Memory

Memory

1. Secondary

   • Optical storage
     • Capacity: CDs hold the least data (around 700MB), then DVD (4.7GB) and Blu Ray holds the most (around 50GB).
     • Each type of media (CD, DVD or Blu Ray) comes in different formats:
       • ROM (Read only memory) - Data is "pressed" onto the disc in factories and cannot be altered
       • R (Recordable) - Data can be "burned" to a disc once only
       • RW (Rewritable) - The contents of the disc can be re-written multiple times (though the whole disc is wiped each time, so not suitable for dragging and dropping files as with a memory stick)
     • • Optical discs are cheap and quick to produce in large numbers. Recordable optical media is cheap per GB although fewer devices have optical drives built in these days and DVDs and CDs have a smaller capacity than USB flash drives.
       • They are also small and very portable.
       • Optical discs offer faster access times than magnetic tape, but have slower transfer rates than Magnetic Hard Disk Drives and Flash Memory devices.
       • Optical discs are prone to scratching which can cause data loss and rewritable (RW) discs are particularly unreliable for long-term storage due to the stability of the chemicals that they are made from.
   • Magnetic storage
     • Magnetic Hard Disks offer a large capacity for a relatively low cost-per-GB
     • they provide reasonably fast transfer rates (around 120MB/sec)
     • Magnetic Hard Disks are generally reliable - they keep working for a long time, however they will eventually develop faults due to wear and tear of the moving parts
     • They are easily damaged if they are dropped or knocked when in operation, meaning that they are not very robust
     • The value of each bit of data (either 1 or 0) is represented as a positively or negatively charged magnetic particle on a magnetic disc (also called a platter)
     • A magnetic read/write head is used to read and write data whilst the platter spins, typically at 5400, 7200 or 10000 revolutions per minute
     • Uses devices such as Hard Disks, Magnetic Tapes, and floppy disks
       • External Hard Disk Drives are often used for home and enterprise backup as they offer high capacity and low cost-per-GB whilst being portable and relatively quick for transferring large amounts of data
       • Hard Disks are often used as the main form of secondary storage in a computer system to hold the Operating System, applications and user data
     • Magnetic tape
       • Magnetic tape comes in cassettes that contain a reel of magnetic tape.
       • Special tape drives are must be installed in a computer or server to read and write magnetic tapes.
       • Magnetic tape offers very high capacity, good durability and a very low cost per GB
       • ideally suited to long-term backup storage
       • Magnetic tapes can store over 300TB (307,200,000 GB) of data in a small, portable cartridge
       • designed to last in storage for up to 30 years.
       • binary data is stored as positively or negatively charged particles on the surface of a long tape stored within the cartridge.
       • The downside of Magnetic Tape is they have very slow access times compared to other forms of secondary storage, this is because Magnetic Tape uses serial access, where the right part of the tape has to be found before the desired data can be read
   • Solid State and Flash Memory
     • Solid State Drives
       • Solid State Drives are replacing Hard Disk Drives as the main form of secondary storage in computer systems due to their excellent speed, robustness and portability.
       • Solid State Drives are made of flash memory and contain no moving parts
       • Much faster transfer rate than magnetic HDDs (around 550MB/s compared to 120MB/s for HDDs)
       • Flash memory has a higher cost-per-GB than Magnetic Hard Disks
       • 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
     • USB Flash Drives, Memory Sticks and SD cards
       • USB Flash Drives (or memory sticks) and SD cards are made of similar flash memory to Solid State Drives,
       • their transfer rates are a lot slower than SSDs (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
       • 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
       • They can suffer from damage if they are removed from devices prematurely, this makes them less reliable

2. Primary