1.2

• RAM is much faster to access than secondary storage such as Hard Disk Drives (HDDs).
• RAM is much more expensive per unit capacity than secondary storage media.
• RAM is volatile. This means that it will lose its contents when power is lost (i.e. if the computer is switched off).
• Random Access means that any part of the memory can be accessed non-sequentially and as quickly as any other part of the memory.
• ROM is very fast to access.
• ROM is typically expensive, so tends to be only small.
• ROM is often made from flash memory. This means that its contents cannot be changed easily.
• ROM is non-volatile. This means that ROM keeps its contents when power is lost.
• Read-Only Memory (ROM) comes on a small, factory-made chip on the motherboard.
• The Basic Input Output System (BIOS) usually:

  1. Loads an operating system
  2. Checks the system for errors
• Flash memory is usually based on a technology called Electrically Erasable Programmable Read-Only Memory (EEPROM). EEPROM can be used to create many flash memory devices.
• SSDs are often used in home computers because of their performance. SSDs are a form of re-writable flash storage.
• SD cards are used to extend the capacity of devices such as:

  • Cameras.
  • Smartphones.
  • Tablets.
  • They provide high capacity for their physical size. This is useful in these smaller devices.
• Flash memory is significantly faster than many other storage media such as Hard Disk Drives (HDDs). This is partly because flash memory does not have any moving parts, and so we do not need to wait for parts to move before we can access the drive.
• Flash memory can only be rewritten a certain number of times before it starts to degrade. We may lose access to the drive once the flash memory has degraded past a point.
• Flash memory is non-volatile. This means that it will keep its contents once power is switched off.
• Flash memory is not easily damaged by shocks. Flash memory is appropriate in scenarios where it might be exposed to lots of vibrations.
• Virtual Memory is a technique where we use secondary storage as if it were RAM.
• Virtual memory is not always ideal since:

  • Virtual Memory can have negative impacts on performance.
  • Virtual Memory can have negative impacts on the health of Solid State Disks (SSDs).
  • We might have limited space in secondary storage.
• Paging


  • Memory can be split into sections called 'pages'.
  • Virtual Memory works by moving these pages between the Random Access Memory (RAM) and the secondary storage.
• Move pages to Hard Disk Drive (HDD)


  • If a page is not being used, then we might want to put it into Virtual Memory.
  • When a page is moved to the HDD and not being used, it is put into a special file called the pagefile.
  • Other processes can then use the memory that we just freed in the RAM.
• Move pages to RAM

  • When we want to use a page again, we first need to move it back to the RAM.
  • We do this by swapping the page that we want with one that is already in the RAM.
• The smallest unit of data that we can store is called a binary digit, or a bit for short. The value of a bit can be either a 0 or a 1.
• • Kilobyte (KB) (1000 bytes).
  • Megabyte (MB) (1000 KB).
  • Gigabyte (GB) (1000 MB).
  • Terabyte (TB) (1000 GB).
  • Petabyte (1,000 TB).
• Accessing data in Solid State Storage

  • Data can be accessed randomly. This means that any piece of data can be accessed in a constant amount of time.
  • This is achievable because there are no moving parts.
• Storing a bit


  • Solid state drives are made of flash memory.
  • Flash memory stores the value of each bit in a semi-conductor chip.
• Solid state drives (SSDs) are robust. They are not easily damaged by shocks because they don't have any moving parts.
• SSDs have a limited number of read/write cycles. After a certain number of re-writes they will stop working.
• Solid state media tends to have very fast read/write times. Some solid state media has slower access time. For example, USB sticks.
• Solid state media has a higher cost-per-GB than magnetic media.
• SSDs have a good capacity, but not on the scale of optical or magnetic media.
• Solid state media is very reliable and robust. Once the read/write cycles are used up, then the drive will fail.
• The most common type of magnetic storage is the Hard Disk Drive (HDD).
• How Magnetic Storage works

  • The value of each bit of data (0 or 1) is represented as a positively or negatively charged magnetic particle.
  • These particles are part of a magnetic disk called the platter.
• Accessing magnetic storage


  • Data could be stored anywhere on the disk, and so to read/write data we first need to find the location on the disk.
  • A magnetic read/write head is used to access the data while the platter spins.
• Reliability of Magnetic storage

  • The HDD has moving parts.
  • Although the HDD is generally reliable, any device with moving parts will be subject to wear-and-tear.
  • HDDs can also be damaged by shocks. This means they are not robust.
• Magnetic media is very low cost per unit capacity.

  This makes it an economical choice for storing large amounts of data.
• Magnetic media needs a high number of moving parts. The high number of moving parts make magnetic media slow to access.
• Magnetic media is reliable and tends to last for a long time. However magnetic media is not robust, and tends to break on impact or during vibrations.
• Magnetic media can store a large quantity of data in a compact way. This makes them a popular choice for archives as well as home computers.