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Computer Memory

What is Memory?

Characteristics of Main Memory:

  • It is faster computer memory as compare to secondary memory.
  • It is semiconductor memories.
  • It is usually a volatile memory.
  • It is the main memory of the computer.
  • A computer system cannot run without primary memory.

In general, memory is of three types:

1. Primary Memory :

  • It is also known as the main memory of the computer system.
  • It is used to store data and programs or instructions during computer operations.
  • It uses semiconductor technology and hence is commonly called semiconductor memory.

Primary memory is of two types :

  • (i) RAM (Random Access Memory) :
    • It is a volatile memory.
    • Volatile memory stores information based on the power supply.
    • If the power supply fails/ interrupted/ stopped, all the data & information on this memory will be lost.
    • RAM is used for booting up or start the computer.
    • It temporarily stores programs/data which has to be executed by the processor.

RAM is of two types :

  • S RAM (Static RAM) :
    • Transistors are used to store information.
    • SRAM has lower access time; therefore it is faster compared to DRAM.
    • It is used in cache memory.
    • Capicitor are not used hence no refreshing is require.
    • SRAMs are low density devices.
    • SRAM's structure is complex than DRAM.
    • SRAM requires constant power supply, which means this type of memory consumes more power.
  • D RAM (Dynamic RAM) :
    • Capacitors are used to store data.
    • DRAM has higher access time; therefore it is slower compared to SRAM.
    • It is used in main memory.
    • To store information for a longer time, capacitor needs to be refreshed periodically.
    • DRAMs are high density devices.
    • DRAM's structure is simpler than DRAM.
    • DRAM offers reduced power consumption, due to the fact that the information is stored in capacitor.

2. ROM (Read Only Memory)

  • It is non-volatile memory.
  • Non-volatile memory stores information even when there is a power supply failed/ interrupted/ stopped.
  • ROM is used to store information that is used to operate the system.
  • As its name refers to read-only memory, we can only read the programs and data that is stored on it.
  • It contains some electronic fuses that can be programmed for a piece of specific information.
  • The information stored in the ROM in binary format.
  • It is also known as permanent memory.

ROM is of four types :

  • MROM (Masked ROM) → Hard-wired devices with a pre-programmed collection of data or instructions were the first ROMs. Masked ROMs are a type of low-cost ROM that works in this way.
  • PROM (Programmable Read Only Memory) → This read-only memory is modifiable once by the user. The user purchases of blank PROM and uses of PROM program to put the required contents into the PROM. Its content can't be erased once written.
  • EPROM (Erasable Programmable Read Only Memory) → It is an extension to PROM where you can erase the content of ROM by exposing it to Ultraviolet rays for nearly 40 minutes.
  • EEPROM (Electrically Erasable Programmable Read Only Memory) → Here the written contents can be erased electrically. You can delete and reprogramme EEPROM up to 10,000 times. Erasing and programming take very little time, i.e, nealy 4-10 ms(milliseconds). Any area in an EEPROM can be wiped and programmed selectively.

Secondary Storage Devices

Why do we need Secondary Storage?

  • That's why secondary storage is needed to keep programs and data long term.
  • Secondary storage is non-volatile and able to keep data as long term storage.
  • They are used for various purposes such as backup data used for future restores or disaster recovery, long-term archiving of data that is not frequently accessed, and storage of non-critical data in lower-performing, less expensive drives.

Characteristics of Secondary Storage Devices

Types of Secondary Storage Device

There are two types of secondary storage devices, i.e., fixed storage and removable storage.

1. Fixed Storage

  • Fixed storage is an internal media device used by a computer system to store data. Usually, these are referred to as the fixed disk drives or Hard Drives.
  • Fixed storage devices are not fixed. These can be removed from the system for reparing work, maintenance purposes, and also for an upgrade, etc. But in general, this can not be done without a proper toolit to open up the computer system to provide physical access, which needs to be done by an engineer.
  • Technically, almost all data, i.e. being processed on a computer system, is stored on some built-in fixed storage device. We have the following types of fixed storage:
    • Internal flash memory (rare)
    • SDD (solid-state disk) units
    • Hard disk drives (HDD)

2. Removable Storage

  • Removable storage is an external media device that is used by a computer system to store data.
  • Usually, these are referred to as the Removable Disks drives or the External Drives.
  • Removalble storage is any storage device than can be removed from a computer system while the system is running.
  • Examples of external devices include CDs, DVDs, Blu-ray disk drives, and diskettes and USB drives.
  • Removable storage makes it easier for a user to transfer data from one computer system to another.
  • The main benefir of removable disks in storage factors is that they can provide the fast transfer rates associated with storage area networks (SANs).
  • We have the following types of Removable Storage:
    • Optical discs (CDs, DVDs, Blu-ray discs)
    • Memory cards
    • Floppy disks
    • Magnetic tapes
    • Disk packs
    • Paper storage (punched tapes, punched cards)

Classification of Secondary Storage Devices

Sequential Access Storage Device

  • It is a class of data storage devices that 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
      • It is a medium for magnetic recording, made of a thin, magnetizable coating on a long, narrow strip of plastic film.
      • Devices that record and play audio and video using magnetic tape are tape recorders and videotape recorders.
      • A device that stores computer data on magnetic tape is known as a tape drive.
      • It was a key technology in early computer development, allowing unparalleled amounts of data to be mechanically created, stored for long periods, and rapidly accessed.

Direct Access Storage Devices

  • A direct-access storage device (DASD) is another name for secondary storage devices that stores data in discrete locations with a unique address, such as hard disk drives, optical drives and most magnetic storage devices.

1. Magnetic disks

  • A magnetic disk is a storage device that uses a magnetization process to write, rewrite and access data.
  • It is covered with a magnetic coating and stores data in the form of tracks, spots and sectors.
  • Hard disks, zip disks and floppy disks are common examples of magnetic disks.

Floppy Disk

  • A floppy disk is a flexible disk with a magnetic coating on it, and it is packaged inside a protective plastic envelope.
  • These are among the oldest portable storage devices that could store up to 1.44 MB of data, but not they are not used due to very little memory storage.

Hard Disk Drive (HDD)

  • Hard disk drive comprises a series of circular disks called platters arranged one over the other almost 1/2 inches apart around a spindle.
  • Disks are made of non-magnetic material like aluminium alloy and coated with 10-20 nm magnetic material.
  • The standard diameter of these disks is 14 inches, and they rotate with speeds varying from 4200 rpm (rotations per minute) for personal computers to 15000 rpm for servers.
  • Data is stored by magnetizing or demagnetizing the magnetic coating.
  • A magnetic reader arm is used to read data from and write data to the disks.
  • A typical modern HDD has a capacity in terabytes (TB).

2. 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 user laser beans to read and write data.

CD Drive

  • CD stands for comact disk.
  • CD's are circular disks that use optical rays, usually lasers, to read and write data.
  • They are very cheap as you can get 700 MB of storage for less than a dollar.
  • CDs are inserted in CD drives built into the CPU cabinet.
  • They are portable as you can eject the drive, remove the CD and carry it with you.
  • There are three type of CDs:
    • CD-ROM (Compact Disk - Read Only Memory) → The manufacturer recorded the data on these CDs. Proprietary software, audio or video are released on CD-ROMs.
    • CD-R (Compact Disk - Recordable) → The user can write data once on the CD-R. It cannot be deleted or modified later.
    • CD-RW (Compact Disk - Rewritable) → Data can repeatedly be written and deleted on these optical disks.

DVD Drive

  • DVD stands for digit video dispay.
  • DVD is an opitcal device that can store 15 times the data held by CDs.
  • They are usually used to store rich multimedia files that need storage capacity.
  • DVDs also come in three varieties → read-only, recordable and rewritable.

Blu Ray Disk

  • Blu Ray Disk (BD) is an optical storage media that stores high definition (HD) video and another multimedia files.
  • DB uses a shorter wavelength lase than CD/DVD, enabling the writing arm to focus more tightly on the disk and pack in more data.
  • BDs can store up to 128 GB of data.

3. Memory Storage Devices

  • A memory device contains trillions of interconnected memory cells that store data.
  • When switched on or off, these cells hold millions of transistors representing 1s and 0s in binary code, allowing a computer to read and write information.
  • It includes USB drives, flash memory devices, SD and memory cards, which you'll recognize as the storage medium used in digital cameras.

Flash Drives

  • A flash drive is a small, ultra-portable storage device.
  • USB flash drives were essential for easily moveing files from one device to another.
  • Flash drives connect to computers and other devices via a built-in USB Type-Aor USB-C plug, making one a USB device and cable combination.
  • Flash drives are often referred to as pen drives, thumb drives, or jump drives.
  • The term USB drive and solid-state drive (SSD) are also sometimes used, but most of the time , those refer to larger, not-so-mobile USB-based storage devices like external hard drives.
  • These days, a USB flash drive can hold up to 2 TB of storage.
  • They're more expensive per gigabyte then an external hard drive, but they have prevailed as simple, convenient solution for storing and transferring smaller files.
  • Pen drive has the following advantages in computer organization, such as:
    • Transfer Files: A pen drive is a device plugged into a USB port of the system that is used to transfer files, documents, and photos to a PC and vice versa.
    • Portability: The lightweight nature and smaller size of a pen drive make it possible to carry it from place to place, making data transportation an easier task.
    • Backup Storage:Most of the pen drives now come with the feature of having password encryption, important information related to family, medical records, and photos can be stored on them as a backup.
    • Transport Data: Professionals or Students can now easily transport large data files and video, audio lectures on a pen drive and access them from anywhere. Independent PC technicians can store work-related utility tools, various programs, and files on a high-speed 64 GB pen drive and move from one site to another.

Memory Card

  • A memory card or memory cartridge is an electronic data storage device used for storing digital information, typically using flash memory.
  • These are commonly used in portable electronic devices, such as digital cameras, mobile phones, laptop computers, tablets, PDAs, portable media players, video game consoles, synthesizers, electronic keyboards and digital pianos, and allow adding memory to such devices without compromising ergonomy, as the card is usually contained within the device rather than protruding like USB flash drives.

Difference between Primary and Secondary Memory

Primary Memory

Secondary Memory

Primary memory is directly accessed by the Central Processing Unit (CPU).

Secondary memory is not accessed directly by the CPU. Instead, data accessed from a secondary memory is first loaded into RAM and then sent to the Processing Unit.

RAM provides a much faster-accessing speed to data then secondary memory. Computers can quickly process data by loading software programs and required files into primary memory (RAM).

Secondary memory is slower in data accessing. Typically primary memory is six times faster than secondary memory.

Primary memory is volatile and gets completely erased when a computer is shut down.

Secondary memory provides a feature of being non-volatile, which means it can hold on to its data with or without an electrical power supply.

References

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Secondary Storage Devices