Memory Hierarchy
The hierarchical arrangement of storage in current computer architectures is called the memory hierarchy. The computer uses a hierarchy of memory that is organized in a manner to enable the fastest speed and largest capacity of memory as shown in the figure. Memory is characterized on the basis of two key factors; capacity and access time.
Parameters of Memory
Some related parameters of memory are as follows
- Storage Capacity: It is representative of the size of memory. The capacity of internal memory or main memory can be expressed in terms of the 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 writes operation till the data is made available or written at the desired location.
- Physical Characteristics: In this respect, the devices can be categorized into four main categories electronic, magnetic, mechanical, and optical.
- Permanence of Storage: Its permanence is high for future use in magnetic materials.
Types of Memory
In general, memory is classified into two categories as follows
- Primary memory or Main memory
- Secondary memory or Auxiliary memory
Primary Memory
The memory unit that communicates directly with the CPU is called main memory or internal memory or primary memory. The primary memory allows the computer to store data for immediate manipulation and to keep track of what is currently being processed. It has limited storage capacity.
The main memory is volatile in nature, which means that when the power is turned OFF, the contents of this memory are lost forever.
Primary memory can be further classified into two categories which are as follows
Random Access Memory (RAM):
It is also known as read/write memory, which allows the 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. There are two categories of RAM as follows
- Dynamic RAM (DRAM): It is made up of memory cells and each cell is composed of one capacitor and one transistor. DRAM must be refreshed continually to store information. DRAM is slower, less- expensive, and occupies less space on the computer’s motherboard.
- Static RAM (SRAM): It retains the data as long as power is provided to the memory chip. SRAM needs not to be refreshed periodically. It uses multiple transistors for each memory cell. It does not use a capacitor. SRAM has often used cache memory due to its high speed. SRAM is more expensive and faster than DRAM.
Read Only Memory (ROM):
It is also known as non-volatile memory or permanent storage. It does not lose its contents when the power is switched OFF. 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. So, it has only read capability, not write. There are three categories of ROM as follows
- 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. These types of memories are found in video game consoles, mobile phones, implantable medical devices, and high-definition multimedia interfaces.
- 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 (UVEPROM).
- 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. It is the most flexible type of ROM and is now commonly used for holding BIOS.
Apart from the above memory, there is also some other memory that helps with primary memory which is as follows
- Cache Memory: It is a storage buffer that stores the data which is used more often, or temporarily, and makes, them available to the CPU at a fast rate. Cache memory is a very high-speed memory placed in between RAM and CPU. It 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. Flash Memory It is a kind of semiconductor-based non-volatile rewritable memory, used in digital cameras, mobile phones, printers, etc.
- Virtual Memory: It is a technique that allows the execution of processes that are not completely in the main memory. One major advantage of this scheme is that programs can be larger than the main memory.
Secondary Memory/ Storage
This 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 memory, i.e. RAM. It is the slower and cheaper form of memory. 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 permanent storage. Secondary memory devices include as follows
- Magnetic Storage: Magnetic Storage is the manipulation of magnetic fields on a medium in order to record audio, video, or other data. It includes a hard disk drive, a floppy disk, and magnetic tape.
- Optical Storage: Optical storage is any storage type in which data is written and read with a laser. It includes CD, DVD, and Blu-ray discs.
- Solid State Storage: Solid state storage is a type of storage technique that employs storage devices built using silicon microchip-based storage architecture. It includes a pen/flash drive, and a memory card.
Hard Disk Drive (HDD)
It is a non-volatile and random access digital data storage device. HDD is a data storage device used for storing and retrieving digital information using rotating disks (platters) coated with magnetic material. All programs on a computer are installed on a hard disk. It is a fixed disk i.e. cannot be removed from the drive.
It consists of a spindle that holds non-magnetic flat circular disks, called platters, which hold the recorded data. Each platter requires two read/write heads, that are used to write and read 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 (Diskette)
It is used to store data but it can store a small amount of data and it is slower to access than hard disks. Floppy disk rounds 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. The Floppy disk is removable from the drive. Floppy disk is available in three sizes; 8 inches, 5.25 inches, and 3.5 inches.
Magnetic Tape
These 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. Magnetic tapes hold the maximum data, which can be accessed sequentially. They are generally used to store backup data or that type of data, which is not frequently used, or to transfer data from one system to another.
Compact Disc (CD)
It is the most popular and the least expensive type of optical disc. A CD is capable of being used as a data storage device along with storing digital audio. The files are stored in these particular contiguous sectors. CDs are categorized into three main types as follows
- CD-ROM (Compact Disc-Read Only Memory)
- CD-R (Compact Disc- Recordable)
- CD-RW (Compact Disc- Rewritable)
Digital Video Disc (DVD)
DVD is also known as Super Density Disc (SDD) or Digital Versatile Disc (DVD). It is an optical disc storage media. DVDs offer higher storage capacity than CDs while having the same dimensions.
Depending upon the disc type, a 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 computer too. They are not rewritable media.
DVDs come in three varieties as follows
- DVD-ROM (Digital Video Disc-Read Only Memory)
- DVD-R (DVD-Recordable)
- DVD-RW (DVD-Rewritable)
The rate at which data is written to disc or read from disc is called the data transfer rate.
The root directory is the main folder of the disc. It contains information about all folders on the disc.
Blu-ray Disc
It is an optical disc storage medium designed to re-capture the data normally in DVD format. Blu-ray disc (BD) contains 25 GB (23.31 GB) per layer space. The name Blu-ray disc refers to the blue laser used to read the disc, which allows information to be stored at a greater density than the longer-wavelength red laser used in DVDs. Blu-ray can hold almost 5 times more data than a single-layer DVD. The variations in the formats are as follows
- BD-ROM (Read only)
- BD-R (Recordable)
- BD-RW (Rewritable)
- BD-RE (Rewritable)
Pen/Thumb Drive
A pen drive is also known as a flash drive. A 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, rewritable, and much smaller than a floppy disk. Today, flash drives are available in various storage capacities as 256MB, 512MB, 1GB, 4GB, and 16GB up to 64 GB. They are widely used as an easy and small medium to transfer and store information from computers. Memory Cards These are data storage devices in a chip shape that can store the data in them. They are commonly used in many electronic devices, including digital cameras, mobile phones, and laptop computers. They are small, re-recordable, easily portable, and very light weighted.
Secondary Memory Devices and their Storage Method and Capacity are as follows
