COMPUTER MEMORY

What is Computer memory ?

A memory is just like a human brain. It is used to store data and instructions. Computer memory is the storage space in the computer, where data is to be processed and instructions required for processing are stored.Computer memory is capable of storing information temporarily, like RAM (random access memory), or permanently, like ROM (read-only memory). Memory devices utilize integrated circuits and are used by operating systems, software, and hardware..

Memory Hierarchy

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

Types of Computer Memory: Primary and Secondary

In general, the memory is classified into two categories 

1. Primary Memory

2. Secondary Memory

Primary memory (RAM)

The memory unit that directly communicates with the processor is called primary memory. The primary memory allows the the computer 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.

Primary memory is further classified in two cateogries 

1. Random Access Memory (RAM)

The acronym RAM stems from the fact that data stored in random access memory can be accessed – as the name suggests – in any random order. Or, put another way, any random bit of data can be accessed just as quickly as any other bit.

The most important things to understand about RAM are that RAM memory is very fast, it can be written to as well as read, it is volatile (so all data stored in RAM memory is lost when it loses power) and, finally, it is very expensive compared to all types of secondary memory in terms of cost per gigabyte. It is because of the relative high cost of RAM compared to secondary memory types that most computer systems use both primary and secondary memory.

Data that is required for imminent processing is moved to RAM where it can be accessed and modified very quickly, so that the CPU is not kept waiting. When the data is no longer required it is shunted out to slower but cheaper secondary memory, and the RAM space that has been freed up is filled with the next chunk of data that is about to be used.     

RAM is used for temporary sturage of data and intermediate results. RAM is a microchip implemented using semiconductors 

There are two categories of RAM

(I) Dynamic RAM(DARM)

Dynamic random access memory (DRAM) is a type of semiconductor memory that is typically used for the data or program code needed by a computer processor to function. DRAM is a common type of random access memory (RAM) that is used in personal computers (PCs), workstations and servers.

Memory is made of bits of data or program code that are arranged in a two-dimensional grid. DRAM will store bits of data in what's called a storage, or memory cell, consisting of a capacitor and a transistor. The storage cells are typically organized in a rectangular configuration. When a charge is sent through a column, the transistor at the column is activated. A DRAM storage cell is dynamic, meaning that it needs to be refreshed or given a new electronic charge every few milliseconds to compensate for charge leaks from the capacitor.  Important point to note down with regard to DRAM is it is volatile and it needs to be refreshed. 

(iI) Static RAM 

In static RAM, a form of flip-flop holds each bit of memory . A flip-flop for a memory cell takes 4 or 6 transistors along with some wiring, but never has to be refreshed. This makes static RAM significantly faster than dynamic RAM. However, because it has more parts, a static memory cell takes a lot more space on a chip than a dynamic memory cell. Therefore you get less memory per chip, and that makes static RAM a lot more expensive.

So static RAM is fast and expensive, and dynamic RAM is less expensive and slower. Therefore static RAM is used to create the CPU's speed-sensitive cache, while dynamic RAM forms the larger system RAM space. Static memory is also a volatile memory that means it does not losse contents as long as it is powered. SRAM doesn't need to be refreshed like DRAM.

2. Read only Memory(ROM)

It is also known as non volatile memory or permamnent storage . It doesn't lose its contents 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 programmed at the time of manufacturing . It can't be reprogrammed or rewritten. 

There are three categories of ROM

(i) Programmable ROM(PROM)

PROM or programmable ROM (programmable read-only memory) is a computer memory chip that can be programmed once after it is created. Once the PROM is programmed, the information written is permanent and cannot be erased or deleted. PROM was first developed by Wen Tsing Chow in 1956. An example of a PROM is a computer BIOS in early computers. Today, PROM in computers has been replaced by EEPROM.

(ii) Erasable Programmable ROM(EPROM)

EPROM (erasable programmable read-only memory) is programmable read-only memory (programmable ROM) that can be erased and re-used. Erasure is caused by shining an intense ultraviolet light through a window that is designed into the memory chip.

(iii) Electically Erasable Programmable ROM(EEPROM)

EEPROM (also E2PROM) stands for electrically erasable programmable read-only memory and is a type of non-volatile memory used in computers, integrated in microcontrollers for smart cards and remote keyless systems, and other electronic devices to store relatively small amounts of data but allowing individual bytes to be erased and reprogrammed. It can be erased by exposing to ultraviolet light, then written. So it is also known as Ultra Violet Erasable Programmable ROM (UVEPROM).

SECONDARY MEMORY
(Auxiliary memory / storage devices)

The secondary memory stores large amount of data for a extended period of time . Data in secondary memory can't be directly processed by the CPU, It must be first transferred to primary memory(RAM) for processing .

Secondary storage is used to store data and programs when they are not being processed . It is also non volatile. 

Secondary memory devices include 

Magnetic Disks

• Hard Disks Drive 
• Floppy Disk
• Memory Stick

Optical Disks

• CD 
• DVD
• Blue Ray Disk

Solid State Disks

• Pen/Flash Drive

HARD DISK DRIVE

A hard disk drive (HDD), hard disk, hard drive, or fixed disk is an electro-mechanical data storage device that uses magnetic storage to store and retrieve digital data using one or more rigid rapidly rotating platters coated with magnetic material. The platters are paired with magnetic heads, usually arranged on a moving actuator arm, which read and write data to the platter surfaces. Data is accessed in a random-access manner, meaning that individual blocks of data can be stored and retrieved in any order. HDDs are a type of non-volatile storage, retaining stored data even when powered off.

Floppy Disk

Floppy disk is a soft magnetic disk. It is called floppy because it flops if you wave it (at least, the 5¼-inch variety does). Unlike most hard disks, floppy disks (often called floppies or diskettes) are portable, because you can remove them from a disk drive. Disk drives for floppy disks are called floppy drives. Floppy disks are slower to access than hard disks and have less storage capacity, but they are much less expensive. And most importantly, they are portable.

Standard Sizes

Floppies come in three basic sizes:

•  8-inch:The first floppy disk design, invented by IBM in the late 1960s and used in the early 1970s as first a read-only format and then as a read-write format. The typical desktop/laptop computer does not use the 8-inch floppy disk.

•  5¼-inch: The common size for PCs made before 1987 and the predecessor to the 8-inch floppy disk. This type of floppy is generally capable of storing between 100K and 1.2MB (megabytes) of data. The most common sizes are 360K and 1.2MB.

•  3½-inch: Floppy is something of a misnomer for these disks, as they are encased in a rigid envelope. Despite their small size, microfloppies have a larger storage capacity than their cousins -- from 400K to 1.4MB of data. The most common sizes for PCs are 720K (double-density) and 1.44MB (high-density). Macintoshes support disks of 400K, 800K, and 1.2MB. 

Compact Disks

It is the most popular and the least expensive type of optical disk. A CD is capable of being used as a data storage device along with storing of digital audio. 

CDs are characterised into three main categories

1. CD-ROM(Compact - Disk  - Read Only Memory)

A CD-ROM is a CD that can be read by a computer with an optical drive. The "ROM" part of the term means the data on the disc is "read-only," or cannot be altered or erased. Because of this feature and their large capacity, CD-ROMs are a great media format for retail software.

2. CD- R (Compact Disk Recordable) 

Data can be written on these disks only once . The data once stored in these disks can not be erased.

3. CD-RW(Compact Disk - Rewirtable) 

It is an erasable disk. CD-RW is used to write multiple times on a disk by the use of format feature.

Digital Video Disk(DVD)

Stands for "Digital Versatile Disc." A DVD is a type of optical media used for storing digital data. It is the same size as a CD, but has a larger storage capacity. Some DVDs are formatted specifically for video playback, while others may contain different types of data, such as software programs and computer files.

Blue - ray Disk

Blu-ray is an optical disc format such as CD and DVD. It was developed for recording and playing back high-definition (HD) video and for storing large amounts of data. While a CD can hold 700 MB of data and a basic DVD can hold 4.7 GB of data, a single Blu-ray disc can hold up to 25 GB of data. Even a double sided, dual layer DVD (which are not common) can only hold 17 GB of data. Dual-layer Blu-ray discs will be able to store 50 GB of data. That is equivalent to 4 hours of HDTV.

Pen / Thumb Drive

Commonly known as USB flash drive it is a portable device which means it can be easily transferred from one location to another due to its compact design it looks like a pen shape and therefore it is named as pen drive.The pen drives are used in a wide range in the world they have easily replaced CD’s, Floppy Disk due to its huge data storing capacity and faster data transferring speed. Pen drives and USB flash drives are connected to USB (Universal Serial Bus) Port which is available on computer motherboards they are used without any additional power supply they receive the power directly from the USB port itself and hence are playing dominant role in the PC world.

Magnetic Tapes 

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 arc durable, can be written, erased and re-written. 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 uge reels.

Memory Management

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 o or 1.

Byte (B)

A byte is approximately one character (letter 'a'. number T. 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 I 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 i 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 24° bytes (21° GB). is approximately a trillion (1012) bytes. 

Petabyte (PB)

One petabyte of information is  equal to 1000 terabytes or 10⁴⁵bytes,

Exabyte (EB)

One exabyte of information equat to 100 petabytes or 10¹⁸ bytes.

Zettabyte (ZB)

One zettabyte of information equal to 1000 exabytes or 10²¹ bytes.

Units of computer memory measurements

I Bit  = Binary Digit
8 Bits = 1 Byte = 2 Nibble
1024 Bytes = 1 KB (Kilo Byte)
1024 KB = 1 MB (Mega Byte)
1024 MB = 1 GB(Giga Byte)
1024 GB = 1 TB(Tera Byte)
1024 TB = 1 PB(Peta Byte)
1024 PB 1 EB(Exa Byte)
1024 EB =  1 ZB(Zetta Byte)
1024 ZB  = 1 YB (Yotta Byte)
1024 YB = 1 (Bronto Byte)
1024 Brontobyte  = 1 (Geop Byte)

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