Secondary Storage & Primary Storage

Secondary Storage & Primary Storage

1. Magnetic Storage

   • These offer large storage with low cost per Gb and 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.
   • Quite portable but not as portable as other primary storage.
   • 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 (platter).

2. Flash Memory

   • 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).
   • 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.
   • 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.

3. Optical Storage

   • Optical storage technology uses laser light to retrieve data from the surface of optical media such as CDs and DVDs
   • Each 1 and 0 of the binary data is represented as microscopic holes (pits) on the surface of the disc.
   • A laser is then shone on the disc surface which either bounces back onto a sensor (if it hits a flat part of the surface) or it doesn't (if the laser hits a pit).
   • This "hit-or-miss" signal is used to reconstruct the 0s and 1s that make up the binary data.
   • CDs hold the least data (around 700MB), then DVD (4.7GB) and Blu Ray holds the most (around 50GB).
   • 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)

4. Primary Storage

   • There are two types of primary storage, or main memory: Random Access Memory (RAM) and Read Only Memory (ROM)
     • RAM
       • Random Access Memory (RAM) temporarily stores program instructions and data while they are being processed by the CPU.
         • RAM is volatile, meaning that its contents is lost when it loses power (i.e. the computer is switched off).
           • RAM is much faster than secondary storage devices such as Hard Disk Drives but much more expensive on a per-GB basis.
             • Because RAM is both volatile and expensive, general-purpose computer systems have secondary storage for long-term storage of programs and data.
     • ROM
       • Read Only Memory (ROM) permanently stores program instructions and data but it cannot have new data written to it.
       • ROM is non-volatile, meaning that its contents is retained when it loses power (i.e. the computer is switched off).
       • ROM is often made from flash memory cannot be altered without being "re-flashed". This means that it tends to contain small programs that get the computer up and running, leaving the big stuff to secondary storage.
       • ROM is typically used to store the boot-up sequence or Basic Input Output System (BIOS) that the CPU executes as soon as it turns on. The BIOS contains instructions that load the operating system from secondary storage into RAM for the computer to continue booting up.

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