Types of Computer Memories

 

Introduction to Memory Unit

Introductions of Memory Unit

A memory unit is the section of the CPU where programs and data are stored after they are read through the input unit. Primary memory is the main storage area of a computer. It stores programs, input values, output values, and intermediate results of processing.

Secondary memory is permanent memory that stores data permanently in a computer system.

Types of Memory

Computer memory is classified into two major categories.

1. Primary Memory
2. Secondary Memory

PRIMARY MEMORY

Primary memory is the main memory of the computer. It stores data and instructions that are currently being processed by the CPU.

Characteristics of Primary Memory

• Directly accessible by the CPU.

• Faster than secondary memory.

• Used during program execution.

• Limited storage capacity.

• Includes RAM and ROM.

Random Access Memory (RAM)

Random Access Memory (RAM) is a volatile memory. It is also called read-and-write memory because it is used to store data and instructions during the execution of a program.

The data stored in RAM is erased when the computer is switched off. Since it is possible to access any location of this memory directly, it is called Random Access Memory.

Types of RAM

1. Dynamic Random Access Memory (DRAM)

The information stored in DRAM has to be refreshed at very short intervals; otherwise, it is erased.

DRAM has a higher storage capacity and is cheaper than SRAM.

2. Static Random Access Memory (SRAM)

The information stored in SRAM does not need to be refreshed, but it remains stable as long as power is supplied.

SRAM is costly but has a higher speed than DRAM.

Read Only Memory (ROM)

A computer does not contain intelligence or memory by itself and requires instructions that are provided by the user or system. Whenever the computer is switched on, it searches for the required instructions.

ROM is permanent and does not lose its contents when the system is switched off. One of the most important programs stored in ROM is the BIOS (Basic Input/Output System), which activates the hardware and enables communication between the system and application software.

Types of ROM

1. Mask Read-Only Memory (Mask ROM)

In Mask ROM, the information is stored at the time of manufacturing. The information stored in this type of ROM cannot normally be changed.

2. Programmable Read-Only Memory (PROM)

PROM is programmed after its manufacture. Once programmed, its information cannot be altered or erased later.

3. Erasable Programmable Read-Only Memory (EPROM)

EPROM is similar to PROM, but the stored information can be erased later using ultraviolet (UV) light. After erasing, it can be reprogrammed.

4. Electrically Erasable Programmable Read-Only Memory (EEPROM)

EEPROM is similar to EPROM, but the stored information can be erased using an electrical process.

5. Electrically Alterable Programmable Read-Only Memory (EAROM)

Compared with EPROM and EEPROM, the stored information in EAROM can be altered later.

Cache Memory

Cache memory is a high-speed memory device used as a buffer or cache by certain processors.

It provides a temporary work area for holding intermediate data and instructions. The restricted use of such memory can increase processing speed without drastically increasing costs.

Cache memory is used in microcomputers to improve processing performance.

SECONDARY STORAGE DEVICES

Secondary storage devices permanently store data and programs.

Types of Secondary Storage Devices

1. Sequential Access Devices.

2. Direct Access Devices.

1. Sequential Access Device

Magnetic Tape

Magnetic tapes are similar to commonly used audio tapes.

A magnetic tape drive consists of a spool on which magnetic tape is wound. The tape is transported across a set of magnetic heads.

The tape is taken up on another spool. Between the two spools, a set of heads records information on the tape.

The width of the tape is standardized. The recording density is measured in bits per inch (BPI). The reference mentions recording densities of 1600, 3200, or 6250 bits per inch.

The tape is read sequentially; that is, data is read one item after another in the order in which it is stored.

Magnetic Tape Diagram — Main Labels

• Supply Head
• Write Head
• Tape-Up Reel
• Control Panel
• Read Head
• Stop Capstan
• Drive Capstan
• Tape Support Tools
• Idler
• Vacuum Column

The reference also notes that the vacuum column absorbs the input due to the stoppage and start of the tape.

2. Direct Access Devices

Direct Access Devices allow the computer to access any data location directly without reading data sequentially.

Direct Access Devices are classified into:

(A) Magnetic Disks

1. Floppy Disk

2. Hard Disk

3. Solid-State Drive (SSD)

(B) Optical Disks

• CD-ROM

• CD-RW

• DVD-ROM

• DVD-RW

(C) External Storage Devices

• Pen Drive

• Flash Drive

Note: SSD has been added here as the third type under Magnetic Disks, as requested.

 

(A) Magnetic Disks

Magnetic disks are storage devices that provide direct access to stored information.

1. Floppy Disk

A floppy disk is a low-cost magnetic storage medium used for secondary data storage.

Floppy disk drives use magnetic media. Different types of floppy disks differ in size and storage capacity.

Examples mentioned in the reference include:

• 360 KB — 5¼ inch
• 1.2 MB — 5¼ inch
• 1.44 MB — 3½ inch

2. Hard Disk (HDD)

A Hard Disk Drive (HDD) is a magnetic storage device that stores data permanently on rotating magnetic platters.

A hard disk is a multilayer printed circuit board (PCB)-based storage device containing magnetized surfaces.

Hard disks are available in different storage capacities. Examples mentioned in the reference include:

• 20 GB

• 40 GB

• 80 GB

• 320 GB

3. Solid-State Drive (SSD)

A Solid-State Drive (SSD) is a modern storage device that stores data using flash memory chips instead of rotating magnetic disks.

A Solid-State Drive (SSD) is a type of secondary storage device that uses flash memory to store data.

Unlike a traditional hard disk, an SSD does not use rotating magnetic platters or mechanical read/write heads. Because it has no moving mechanical parts, an SSD generally provides faster data access and better resistance to physical shock than a conventional hard disk.

Main Features of SSD

• Uses flash memory for data storage.

• Has no moving mechanical parts.

• Provides fast data access.

• Provides faster booting and application loading compared with traditional hard disks in many systems.

• Operates silently because it does not contain rotating disks.

• Generally consumes less power than mechanical hard disks.

• Available in different storage capacities.

SSD vs. Hard Disk

Feature	Hard Disk (HDD)	Solid-State Drive (SSD)
Storage Technology	Magnetic	Flash memory
Moving Parts	Yes	No
Data Access	Relatively slower	Generally faster
Noise	May produce mechanical noise	Silent
Shock Resistance	Lower	Generally higher
Power Consumption	Generally higher	Generally lower
Common Use	Desktop and large-capacity storage	Computers, laptops, and high-speed storage

(B) Optical Disks

Optical disks store data using laser technology.

1. Compact Disc (CD)

A Compact Disc (CD) is an optical storage medium used for storing digital data, software, audio, multimedia, and other information.

The reference mentions:

• CD-ROM
• CD-R
• CD-RW

CDs became popular as storage devices for computers and were used for storing software, multimedia, audio, and graphics.

The reference mentions a storage capacity of approximately 700/720 MB.

2. Digital Versatile Disc (DVD)

Digital Versatile Disc (DVD) is an optical storage medium with a higher storage capacity than a conventional CD.

The reference mentions DVDs with capacities of approximately 4.7 GB and 5.2 GB and describes their use for storing large amounts of information, particularly for multimedia applications.

Examples include:

• DVD-ROM
• DVD-R
• DVD-RW

(C) External Storage Devices

External storage devices are portable devices used to store and transfer data.

1. Pen Drive

A pen drive is a small, portable storage device used to store and transfer digital data.

It normally connects to a computer through a USB (Universal Serial Bus) port.

Pen drives are available in different storage capacities.

The reference mentions examples such as:

• 1 GB
• 2 GB
• 4 GB
• 8 GB
• 16 GB
• 32 GB
• etc.

2. Flash Drive

A flash drive is a portable storage device that uses flash memory for storing digital data.

It can be used for transferring files between computers and other compatible devices.

The reference includes an illustration of a small USB storage device.

3. Memory Cards

A Memory Card is a small, removable storage device that uses flash memory to store digital data.

Memory cards are commonly used in digital cameras, smartphones, tablets, video cameras, and other electronic devices.

Common Types of Memory Cards

• SD (Secure Digital) Card
• microSD Card
• miniSD Card
• CF (CompactFlash) Card

Features

• Small and lightweight.
• Portable and removable.
• Uses flash memory.
• Does not contain moving mechanical parts.
• Available in different storage capacities.
• Suitable for storing photographs, videos, music, documents, and other digital files.

Uses

• Storing photographs and videos in digital cameras.
• Expanding storage capacity in compatible smartphones and tablets.
• Storing multimedia files.
• Transferring data between compatible devices.

MEMORY UNITS

Memory Unit	Equivalent
Bit	A Binary Digit
Nibble	4 Bits
Byte	8 Bits
Kilobyte (KB)	1024 Bytes
Megabyte (MB)	1024 KB
Gigabyte (GB)	1024 MB
Terabyte (TB)	1024 GB
Petabyte (PB)	1024 TB