1.3 Storage

1.3 Storage

1. Secondary Storage

   • Permanently stores programs and data whilst they are not in use by the CPU. This is often provided by a Hard Disk Drive or Solid State Drive in a computer but can also take the form of removable media such as USB memory sticks and optical (CD/DVD) discs.
   • The need for secondary storage
     • Secondary storage is used for storing backups of data in case the originals are damaged or destroyed.
     • Secondary storage is used for permanently storing data and programs when they are not immediately needed by the computer.

2. Primary Storage

   • Made up of RAM and ROM, this stores programs and instructions for the CPU to process.

3. Storage capacity

   • The smallest unit of data that a computer can represent is a bit (b), it's value can be either 0 or 1.
   • Note that each unit is 1024 times larger than the next smallest unit and not 1000 times. This is because 1024 is more easily represented in binary than 1000.
   • A group of four bits is a nibble and a group of eight bits is a byte (B).
   • Because these units are very small, larger units are typically used to describe the size of data, including kilobytes (KB, 1KB = 1024 bytes), megabytes (MB, 1MB = 1024KB), gigabytes (GB, 1GB = 1024MB) and terabytes (1TB = 1024GB).

4. Solid State Drives (SSDs)

   • Solid State Drives are made of flash memory, contain no moving parts and offer a much faster transfer rate than magnetic HDDs (around 550MB/s compared to 120MB/s for HDDs).
   • Solid State Drives are replacing Hard Disk Drives as the main form of secondary storage in computer systems, particularly in laptops due to their excellent speed, robustness and portability.
   • Example
     • USB Flash Drives, Memory Sticks and SD cards
       • USB Flash Drives ("memory sticks") and SD cards are made of similar flash memory to Solid State Drives, though their transfer rates are a lot slower (typically around 6MB/sec to 30MB/s, but potentially up to 90MB/s).
       • They offer good capacity (typically around 16GB to 64GB) for a reasonable cost-per-GB, though not as cheap-per-GB as Hard Disk Drives or optical media.
       • Their good capacity and cost, combined with being extremely portable makes them suitable for transferring documents and making short-term backups of files.
       • Good quality USB Flash Drives are also quite robust, surviving well in a bag or on a keyring, but they can suffer from damage if they are removed from devices prematurely, making them less reliable.
   • Flash memory has a higher cost-per-GB than Magnetic Hard Disks and can only be re-written a limited number of times, meaning that SSDs will start to degrade sooner than a Hard Disk would typically wear out, reducing their durability.
   • Mobile devices such as smartphones and tablets use flash memory exclusively for their secondary storage.