Computer Data Storage

 Memory

• Memory is also referred to as primary storage or volatile storage; its contents are lost when the computer is shut down; very fast access time (nanoseconds)
• Memory is managed in units (cells)—each 8 bits (1 byte)
• Typically measured in units of 2²⁰ bytes or megabytes (1,048,576 bytes is 1 MB); this is not the same as 1,000,000 bytes
• Each cell has an address—numbered from 0 to the number of bytes of memory in the computer
• A byte is the smallest addressable unit

 Mass Storage

• Also referred to as secondary storage; its contents remain in tact after the computer is shut down; access is slower than memory (milliseconds)
• Magnetic disks; hard drives, floppy drives
• Data is stored in sectors which are stored on tracks; concentric rings on the surface of a disk
• Slower because they are mechanical (i.e. moving parts); seek time, rotational delay (latency), transfer time
• Although disks are faster and (cheaper!) than a few years ago, their progress has lagged behind processor speeds
• Tape drives are slower still; sequential access; immense storage capabilities; cheap

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