circuitry. Standard hard disk sizes are: 3½ and 5½ inch diameter.

The speed of a hard disk is in the range of 1000 to 3600 rpm(7200 to 10000 rpm now days). the high speed also makes reading and writing of data faster. The access time is about 20ms. Once the desired track is located, the data are transferred at the speed of 10mb per second.

Access time is the time delay or latency between a request to an electronic system, and the access being completed or the requested data returned.

Optical Disk

Optical disks are used to store huge amount of digital data for computers. 1 Giga bytes of data can be stored on one side of a 12inch optical disk. In most cases disk is removable. Some unit of optical disks rotates a constant speed of 700 to 1200 rpm. In some system where compact disks are used speed is adjustable. In such units disks rotate more slowly when outer track are read. There are three type of optical disk: read only, write once/read, and read/write. A read only system uses prerecorded disks. In a write-once/read system data can be written only once. Reading can be done as many times desired. In this type of unit erasing and rewriting is not possible. In read/write system writing, erasing and writing new data again is possible.

The draw back of an optical disk is the slow average seek time of the drive.

Seek time is one of the three delays associated with reading or writing data on a computer's disk drive, and somewhat similar for CD or DVD drives. The others are rotational delay and transfer time. In order to read or write data in a particular place on the disk, the read/write head of the disk needs to be physically moved to the correct place. This process is known as seeking, and the time it takes for the head to move to the right place is the seek time. Seek time for a given disk varies depending on how far the head's destination is from its origin at the time of each read or write instruction; usually one discusses a disk's average seek time.

Magnetic bubble type memories

Magnetic bubble type memories is a type of non-volatile computer memory that uses a thin film of a magnetic material to hold small magnetized areas, known as bubbles or domains, each of which stores one bit of data. Bubble memory started out as a promising technology in the 1970s, but failed commercially as hard disk prices fell rapidly in the 1980s. Magnetic bubble memory is a solid state device having high reliability, small size, light weight and limited power dissipation. It is a static device. It has no rotating parts. It has very high storage capacity in compact space. It is nonvolatile semi random access type memory.

Cache memory

Cache is pronounced as “cash”. It is placed in between CPU and main memory. It is high speed semiconductor memory. It is faster than main memory. Its access time is 15-20ns while that of the main memory is 80ns. It is very expensive. It is not accessible to programmers. It stores instructions which are going to be executed immediately. Earlier cache memory was provided in a larger computer, now it is also provided in minicomputers and PCs.

Virtual Memory

The term virtual refers to something which appear to be present but actually it is not. The virtual memory technique that permit user to use more memory than a computer actually has. The main memory actually available in a computer system is called as real or physical memory. The secondary memory is provided in a computer system for bulk storage. The CPU does not address secondary memory directly. The CPU executes programs which are in the main memory. Program and data are transferred from the secondary memory to the main memory for processing when required. The result and modified information are again stored in the secondary memory.

A programmer can prepare a program which uses more memory space than the capacity of the main memory. Such a program is executed by virtual memory technique. The program is stored in the secondary memory. The memory management unit transfers a part of the program from secondary memory to the primary memory, which is currently required. This part of program is executed. After executing this part of program it is sent back to the secondary memory. Thereafter, the CPU takes another part of program for execution. Thus the primary memory keeps only currently needed portion of the program. The part of the program that is no longer needed is sent back to secondary memory. Thus by swapping technique a program using more memory space than the capacity of the main memory is executed. This concept is knows as virtual memory technique.

Buses

various input/output devices and memories are connected to the CPU by a group of lines called a bus. There are three types of buses: address bus, data bus and control bus. The bus architecture of a microprocessor has been shown in figure below. The address bus carries the address of memory location or I/O device that a CPU has to access. When an address is sent by the processor all devices connected to it receive this address but only that device will respond which also received chip has enabled from the CPU. The address bus is unidirectional. Data and control buses are bidirectional. A data bus is used for the transfer of data between the processor and memory or I/O devices. The control bus is used to carry control signals between CPU and various devices connected to it.