Computer Memory - Unit II

Computer Memory: 

1. Representation of information: BIT, BYTE, Memory

2. Memory size / Units of measurement of storage

3. Main memory: main memory organization, RAM, ROM, PROM, EPROM

4. Secondary storage devices: Sequential Access Memory, Direct Access Memory 

Magnetic Tapes, Magnetic disks, 

5. Optical Disc

6. Memory storage devices: Flash Drive, Memory card

1. Basic Memory Units in Computer

The following are the basic memory units in a computer system:

Bit (Binary Digit)

A computer system uses electrical components (e.g., integrated circuits and semiconductors) to handle data or information stored in it. Such components can only understand and recognize the presence or absence of electrical signals. That is why a computer uses two different states, such as 0 (zero) and 1 (one). 

The two symbols, 0 and 1, are known as bits or binary digits. While 0 indicates the absence (passive state) of the signal, 1 indicates the presence (active state) of the signal. However, a bit at a time can only store one of these two binary values, i.e., either 0 or 1. Therefore, multiple bits in a combined form are capable of holding a greater range of values.

Nibble

A nibble is a collection of 4 bits.

Byte

A byte is a collection of eight bits sequenced together to create a single computer alphabetical or numerical character. One byte, or eight bits, was the original amount of information needed to encode the characters of the text. The number was later standardized after upgrades in computer hardware.

A byte is referred to as the fundamental unit of measurement for the data on a computer system. It is because bytes can be more often represented in larger multiples, such as Kilobytes, Megabytes, Gigabytes, Terabytes, and Petabytes. Bytes are stored digitally (such as disk, tape, etc.) to define the amount of data on storage. Additionally, bytes also help in measuring memory and document size.

2. Memory size / Units of measurement of storage 

Memory is the electronic holding place for the instructions and data a computer needs to reach quickly. It's where information is stored for immediate use. Memory is one of the basic functions of a computer, because without it, a computer would not be able to function properly. Memory is also used by a computer's operating system, hardware and software.

If a CPU had to only use a secondary storage device, computers would become much slower. In general, the more memory (primary memory) a computing device has, the less frequently the computer must access instructions and data from slower (secondary) forms of storage.

Memory vs. storage

The concept of memory and storage can be easily conflated as the same concept; however, there are some distinct and important differences. Put succinctly, memory is primary memory, while storage is secondary memory. Memory refers to the location of short-term data, while storage refers to the location of data stored on a long-term basis.

Memory is most often referred to as the primary storage on a computer, such as RAM. Memory is also where information is processed.

The term storage refers to secondary memory and is where data in a computer is kept. An example of storage is a hard drive or a hard disk drive (HDD). Storage is nonvolatile, meaning the information is still there after the computer is turned off and then back on. 

3. Main Memory: RAM, ROM, PROM, EPROM

Main memory is where programs and data are kept when the processor is actively using them. When programs and data become active, they are copied from secondary memory into main memory where the processor can interact with them. A copy remains in secondary memory.

Main memory is intimately connected to the processor, so moving instructions and data into and out of the processor is very fast.

Main memory is sometimes called RAM. RAM stands for Random Access Memory. "Random" means that the memory cells can be accessed in any order. However, properly speaking, "RAM" means the type of silicon chip used to implement main memory.

When people say that a computer has "512 megabytes of RAM" they are talking about how big its main memory is. One megabyte of memory is enough to hold approximately one million (106) characters of a word processing document. (There will be more about bytes and megabytes later on in these notes.)

Nothing permanent is kept in main memory. Sometimes data are placed in main memory for just a few seconds, only as long as they are needed.

RAM is the main memory. Integrated circuit Random Access Memory (RAM) chips are applicable in two possible operating modes are as follows −

Static − It consists of internal flip-flops, which store the binary information. The stored data remains solid considering power is provided to the unit. The static RAM is simple to use and has smaller read and write cycles.

Dynamic − It saves the binary data in the structure of electric charges that are used to capacitors. The capacitors are made available inside the chip by Metal Oxide Semiconductor (MOS) transistors. The stored value on the capacitors contributes to discharge with time and thus, the capacitors should be regularly recharged through stimulating the dynamic memory.

Random Access Memory

The term Random Access Memory or RAM is typically used to refer to memory that is easily read from and written to by the microprocessor. For a memory to be called random access, it should be possible to access any address at any time. This differentiates RAM from storage devices such as tapes or hard drives where the data is accessed sequentially.

RAM is the main memory of a computer. Its objective is to store data and applications that are currently in use. The operating system controls the usage of this memory. It gives instructions like when the items are to be loaded into RAM, where they are to be located in RAM, and when they need to be removed from RAM.

Read-Only Memory (ROM)

In each computer system, there should be a segment of memory that is fixed and unaffected by power failure. This type of memory is known as Read-Only Memory or ROM.

PROM (Programmable Read-Only Memory) is a type of ROM that is written only once. It was meant to fulfill the requirement of a group of ROMs which may contain a selected memory content. It’s memory is written just the once and programmed electrically by the user at the time or when the initial chip fabrication. 

EPROM (Erasable Programmable Read-Only Memory) is a type of ROM that is read and written optically. To write associate EPROM, its storage cells should stay within the same initial state.

 4. Secondary storage devices: Sequential Access Memory, Direct Access Memory 

A secondary storage device refers to any non-volatile storage device that is internal or external to the computer. A secondary storage device is also known as an auxiliary storage device, backup storage device, tier 2 storage, or external storage. These devices store virtually all programs and applications on a computer, including the operating system, device drivers, applications and general user data.

The Secondary storage media can be fixed or removable. Fixed Storage media is an internal storage medium like a hard disk that is fixed inside the computer. A storage medium that is portable and can be taken outside the computer is termed removable storage media. 

Characteristics of Secondary Storage Devices

These are some characteristics of secondary memory, which distinguish it from primary memory, such as:

It is non-volatile, which means it retains data when power is switched off

It allows for the storage of data ranging from a few megabytes to petabytes.
It is cheaper as compared to primary memory.
Secondary storage devices like CDs and flash drives can transfer the data from one device to another.

Classification of Secondary Storage Devices

The following image shows the classification of commonly used secondary storage devices.

Sequential Access Storage Device

It is a class of data storage devices that read stored data in a sequence. This is in contrast to random access memory (RAM), where data can access in any order, and magnetic tape is the common sequential access storage device.

Magnetic tape: Magnetic tapes are used for large computers like mainframe computers where large volume of data is stored for a longer time. In PC also you can use tapes in the form of cassettes. The cost of storing data in tapes is inexpensive. Tapes consist of magnetic materials that store data permanently. 

Advantages of Magnetic Tape:

 

a. Compact: A 10-inch diameter reel of tape is 2400 feet long and is able to hold 800, 1600 or 6250 characters in each inch of its length. The maximum capacity of such tape is 180 million characters. Thus data are stored much more compactly on tape.

b. Economical: The cost of storing characters is very less as compared to other storage devices.

c. Fast: Copying of data is easier and fast.

d. Long term Storage and Re-usability: Magnetic tapes can be used for long term storage and a tape can be used repeatedly without loss of data.

Magnetic Disk

A magnetic Disk is a type of secondary memory that is a flat disc covered with a magnetic coating to hold information. It is used to store various programs and files.

There are various advantages and disadvantages of magnetic disk memory. 

Advantages:- 

These are economical memory 

Easy and direct access to data is possible. 

It can store large amounts of data. 

It has a better data transfer rate than magnetic tapes. 

It has less prone to corruption of data as compared to tapes. 

Disadvantages:- 

These are less expensive than RAM but more expensive than magnetic tape memories. 

It needs a clean and dust-free environment to store. 

These are not suitable for sequential access.

5. Optical Disc

An optical disc is an electronic data storage medium that is also referred to as an optical disk, optical storage, optical media, Optical disc drive, disc drive, which reads and writes data by using optical storage techniques and technology.

An optical disc can store more data and has a longer lifespan than the preceding generation of magnetic storage medium. To read and write to CDs and DVDs, computers use 

a CD writer or DVD writer drive, and to read and write to Blu-ray discs, they require a Blu-ray drive.

The CDs, Blu-ray, and DVDs are the most common types of optical media, which are usually used to:

They are used to transfer data to various devices or computers.

These media are used to deliver the software to others.

They help users to hold large amounts of data, like videos, photos, music, and more.

Also, optical media are used to get back up from a local machine.

With the introduction of an all-new generation of optical media, the storage capacity to store data has increased. CDs have the potential to store 700 MB of data, whereas DVDs allow you to store up to 8.4 GB of data. Blu-ray discs, the newest type of optical media, can hold up to 50 GB of data. 

CD-ROM

CD-ROM, short for compact disk read-only memory, was the first disk on the basis of drives for the latest PCs. CD-ROM devices populate Compact Disk Filing System discs with data encoded in ISO 9660. To reduce noise and increase stability, most CD-ROM drives in computers run at a slower speed, and if the drive experiences read errors, it will only speed up for larger data files. However, the newest CD-ROM drives have the potential to achieve read speeds of 60 revolutions in a second (60x).

DVD-ROM

DVD-ROM drives, which stand for Digital Versatile Disk Read Only Memory and are a direct evolution from CD-ROM drives, have significantly more performance and capacity than their CD counterparts while maintaining the same physical dimensions. The DVD Forum is a non-profit organization that establishes several standards for DVD functionality and construction, as well as overseeing DVD development.

6. Memory storage devices: Flash Drive, Memory card

Flash Drive

A flash drive is a small, ultra-portable storage device which, unlike an optical drive or a traditional hard drive, has no moving parts.

Flash drives connect to computers and other devices via a built-in USB Type-A or USB-C plug, making one a kind of combination USB device and cable.

Flash drives are  referred to as pen drives. Flash drives can be written and rewritten to an almost unlimited number of times, similar to hard drives.

Memory Card

A memory card is a type of storage device that is used for storing media and data files. It provides a permanent and non-volatile medium to store data and files from the attached device. Memory cards are commonly used in small, portable devices, such as cameras and phones.

A memory card is also known as a flash card.

Some of the most popular forms of memory cards are:

Secure Digital (SD) card

CompactFlash (CF) card

SmartMedia

Memory Stick

MultiMediaCard (MMC)