Secondary Storage

A. Introduction
   1. Memory vs. Storage
   2. Storage Terms: Kilobyte, Megabyte, Gigabyte, Terabyte
   3. Speed (Fast to Slow): Memory > Hard Disk > CD > Floppy Disk > Tape

B. Types of Secondary Storage

   1. Floppy disks
      a. Called floppies because the inside plastic disk is flexible
      b. 3.5 inch sizes (nowadays)
      c. Disk drives
         i. reads and writes floppies
         ii. floppies turn at 300 revs per minute
         iii. read and write heads located on access arm
         iv. seek and search
         v. drives A and B
      d. Parts of a floppy
         i. data access area under metal shutter
         ii. hub in center
         iii. write protect notch in corner
         iv. tracks  - concentric, not grooves, divided into sectors
         v. formatting (initializing) puts tracks and sectors on
         vi. capacity - HD (1.44 M), DD (720K)
         viii. File Allocation Table (FAT)
      e. Care and maintenance
         i. don't bend or put heavy objects on them (put in typewriter)
         ii. dont touch the disk itself
         iii. keep away from strong magnetic fields (motors, telephones)
         iv. keep away from heat and chemicals (alcohol and solvents)
   2. Hard Disks
      a. Internal/external/removable
      b. Metal platter(s) sealed inside a container
      c. Mechanism consists of a motor, access arm and read and write heads
      d. Advantages over floppies
         i. hold more data (120M = 150 3.5, DD floppies)
         ii. spin 10 times faster (quicker access)
      e. Disadvantages - not transportable
      f. Hard disk cartridges get around this disadvantage
      g. External hard drives may also be moved
      h. Disk packs
         i. platters aligned vertically with many access arms
         ii. used for large volume, multiple user storage
         iii. access time depends on
            A. seek time (getting the head over the right track)
            B. switching time (activating the read/write head)
            C. rotational delay time (time to rotate under the head)
            D. data transfer time (transfer from disk track to memory)
      i. Sealed disks prevent contamination and head crash
         i. read/write heads ride on a cushion of air 0.000001 inch thick
         ii. head crash happens when head contacts surface of disk
      j. Performance enhancements
         i. disk caching
              reading data from disk to memory before it is actually
              needed by CPU
         ii. data compression and decompression
         iii. redundant arrays of independent disks (RAIDs)
   3. Optical Disks (CDs)
      a. Can hold 650 M
      b. Laser alters plastic or metal surface to store data
      c. Laser scan read the alterations (indentations)
      d. usually read-only (CD-ROMs), though some CD-R (recordable once) and
         CD-RW (rewritable many times) becoming more popular
      e. also DVD
   4. Magnetic Tape
      a. Used primarily as backup storage
      b. Sequential access