1.2 - Memory and storage

• The need for primary storage

  • A computer system needs primary storage for any data that it needs to access quickly
  • This includes the start-up instructions, the operating system, programs that are running and any associated data
• Random access memory (RAM)

  • Part of the main memory in a computer system
  • Holds the operating system, applications that are running and any associated data while the computer is on and in use
• How data gets into RAM

  1. When a program is loaded, it is copied from secondary storage, such as a hard disk drive (HDD), into RAM

  2. Any data associated with the program will also be stored in RAM so that the CPU can access both the data and instructions
• More RAM available


  • More data and applications can be stored in it
  • Better performance of the system
• Read-only memory (ROM)


  • Stores the instructions and data needed to get the system up and running and ready to load the operating system from secondary storage
  • This special program stored on ROM is called the Bootstrap Loader and we say the process 'boots' the computer
• Differences between RAM and ROM

  • RAM is volatile and needs power to maintain its content
  • RAM is read and write - data can be read from and written to RAM by the computer
  • ROM is non-volatile and does not require power to maintain the content
  • ROM is read-only - the computer cannot overwrite its content
• Virtual memory


  Area of secondary storage used to temporarily store data from RAM when there is insufficient RAM to hold all the data and instructions needed
• More RAM installed


  • Less need for virtual memory
  • Improved performance
• The need for secondary storage

  • To store the operating system, data, images, programs, documents and various other files so they are available when the computer is switched on
• Factors affecting choice of secondary storage

  • Capacity
  • Speed
  • Portability
  • Durability
  • Reliability
  • Cost
• Magnetic storage


  • Uses the principle of magnetism to store data
  • Hard disk drives (HDDs) are the most common type

  
• Advantages of HDDs


  • Reliable and cost-effective
  • High capacity at low cost
• Disadvantages of HDDs


  • Less portable than solid-state drives or optical disks
  • Subject to damage if dropped or brought near to strong electric or magnetic fields
• Solid-state storage


  • Uses flash memory technology
  • Very fast data access times compared to HDDs
  • Relatively expensive compared to HDDs
  • Typically has lower capacity than HDDs

  
• Advantages of SSDs


  • Lower latency times because there are no moving parts
  • Lower power requirements and do not generate heat or noise
  • Not susceptible to problems caused by sudden movements
  • Lighter, smaller and thinner than HDDs
• Combining SSD and HDD

  • Frequently accessed data stored on SSD
  • Large, less frequently required data stored on HDD
  • Provides speed advantage of SSD with capacity advantage of HDD at reasonable cost
• Optical storage


  • Data stored using the properties of light
  • Typical media include CDs, DVDs and Blu-Ray disks
• Types of optical storage

  • CD/DVD-ROM: Read-only
  • CD/DVD-R: Recordable
  • CD/DVD-RW: Rewritable
• Flash memory


  A method of storing data that is based on electronics

  
• Latency


  A delay before data can be transferred
• Fragmented data


  Data stored in different physical locations across the disk
• Optical storage


  • Data can be stored by using the properties of light
  • Typical optical storage media include CDs, DVDs and Blu-Ray disks
  • They are written to and read from using laser light
• Types of optical storage media

  • CD/DVD-ROM: Read-only device with the data stored at the manufacturing stage
  • CD/DVD-R: Recordable media that can be written to once using a suitable drive
  • CD/DVD-RW: Rewriteable media that can be written to several times
• CD


  • Holds around 700 MB of data and costs pennies
  • Used to distribute data and programs or make semi-permanent copies of data
• DVD


  • Has a larger capacity of 4.7–8.5 GB
  • Can store more data than CDs, such as standard resolution movies
  • Has a faster access time than a CD and costs a little more, but is still only pennies
• Blu-Ray


  • Has significantly larger capacity (25–50 GB) and much faster access speeds
  • Can be used to store large amounts of data and the much higher access speed makes them particularly suitable for high-resolution movies and console games
  • Slightly more expensive than DVDs but still reasonably inexpensive
• Comparison of optical devices

  • CD: Typical cost 18p, Capacity 700 MB
  • DVD: Typical cost 60p-80p, Capacity 4.7 GB single layer, 8.5 GB dual layer
  • Blu-Ray: Typical cost £1.80-£3.00, Capacity 25 GB single layer, 50 GB dual layer
• Factors to consider when choosing secondary storage media

  • Capacity: How much data does it need to store?
  • Speed: How quickly does the data need to be accessed?
  • Portability: Does the device or medium need to be transported?
  • Durability: Will the device or medium be used in a hostile environment?
  • Reliability: Does it need to be used repeatedly without failing?
  • Cost: What is the cost per unit of storage related to the value of the data?
• Capacity and cost of storage media

  • Magnetic hard disk: Up to 15 TB, Typical cost 3p per GB
  • SSD: 250 MB up to 2 TB, Typical cost 15p per GB
  • DVD: 8.5 GB, Typical cost 9p per GB
  • Blu-Ray: 50 GB, Typical cost 6p per GB
  • CD: 700 MB, Typical cost 23p per GB
• Access speeds of storage media

  • SSD: 200-550 MB/s
  • Magnetic hard disk: 50-120 MB/s
  • Blu-Ray disk: 72 MB/s
  • DVD: 1.32 MB/s
  • CD: 0.146 MB/s

  
• Portability, durability and reliability of storage media

  • SSD: Small, low power, very portable, not subject to damage from sudden shocks, reliable for a very long time
  • Magnetic hard disk: Higher power, subject to damage from drops/magnetic fields, reliable for 5-7 years
  • CD/DVD/Blu-Ray: Light, small, portable, reasonably robust but easily damaged, CDs/DVDs fail after 10 years, Blu-Ray after 20 years
• Bit


  One binary digit, the smallest unit of data storage
• Byte


  8 bits grouped together, a unit of data storage
• Nibble


  4 bits grouped together
• Data storage units


  • 1 bit (b)
  • 1 byte (B) = 8 bits
  • 1 kilobyte (KB) = 1000 bytes
  • 1 megabyte (MB) = 1000 kilobytes
  • 1 gigabyte (GB) = 1000 megabytes
  • 1 terabyte (TB) = 1000 gigabytes
  • 1 petabyte (PB) = 1000 terabytes
• Binary number system


  A number system that only uses the two values 1 and 0 to represent the on/off status of electronic switches
• Binary place values


  • 128
  • 64
  • 32
  • 16
  • 8
  • 4
  • 2
  • 1
• Most significant bit (MSB)


  The leftmost digit in a binary number, with the highest place value
• Least significant bit (LSB)


  The rightmost digit in a binary number, with the lowest place value
• Computers represent all data (numbers, characters, sounds and images) in binary