1.2 - Memory and Storage

• Primary Memory is the component of the computer that holds data, programs, and instructions that are currently in use
• Two types of primary memory:

  • RAM (Random Access Memory)
  • ROM (Read Only Memory)
• Random Access Memory:

  • Volatile - All data is lost when the computer is turned off
  • Used for the computer's working memory, holds instructions currently in use
  • Can be written to and read from
• Read Only Memory:

  • Non-volatile - Data is is kept when computer is switched off
  • Used to hold the Basic Input Output System (BIOS)
  • Used to hold the boot-up instructions
  • Used in embedded systems
  • Read only, so data cannot be written to it
• Main memory is needed in order to hold instructions, programs or data the computer needs while it is in operation.
• Data, instructions and programs are all copied from the hard disk to main memory as it would be too slow to access everything directly from the hard disk.
• For even faster access, cache is used to store frequently used instructions to boost performance.
• Differences between RAM and ROM:

  • RAM is volatile, ROM is non-volatile
  • RAM is read-write, ROM is read-only
• Virtual memory is the use of secondary storage as additional primary memory, it is used when there isn't enough space in RAM to hold all programs currently in use so that the computer doesn't crash
• A small section of the hard disk is dedicated to act as RAM:

  • When a program in virtual memory is needed, it is swapped with something else in RAM
  • This will cause the computer to slow down since access to the hard disk is much slower
• Secondary storage is non-volatile, long term storage used to keep programs and data indefinitely. It includes hard disks, USB flash drives and CDs
• Secondary storage is needed for the long term storage of files and data since it is non-volatile. External devices are portable and tend to have very large capacities
• Solid State Drives (SSD):

  • Require very little power and create very little heat
  • Suitable for laptops, tablets, mobile phones etc.
  • They are very thin and reliable due to no moving parts
  • Much faster than hard disk drives
• Hard Disk Drive (HDD):

  • Commonly found in desktop computers
  • Much slower than solid state drives
• 3 main types of Secondary Storage:

  • Optical Storage (CDs, DVDs)
  • Magnetic Storage (HDDs)
  • Solid State Storage (SSDs)
• Optical Characteristics:

  • Capacity: From 650MB (CDs) to 3.5GB (Blu-Ray Dual Layer)
  • Speed: Up to 50MB/s, limited due to the moving parts
  • Portability: Highly Portable and lightweight
  • Durability: Susceptible to scratches and will degrade overtime
  • Reliability: Good in the medium term
  • Cost: 50GB for 45p
• Magnetic Characteristics:

  • Capacity: Up to 16TB
  • Speed :Up to 200MB/s moving parts means it is slower compared to SSD
  • Portability: Internal drives aren't portable, External drives are similar size to a large smartphone
  • Durability: Good when not in use, Can be affected by magnetic fields and heat
  • Reliability: Very reliable
  • Cost: 8TB for £120
• Solid State Characteristics:

  • Capacity: Up to 4TB for an SSD, 256GB for a USB memory stick
  • Speed: Up to 3.5GB/s due to no moving parts
  • Portability: Flash drives and memory card are very portable, Internal SSDs are not intended to be portable, but are very lightweight to be used in laptops and tablets
  • Durability: Extremely durable
  • Reliability: Extremely reliable
  • Cost: 4TB for £400
• Examples of magnetic application:

  Database server in a busy office

  • HDD has very high capacity
  • Relatively inexpensive compared to SSDs
  • Fast, durable and reliable
• Examples of optical application:

  Event photographs sent by post to a company from a photographer

  • CD/DVD are very inexpensive
  • They are portable and lightweight so easy to post
  • Will only be used once
• Examples of solid state application:

  Regular transfer of files between home and work

  • USB flash drive has sufficient capacity and speed
  • Very portable, durable and reliable
  • Inexpensive
• Magnetic devices such as hard disk drives use magnetic fields to magnetise tiny individual sections of a metal spinning disk. Each tiny section represents one bit.

  A magnetised section represents a binary '1' and a demagnetised section represents a binary '0'. These sections are so tiny that disks can contain terabytes (TB) of data.
• For magnetic storage: As the disk is spinning, a read/write head moves across its surface. To write data, the head magnetises or demagnetises a section of the disk that is spinning under it. To read data, the head makes a note of whether the section is magnetised or not.
• Optical devices use a laser to scan the surface of a spinning disc made from metal and plastic. The disc surface is divided into tracks, with each track containing many flat areas and hollows. The flat areas are known as lands and the hollows as pits.
• In optical storage: When the laser shines on the disc surface, lands reflect the light back, whereas pits scatter the laser beam. A sensor looks for the reflected light. Reflected light (lands) represents a binary '1', and no reflection (pits) represents a binary '0'.
• Solid state devices use non-volatile random access memory (RAM) to store data indefinitely. They tend to have much faster access times than other types of device and, because they have no moving parts, are more durable.
• Since this type of memory is expensive, solid state devices tend to be smaller in capacity than other types. For example, a solid-state drive that holds 256 GB might be of a similar cost to a hard disk with several terabytes capacity.
• Units of data storage:

  • Bit
  • Nibble (4 Bits)
  • Byte (8 Bits)
  • Kilobyte (1,000 Bytes or 1 KB)
  • Megabyte (1,000 KB)
  • Gigabyte (1,000 MB)
  • Terabyte (1,000 GB)
  • Petabyte (1,000 TB)
• Analogue data is a real-life signal that can vary greatly in value. Examples include:

  • Sound waves
  • Pressure
  • Temperature
• Digital data is binary data which represents analogue data. Computers work with digital data.
• Analogue data must be converted to digital before a computer can use it. A device known as an analogue-to-digital convertor (ADC) is used to generate digital data from analogue signals.
• In the same way as an ADC, a digital signal can be converted back to an analogue signal using a digital-to-analogue convertor (DAC).
• sound file size = sample rate * duration (s) * bit depth
• image file size = colour depth * image height (px) * image width (px)
• text file size = bits per character * number of characters
• Data is stored in binary format because computers use on and off signals, binary has two states 0 and 1, the binary 0 and 1 state can therefore represent the on and off electrical signals.
• Binary has just two units, 0 and 1. The value of each binary place value is calculated by multiplying the previous place value by two. The first eight binary place values are:

  128, 64, 32, 16, 8, 4, 2, 1
• Binary addition rules:

  0 + 0 = 0

  1 + 0 = 1

  1 + 1 = 10 (carry 1)

  1 + 1 + 1 = 11 (carry 1)
• Overflow occurs when the result of a calculation requires more bits (place values) than are in the available range. (typically occurs when the sum of 2 numbers is more than 255)
• The most significant bit is the one furthest to the left (binary, denary or hexadecimal)

  The least significant bit is the one furthest to the right (binary, denary or hexadecimal)