Computer Memory types and explanation with example

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1. Computer Memory

2. Computer memory Computer memory is a generic term for all of the different types of data storage technology that a computer may use, including RAM, ROM, and  flash memory. Some types of computer memory are designed to be very fast, meaning that the central processing unit (CPU) can access data stored there very quickly. Other types are designed to be very low cost, so that large amounts of data can be stored there economically.

3. Another way that computer memory can vary is that some types are  non-volatile , which means they can store data on a long term basis even when there is no power. And some types are  volatile , which are often faster, but which lose all the data stored on them as soon as the power is switched off.  Memory system can be divided into 4 categories: CPU register Cache memory Primary / Main memory Secondary Memory / Mass Storage

4. Primary and Secondary Memory PM includes ROM and RAM, and is located close to the CPU on the computer motherboard, enabling the CPU to read data from primary memory very quickly indeed. PM is used to store data that the CPU needs imminently so that it does not have to wait for it to be delivered. Secondary memory  by contrast, is usually physically located within a separate storage device, such as a hard disk drive or solid state drive (SSD), which is connected to the computer system either directly or over a network. The cost per gigabyte of secondary memory is much lower, but the read and write speeds are significantly slower.

5. Primary Memory Primary memory is computer memory that a processor or computer accesses first or directly. It allows a processor to access running execution applications and services that are temporarily stored in a specific memory location. Primary memory is also known as primary storage or main memory. Primary memory is a computer system's volatile storage mechanism. It may be random access memory (RAM), cache memory or data buses, but is primarily associated with RAM.

6. As soon as a computer starts, primary memory loads all running applications, including the base operating system (OS), user interface and any user-installed and running software utility. A program/application that is opened in primary memory interacts with the system processor to perform all application-specific tasks. Primary memory is considered faster than secondary memory.

7. Types of Primary Memory There are two key types of primary memory: RAM, or random access memory ROM, or read-only memory RAM 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), 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.

8. 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 (pushed or pulled) 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.

9. Types of RAM DRAM : DRAM stands for Dynamic RAM, and it is the most common type of RAM used in computers. The oldest type is known as single data rate (SDR) DRAM, but newer computers use faster dual data rate (DDR) DRAM. DDR comes in several versions including DDR2 , DDR3, and DDR4, which offer better performance and are more energy efficient than DDR. However different versions are incompatible, so it is not possible to mix DDR2 with DDR3 DRAM in a computer system. DRAM consists of a transistor and a capacitor in each cell.

10. SRAM : SRAM stands for Static RAM, and it is a particular type of RAM which is faster than DRAM, but more expensive and bulker, having six transistors in each cell. For those reasons SRAM is generally only used as a data cache within a CPU itself or as RAM in very high-end server systems. A small SRAM cache of the most imminently-needed data can result in significant speed improvements in a system

11. ROM ROM stands for read-only memory, and the name stems from the fact that while data can be read from this type of computer memory, data cannot normally be written to it. It is a very fast type of computer memory which is usually installed close to the CPU on the motherboard. ROM is a type of non-volatile memory, which means that the data stored in ROM persists in the memory even when it receives no power – for example when the computer is turned off. In that sense it is similar to secondary memory, which is used for long term storage.

12. When a computer is turned on, the CPU can begin reading information stored in ROM without the need for drivers or other complex software to help it communicate. The ROM usually contains "bootstrap code" which is the basic set of instructions a computer needs to carry out to become aware of the operating system stored in secondary memory, and to load parts of the operating system into primary memory so that it can start up and become ready to use. ROM is also used in simpler electronic devices to store firmware which runs as soon as the device is switched on.

13. There are four basic ROM types: MROM PROM EPROM EEPROM

14. MROM (Masked ROM) The very first ROMs were hard-wired devices that contained a pre-programmed set of data or instructions. These kind of ROMs are known as masked ROMs, which are inexpensive. PROM PROM is read-only memory that can be modified only once by a user. The user buys a blank PROM and enters the desired contents using a PROM program. Inside the PROM chip, there are small fuses which are burnt open during programming. It can be programmed only once and is not erasable.

15. EPROM Erasable and Programmable Read Only Memory EPROM can be erased by exposing it to ultra-violet light for a duration of up to 40 minutes. Usually, an EPROM eraser achieves this function. The charge is retained for more than 10 years because the charge has no leakage path. For erasing this charge, ultra-violet light is passed through a quartz crystal window (lid). This exposure to ultra-violet light dissipates the charge. During normal use, the quartz lid is sealed with a sticker.

16. EEPROM Electrically Erasable and Programmable Read Only Memory EEPROM is programmed and erased electrically. It can be erased and reprogrammed about ten thousand times. Both erasing and programming take about 4 to 10 ms (millisecond). In EEPROM, any location can be selectively erased and programmed. EEPROMs can be erased one byte at a time, rather than erasing the entire chip. Hence, the process of reprogramming is flexible but slow.

17. Hard disk drive A  hard disk drive  (sometimes abbreviated as a  hard drive ,  HD , or  HDD ) is a non-volatile data storage device. It is usually installed internally in a computer, attached directly to the disk controller of the computer's motherboard. It contains one or more platters, housed inside of an air-sealed casing. Data is written to the platters using a magnetic head, which moves rapidly over them as they spin.

18. Why does a computer need a hard drive? A computer requires an operating system to allow users to interact with and use it. The operating system interprets keyboard and mouse movements and allows for the use of software, like an Internet browser, word processor, and video games. To install a computer operating system, a hard drive is required. A hard drive is also required for the installation of any programs or other files you want to keep on your computer. 

19. Components of HDD Platters The platters are the circular discs inside the hard drive where the 1s and 0s that make up your files are stored. Platters are made out of aluminum, glass or ceramic and have a magnetic surface in order to permanently store data. On larger hard drives, several platters are used to increase the overall capacity of the drive. Data is stored on the platters in tracks, sectors and cylinders to keep it organized and easier to find.

20. The Spindle The spindle keeps the platters in position and rotates them as required. The revolutions-per-minute rating determines how fast data can be written to and read from the hard drive. A typical internal desktop drive runs at 7,200 RPM, though faster and slower speeds are available. The spindle keeps the platters at a fixed distance apart from each other to enable the read/write arm to gain access.

21. The Read/Write Arm The read/write arm controls the movement of the read/write heads, which do the actual reading and writing on the disk platters by converting the magnetic surface into an electric current. The arm makes sure the heads are in the right position based on the data that needs to be accessed or written; it's also known as the head arm or actuator arm. There is typically one read/write head for every platter side, which floats 3 to 20 millionths of an inch above the platter surface.

22. Actuator The actuator or head actuator is a small motor that takes instructions from the drive's circuit board to control the movement of the read/write arm and supervise the transfer of data to and from the platters. It's responsible for ensuring the read/write heads are in exactly the right place at all times.

23. Solid State drive A solid-state drive (SSD) is a new generation of storage device used in computers. SSDs replace traditional mechanical hard disks by using flash-based memory, which is significantly faster. Older hard-disk storage technologies run slower, which often makes your computer run slower than it should. SSDs speed up computers significantly due to their low read-access times and fast throughputs. They use a simple memory chip called NAND flash memory, which has no moving parts and near-instant access times.

24. The memory chips on an SSD are comparable to random access memory (RAM). Instead of a magnetic platter, files are saved on a grid of NAND flash cells. Each grid (also called blocks) can store between 256 KB and 4MB. The controller of an SSD has the exact address of the blocks, so that when your PC requests a file it is (almost) instantly available. There’s no waiting for a read/write head to find the information it needs. SSD access times are thus measured in nanoseconds.

25. SSDs have specific benefits in the following areas : Business :  Gaming : Mobility : Servers

26. Optical disk An optical disk is any computer disk that uses optical storage techniques and technology to read and write data. It is a computer storage disk that stores data digitally and uses laser beams (transmitted from a laser head mounted on an optical disk drive) to read and write data.  Compact disks (CD), digital versatile/video disks (DVD) and Blu -ray disks are currently the most commonly used forms of optical disks.

27. These disks are generally used to: Distribute software to customers Store large amounts of data such as music, images and videos Transfer data to different computers or devices Back up data from a local machine

28. An optical disk drive uses a laser to read and write data. A laser in this context means an electromagnetic wave with a very specific wavelength within or near the visible light spectrum. Different types of discs require different wavelengths. For compact discs, or CDs, a wavelength of 780 nanometers (nm) is used, which is in the infrared range. For digital video discs, or DVDs, a wavelength of 650 nm (red) is used, while for Blu -ray discs a wavelength of 405 nm (violet) is used.

29. Compact Disk A compact disc is a portable storage medium that can be used to record, store and play back audio, video and other data in digital form. A standard compact disc measures 4.7 inches, or 120 millimeters (mm), across, is 1.2 mm thick, weighs between 15 grams and 20 grams, and has a capacity of 80 minutes of audio, or 650 megabytes (MB) to 700 MB of data. CDs are fragile and prone to scratches; they can be repaired, but disc readability may be affected.

30. Advantages of CD Large storage capacity Portability Data cannot be changed Sturdiness Special Capabilities

31. Disadvantages of CD Fairly fragile, easy to snap or scratch Smaller storage capacity than a hard drive or DVD Slower to access than the hard disk.

32. 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. A standard DVD can hold 4.7 GB of data, but variations of the original DVD format have greater capacities. For example, a dual-layer DVD (which has two layers of data on a single side of the disc) can store 8.5 GB of data. A dual-sided DVD can store 9.4 GB of data (4.7 x 2). A dual-layer, dual-sided DVD can store 17.1 GB of data.

33. Blu -ray 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.

34. Blu -ray discs can hold more information than other optical media because of the blue lasers the drives use. The laser is actually blue-violet, but " Blu -ray" rolls off the tongue a little easier than " Blu -violet-ray." The blue-violet laser has a shorter wavelength than the red lasers used for CDs and DVDs (405nm compared to 650nm). This allows the laser to focus on a smaller area, which makes it possible to fill significantly more data on a disc the same size as a CD or DVD.

35. Floppy disk  floppy disk is a storage medium that consists of a thin and flexible magnetic disk inside a plastic carrier. Widely used since the 1970s until the early 2000s, they have gradually been replaced by other storage devices with greater capacity. In order to read and write data from a floppy disk, a computer system must have a floppy disk drive (FDD). A floppy disk is also referred to simply as a floppy.  Since the early days of personal computing, floppy disks were widely used to distribute software, transfer files, and create back-up copies of data. When hard drives were still very expensive, floppy disks were also used to store the operating system of a computer.

36. Advantages of a floppy disk : Small  Easy to carry Cheap Easy to transport and handle Data on a floppy disk device can be write-protected from being changed accidentally Floppy disk can be used to transfer data from one computer to another computer Portable and inexpensive Data on a floppy disk can be accessed randomly manner Useful for transferring a small file Security tab to stop data from being overwritten

37. Disadvantages of a floppy disk : Easy to be damaged The access time of floppy disk is low They have to handle more carefully Floppies can get affected by heat Small storage capacity Many new computers don't have any floppy disk drive Data can be erased if the disk comes into contact with a magnetic field side Slow to access and retrieve data when compared to a hard-disk device

38. Magnetic tape Magnetic tape is a type of physical storage media for different kinds of data. It is considered an analog solution, in contrast to more recent types of storage media, such as solid state disk (SSD) drives. Magnetic tape has been a major vehicle for audio and binary data storage for several decades, and is still part of data storage for some systems. A  magnetic tape , in computer terminology, is a storage medium that allows for data archiving, collection, and backup. At first, the tapes were wound in wheel-like reels, but then cassettes and cartridges came along, which offered more protection for the tape inside.

39. One side of the tape is coated with a magnetic material. Data on the tape is written and read sequentially. Finding a specific record takes time because the machine has to read every record in front of it. Most tapes are used for archival purposes, rather than ad-hoc writing and reading. Data is written into 'tracks' on the medium. Some run along the edge of the tape, which is called  linear recording , while others are written diagonally, which is called  helical recording . Older magnetic tapes used eight tracks, while more modern ones can handle 128 or more tracks.