Computer Memory Overview

The computer memory is one of the most important elements in a computer system. It stores data and instructions required during the processing of data and output results. Storage may be required for a limited period of time, instantly or for an extended period of time. Computer memory refers to the electronic holding place for instructions and data where the processor can read quickly.

Memory Hierarchy

The memory is characterised on the basis of two key factors; capacity and access time. The lesser the access time, the faster is the speed of memory.

The computer uses a hierarchy of memory that is organised in a manner to enable the fastest speed and largest capacity of memory as shown in figure.

Types of Memory:

In general, the memory is classified into two categories.

1. Primary memory or Main memory.
2. Secondary memory or Auxiliary memory.

Parameters of Memory:

The following terms are most commonly used for identifying comparative behaviour of various memory devices and technologies.
Storage Capacity It is representative of the size of the memory. The capacity of internal memory and main memory can be expressed in terms of number of words or bytes.
Access Modes A memory is comprised of various memory locations The information from these memory locations can be accessed randomly. sequentially and directly.
Access Time The access time is the time required between the desired modes for a read or write operation till the data is made available or written at the desired location.
Physical Characteristics In this respect. the devices can be categorised into four main categories electronic, magnetic, mechanical and optical.
Permanence of Storage Its permanence is high for future use in magnetic materials.

Primary Memory(Main Memory):

The memory unit that communicates directly with the CPU is called main memory. The primary memory allows the computer to store data for immediate manipulation and to keep track of what is currently being processed. It is volatile in nature, it means that when the power is turned off, the contents of the primary memory are lost forever.

Primary memory further classified in two categories.

1. Random Access Memory (RAM):

It is also known as read/write memory, that allows CPU to read as well as write data and instructions into it.
RAM is used for the temporary storage of input data, output data and intermediate results. RAM is a microchip implemented using semiconductors.

There are two categories of RAM
(i)Dynamic RAM (DRAM) It is made up of memory cells where each cell is composed of one capacitor and one transistor. DRAM must be refreshed continually to store information. The refresh operation occurs automatically thousands of times per second DRAM is slower, less-expensive and occupies less space on the computer’s motherboard.

(ii) Static RAM (SRAM) It retains the data as long as power is provided to the memory chip. It needs not be ‘refreshed’ periodically. SRAM uses multiple transistors for each memory cell. It does not use capacitor. SRAM is often used as cache memory due to its high speed. SRAM is more expensive than DRAM.

Extended Data Output Dynamic RAM (EDO DRAM) It is a type of RAM chip. It is used to improve the time to read content from memory and enhance the method of access.

Cache Memory
Cache memory is a storage buffer that stores the data that is used more often. temporarily and makes them available to CPU at a fast rate.

The data and instructions that are required during the processing of data are brought from the secondary storage devices and stored in the RAM. For processing it is required that the data and instructions are accessed from the RAM and stored in the registers.

Cache memory is a very high speed memory placed in between RAM and CPU. Cache memory increases the speed of processing.

Cache memory is very expensive, so it is smaller in size. Generally. computers have cache memory of sizes 256 KB to 2 MB.

2. Read Only Memory (ROM):

It is also known as non-volatile memory or permanent storage. It does not lose its content when the power is switched off.ROM has only read capability, no write capability. ROM can have data and instructions written to it only one time. Once a ROM chip is programmed at the time of manufacturing, it cannot be reprogrammed or rewritten.

There are three categories of ROM.
(i) Programmable ROM (PROM) It is also non-volatile in nature. Once a PROM has been programmed, its contents can never be changed. It is a one-time programmable device. PROMs are manufactured blank and can be programmed at buffer, Final test or in system.

These type of memories are found in video game consoles, mobile phones, implantable medical devices and high definition multimedia interfaces. PROM was invented by Wen Tsing Chow in 1956.

(ii) Erasable Programmable ROM (EPROM) It is similar to PROM, but it can be erased by exposure to strong ultraviolet light,then rewritten. So,it is also known as Ultraviolet Erasable Programmable ROM (UV EPROM).EPROM was invented by Dov Frohman of Intel in 1971.

(iii) Electrically Erasable Programmable ROM (EEPROM) It is similar to EPROM, but it can be erased electrically, then rewritten electrically and the burning process is reversible by exposure to electric pulses.

Tit-Bits

• Flash Memory is a kind of semiconductor-based nonvolatile rewritable memory. used in digital camera. mobile phone. Printer.etc.
• Virtual Memory is a technique that allows the execution of processes that are not completely in main memory. One major advantage of this scheme is that programs can be larger than main memory. This technique frees programmers from the concems of memory storage limitations.
• Buffer is a temporary physical storage used to hold data during execution of process from one place to another.

Secondary Memory (Auxiliary Memory/Storage Devices)

The secondary memory stores much larger amounts of data and information for extended periods of time. Data in secondary memory cannot be processed directly by the CPU, it must first be copied into primary storage i.e…, RAM.

Secondary storage is used to store data and programs when they are not being processed. It is also non-volatile in nature. Due to this, the data remain in the secondary storage as long as it is not overwritten or deleted by the user. It is a permanent storage i.e., device.

Secondary memory devices include.
Magnetic Disks :

• Hard Disk Drive.
• Floppy Disk.
• Memory Stick.

Optical Disk:

• CD
• DVD
• Blue-ray Disk

Solid State Disks:

• Pen/Flash Drive

Hard Disk Drive (HDD):

It is a non-volatile, random access digital data storage device. It is a data storage device used for storing and retrieving digital information using rotating disks (platters) coated with magnetic material. All programs of a computer are installed in hard disk.

It consists of a spindle that hold non-magnetic flat circular disks, called platters, which hold the recorded data. Each platter requires two read/write heads, that is used to write and read the information from a platter. All the read/ write heads are attached to a single access arm so that they cannot move independently.

The information is recorded in bands; each band of information is called a track. Each platter has the same number of tracks and a track location that cuts across all platters is called a cylinder. The tracks are divided into pie- shaped sections known as sectors.

Floppy Disk:

It is used to store data but it can store small amount of data and it is slower to access than hard disks. It is round in shape and a thin plastic disk coated with iron oxide. Data is retrieved or recorded on the surface of the disk through a slot on the envelope. Floppy disks is removable from the drive. Floppy disk is available in three sizes; 8 inch,5 1/4 inch and 3 1/2 inch.

• 5 1/4 inch floppy disk has a capacity of 1.2 MB.
• 3 1/4 inch floppy disk has a capacity of 1.44 MB.

Compact Disk (CD):

It is the most popular and least expensive type of optical disk. A CD is capable of being used as a data storage device along with storing of digital audio. The files are stored on this particular contiguous sectors.

CDs are categorised into three main types.

1. CD-ROM(Compact Disk – Read Only Memory) It is designed to store computer data in the form of text and graphics, as well as hi- fi stereo sound. It is capable of storing large amounts of data- up to 1GB, although the most common storage capacity is 700 MB. Data is recorded permanently on the surface of the optical disk through the use of laser. The recorded content cannot be changed or erased by users. It is also called WORM (Write Once Read Many) disk.
2. CD-R (Compact Disk – Recordable) Data can be written on these disks only once. The data once stored in these disks cannot be erased.
3. CD-RW (Compact Disk- Rewritable) It is an erasable disk . CD-RW is used to write data multiple times on a disk by the use of format feature.

Digital Video Disk (DVD):

DVD is also known as super Density Disk (SD). A DVD is an optical disk storage media manufactured by Philips, Sony, Toshiba and Panasonic in 1995.

DVDs offer higher storage capacity than Compact discs while having the same dimensions.

Depending upon the disk type, DVD can store several Gigabytes of data (4.7 GB -17.08 GB) DVDs are primarily used to store music or 6 movies and can be played back on your television or the computer too. They are not rewritable media.

DVDs come in three varieties.
1. DVD-ROM (Digital Video Disk-Read Only Memory).
2. DVD-R (DVD – Record Able).
3. DVD-RW (DVD – Re Writable).

Blue-ray Disk:

Blue-ray disk (official abbreviation BD) is an optical disk storage medium designed to recapture the data normally in DVD format. Blu-ray discs contain 25 GB(23.31 GB) Per Layer space.

The name Blue-ray disk refers to the blue laser used to read the disk, which allows information to be stored at a greater density than the longer-wavelength red laser used is DVDs.

Blu-ray can hold almost 5 times more data than a single layer DVD.

The variations in the formats are as follows
1. BD-ROM (Read only) for pre-recorded content
2. BD-R (Recordable) for PC data storage
3. BD-RW (Rewritable) for PC data storage
4. D-re (Rewritable)for HDTV recording

Pen/Thumb Drive:

Pen drive is also known as flash drive. As flash drive is a data storage device that consists of flash memory (key memory) with a portable USB (universal Serial Bus) interface. USB flash drives are typically removable, re writable and much smaller than a floppy disk. A USB flash drive is same as the size of thumb that plugs into a USB port on the computer.

Today, flash drives are available in various storage capacities as 256MB, 512MB, 1GB, 4GB, 16GB upto 64GB. They are widely used as an easy and small medium to transfer and store the information from their computer.

Memory Stick: 

It is a USB-based flash memory drive. A family of flash memory cards from sony designed for digital storage in cameras. camcorders and other handheld devices. Capacity of memory stick varies from 4 MB to 256GB.

Magnetic Tape

Magnetic tapes are made of a plastic film-type material coated with magnetic materials to store data permanently.Data can be read as well as recorded. It is usually 12.5 mm to 25 mm wide and 500 m to 1200 m long. These can store data in a sequential manner. The data stored in magnetic tape is in the form of tiny segments of magnetised and demagnetised portion on the surface of the material. Magnetic tapes are durable, can be written. erased and rewritten. Magnetic tapes hold the maximum data, which can be accessed sequentially.

Types of Magnetic Tape:

There are mainly two types of magnetic tape as Tape Reel and Tape Cassette. Each of the type has its own requirements. The older systems designed for networks use reel-to-reel tapes. Newer systems use cassettes holding more data than that of the huge reels.

Tit-Bits:

• The rate at which data is written to disk or read from disk is called data transfer rate.
• Track it records data bits as tiny magnetic spots.
• Sector it holds a block of data that is read or written at one time.
• Root directory. is the main folder of disk . it contains information about all folders on the disk.
• Hard disk is a fixed disk i.e., cannot be removed from the drive.

Secondary Memory Device and their Storage Method and Capacity.

Memory Measurement:

When you use a RAM,ROM. Floppy disk or hard disk. the data is measured using some unit. In computer terminology. They are called nibble. Bit, Byte, Kilobyte, Megabyte, Gigabyte, etc.

Bit It stands for a Binary Digit. Which is either 0 or 1.

Byte (B) A byte is approximately one character (letter ’a’. number ‘1’. Symbol’?’. etc…). Also. a group of 8 bits is called a byte.

Nibble 4 bits make one nibble.

Kilobyte (KB) In memory. a group of 1024 bytes is called a Kilobyte.

Megabyte (MB) In memory. a group of 1024 Kilobytes is called a Megabyte. It is sometimes used . less precisely, to mean 1 million bytes or 1000 KB.

Gigabyte (GB) In memory , a group of 1024 megabytes is called a Gigabyte. It is sometimes used, less precisely, to mean 1 billion bytes or 1000 MB . Now, a number of companies manufacture memory chips in terms of Megabyte such as 64 MB, 128 MB, 256 MB, 1.2 GB etc.

Terabyte (TB) A terabyte , exactly 2 bytes (2 GB).is approximately a trillion (10 ) bytes.

Petabyte (PB) one petabyte of information equal to 1000 terabytes or 10 bytes.

Exabyte (EB) One Exabyte of information equal to 1000 petabytes or 10 bytes.

Zettabyte (ZB) One zettabyte of information equal to 1000 exabytes or 10 bytes.

Units of computer memory measurements.

1Bit = Binary Digit
8 Bits = 1 Byte = 2 Nibble
1024 Bytes = 1 KB (Kilobyte)
1024 KB = 1 MB (Megabyte)
1024 MB = 1 GB (Giga Byte)
1024 GB = 1 TB (Terabyte)
1024 TB = 1 PB (Petabyte)
1024 PB = 1 EB (Exabyte)
1024 EB = 1 ZB (Zettabyte)
1024 ZB = 1 YB (Yottabyte)
1024 YB = 1 ( Brontobyte)
1024 Brontobyte = 1 (Geop Byte)

• Bit is the smallest memory measurement unit.
• Geop Byte is the highest memory measurement unit.