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Computer Memory

Sikitaran Magar March 19, 2025

 

Computer Memory

Memory is the storage location in computer where data/instructions or information are stored  temporarily or permanently. Memory is useful for backup device. 
Function of memory
- Stores all the data and instructions required for processing
- Stores intermediate result of processing
- Stores the final results of processing before these results are released to an output.

Types of memory

  1. Primary Memory

    • Also known as main memory, internal memory
    • Memory that holdas data or instruction
    • Generally temporary in nature
    • Directly accessible by CPU

Technology

  • Magnetic Core memory
  • Bubble memory
  • Semiconductor memory

Semiconductor memory

    • Stored digital data
    • Each bits of binary data is stored in a tiny circuit called memory cell

  • Types of semiconductor memory

    • ROM

      • Read Only Memory
      • Data can only be read from the memory but cannot be modified
      • Non-volatile in nature because the data doesn't lose when power is turned off
      • ROM in computer contains BIOS
      • BIOS performs POST(Power On Self Test) operation 

    • Types of ROM

      • PROM

        • Programmable Read Only Memory
        • Developed by Wen Tsing Chow in 1956
        • Can be programmed only once at the time of manufacture
        • Also known as One Time Programmable (OTP) chip
        • Cannot be reprogrammed
        • Application of PROM
          • Radio-Frequency Identification(RFID) tags
          • HDMI
          • Early computer BIOS
          • Video games consoles
          • TV remote

      • EPROM

        • Erasable Programmable Read Only Memory
        • Developed by Dov Frohman in 1971
        • First EPROM: Intel 1702, 2048 bit
        • Program/data can be erased with the use of Ultra Violet Ray
        • Can be reprogrammed
        • Application of EPROM
          • Modem
          • Video card
          • Compact flash
          • Smart memory
          • Memory stick
          • Computer BIOS chip

      • EEPROM

        • Electrically Erasable Programmable Read Only Memory
        • Developed by George Perlegos in 1978
        • Program/data can be erased with high voltage power supply
        • Can be reprogrammed 
        • Erased and written at byte level
        • Most flexible types of ROM and commonly used for holding BIOS
        • Application of EEPROM
          • Computer BIOS chip
          • Microcontrollers for smart cards
          • Telecom, consumer, automatice and industrial applications
          • Wireless and portable market

      • EAPROM

        • Electrically Alterable Programmable Read Only Memory
        • Content of selected memory location can be changed with high voltage power supply
        • Can be reprogrammed

    • RAM

      • Random Access Memory
      • Volatile in nature, it means all data get lost if power supply is off
      • Read/write memory
      • Also called Workin memory or user memory
      • RAM is used to store
        • Instruction waiting to be executed
        • Instruction currently being executed
        • Data waiting for processing
        • Data currently being processed
        • Data result from processing

    • Types of RAM

      • SRAM

        • Static Random Access Memory
        • No need to refreshed frequentlly 
        • Faster than DRAM
        • Uses transistor to store information
        • Can store less data
        • More costlier
        • Used in cache memory

      • DRAM

        • Dynamic Random Access Memory
        • Needs to be refreshed
        • Slower than SRAM
        • Uses capacitor to store information
        • Can store more data
        • Less costlier
        • Used in computer as main memory

    2. Secondary Memory

    • Also known as auxiliary memory, external memory
    • Storage device
    • Memory that stores information permanently for future use
    • Permanent, non-volatile, stable, persistent
    • Not directly accessible by CPU
    • Slower and cheaper than primary memory 

Technology

  • Magnetic Technology: Magnetic Drum, Magnetic Tape, Floppy Disk, HDD, Zip disk
  • Optical Disk: CD, DVD, Blu-ray disc

Magnetic Tape

  • Sequential Access Memory
  • Coated with magnetic oxide
  • Stores analog and digital data/information
  • More reliable but slower read/write speed
  • Data Density or capacity is measured in BPI(Bits per Inch)

Floppy Disk

  • Also known as Diskette
  • Developed by IBM company
  • Information is stored in circular tracks
  • Common capacity used in floppy disk is 1.44 MB
  • Size of floppy disk: 8", 5.25", 3.5"

Size and type

Speed

Track

Sector

Capacity

5.25” (Double Density)

300 rpm

40

9

360 KB

5.25” (High Density)

360 rpm

80

15

1.2 MB

3.5” (Double Density)

300 rpm

80

9

720 KB

3.5” (High Density)

300 rpm

80

18

1.44 MB

3.5” (Extra HD)

300 rpm

80

36

2.88 MB

Hard Disk

  • Random Access Memory
  • Electro mechanical device
  • Metal disk which is coated in a magnetic material and encased in a hard disk drive
  • Divided into tracks, sectors and clusters

Physical structure of hard disk

    • Platters

      • A circular magnetic plate used to store data in harddisk
      • Harddisk contain several platters
      • Each platters has two working surface
      • Logical parts of platters: Tracks, Sector and clusters

                Logical part of platters

      • Tracks

        • Platters are divided into concentric circles called tracks.
        • Circular ring where data is stored
        • Used to store, organized data on the disk

      • Sectors

        • Sub division of tracks
        • The smallest addressable units on a hard disk
        • Data are read from and write to disk in sector sized chunks 

      • Clusters

        • Sets or groups of sectors
        • Size of cluster is depends on file system
        • Cluster size in NTFS is 4 KB
        • Cluster size in FAT is 32 KB

      • Cylinder

        • Set of matched tracks
        • Collection of all tracks that are in same distance from the edge of the platter
        • Sets of tracks that can be accessed simultaneously without moving RW head

    • Spindle

      • Parts of harddisk at the center of platters which provides support for disc
      • Rotates the platters and Read/Write head moves up and down to read or write data from desired sector of a cluster of a track

    • Read/Write head

      • Parts of harddisk that is responsible for reading and writing data/information to disk
      • Moves rapidly across the platter's surface to access data from disk

    • Read/Write Arm

      • Parts of harddisk that controls the movement of RW head

    • Actuator

      • Parts of harddisk that provides support for RW head

Factor affecting speed or performance of Harddisk

  • Seek time: Time taken by RW head to move from its current location to target track where data is stored
  • Latency: Time taken by RW head to position over data
  • Access time: Sum of Seek time and latency

CD(Compact Disc)

  • Random Access memory
  • Optical media 
  • Use laser technology
  • Invented by James Russell
  • Types
    • CD-R: Compact Disc Recordable, storage capacity upto 700 MB
    • CD-ROM: Readymade CD of movies, games, software,etc
    • CD-RW: Compact Disc Rewriteable
    • DVD: Digital Versatile/Video Disc, Storage capacity up to 4.7 GB
    • Blu-Ray Disc
      • One sided: Capacity up to 25 GB
      • Dual sided: capacity up to 50 GB


Types of Memory on different bases
1. Based on Data Access

  • Sequential Access: Data access time depends on the location of data in memory. Eg: Magnetic tape, cassette tape
  • Random Access: Data access time doesn't depend on the location of data in memory. Eg: HDD, floppy disk, CD, RAM, ROM, DVD, SSD, etc

2. Based on Technology

  • Semiconductor technology: RAM, ROM
  • Magnetic technology:  Hard disk drive, floppy disk
  • Optical technology: CD, DVD, Blu-Ray Disc
  • Magnetic Optical: MO disk
  • Paper technology: Punched card, paper tape

3. Based on handling

  • Fixed disk: Attached to computer. Eg: RAM, Harddisk
  • Removable disk: CD, pen drive, floppy disk, portable HDD

4. Based on volume of data

  • Limited storage: Holds limited data. Eg: floppy disk, smart card, etc
  • Mass storage: Holds large amound of data. Eg: Harddisk, magnetic tape, etc

Other memory

  • Flash memory

    • Solid state, non-volatile, permanent, function like RAM and harddisk
    • Used in cellular phone, camera, printer, audio recorder, etc
    • Introduced by Toshiba in 1984
    • Flasm memory was developed from EEPROM
    • Eg: pendrive, memory card, etc

  • Cache memory

    • Very high speed memory
    • Also called buffer memory
    • Placed between RAM and CPU to manage the speed  difference
    • Used to temporarily store frequently accessed data or information for faster access by CPU
    • Used to reduce average time to access data from the main memory
    • Cache Hit: when CPU request data from cache and data is successfully retrieved from cache, then it is cache hit
    • Cache Miss: An event in which a system or CPU request a data from a cache but that data is not currently in cache memory is cache miss.
    • Types or level of cache memory
      • L1(Level 1) cache
        • Smallest and fastest cache
        • Located closest or embedded to the CPU cores. 
        • Also known as primary cache or internal cache
        • Stores frequently used instruction or data accessed by CPU
        • Common size is upto 8 KB to 64 KB
      • L2(Level 2) cache
        • Larger but slightly slower than L1 cache
        • Located between L1 cache and RAM or very close to CPU chip
        • Serves as a backup to the L1 cache and holds more data
      • L3 (Level 3) cache
        • Largest cache memory 
        • Located on motherboard or separate from CPU chip
        • Shared among multiple CPU cores
        • Improve the overall performance of multi-core processors by providing a shared pool of cache memory

  • Virtual memory

    • Memory management technique where secondary memory can be used as extension of its main memory
    • Used when primary memory is not enough to perform certain task
    • Also called swap file or page file
    • Managed by operating system

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