Last updated: 4/23/03
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Computer Storage and Connections

  1. Computer Storage - Secondary Storage
    1. Background
      1. Disks are divided into tracks (concentric circles like the rings in a slice of onion) and sectors (wedges like pie slices). The Figure 4-17 on Pg. 4-18 in Computers, Technology, and Society illustrates this. Note in the Figure that "sector" can sometimes refer to the whole pie slice, and sometimes to just one section of a track. Further, on large disks such as hard drives, sectors are grouped into "clusters." On smaller drives, such as floppy diskettes, a cluster may be the same as a sector. Here is the point: once any data is written to a sector, the sector is not used for any other data, even if we only store a single bit in the cluster. So large clusters can be inefficient in using disk storage space. On the other hand, hard drive capacity is so cheap today that this is not a big concern, although it used to be.
      2. A file is spread out over many sectors, possibly in a random pattern A disk that has seen a lot of activity in erasing and storing files will have many files, each spread out of a wide range of sectors. This slows down reading the files, since the read head must frequently be moved to another track, which takes a relatively long time. This is one reason why computers can seem to slow down over time. Such a disk should be "defragmented" to recover the lost speed.
      3. The capacity in bytes of a secondary storage device is the number of clusters times the size of a single cluster, in bytes. Note that if clusters are used inefficiently (many small files), the actual capacity will be less.
      4. The File Allocation Table (FAT) or its equivalent, keeps track of which clusters are in use. The Directory file contains the file name and starting cluster, and each entry in the FAT tells what the next cluster in the file is. The scheme is illustrated on Pg . 4-28 of  Computers, Technology, and Society. The FAT can take up almost one-third of the gross disk capacity. The unformatted capacity of a floppy diskette, without the Directory and FAT, is 2 MB, which the formatted capacity, with the Directory and FAT, is 1.44 MB, a decrease of 28%.
      5. A CD-ROM has a single spiral track divided into sectors.
    2. Currently Popular Storage Devices
      Type Capacity
      Floppy 1.44 MB
      Hard drive Increasing steadily, currently 300 GB max
      Zip Drive 100 MB, 250 MB and new 750 MB versions
      CD-ROM, CD-R and CD-RW 640 MB to 690 MB. Various formats.
      DVD-ROM, DVD-R and DVD-RW Normally 4.7 GB, some smaller formats (over 7 times CD-ROM)
      Tape drives (used for backup) Various, up to hard drive sizes
      RAM Drives Up to 1 GB removeable
      Removable hard drives various sizes
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  2. Computer Connections
    1. Background. Computer data flow can be either serial (one wire, bits sent one by one) or parallel (as many wires as bits in a unit (8 bits for a byte, for example), all bits sent at the same time). The printer port is parallel; the serial port is serial. Ethernet (networking) and optical fiber are serial. Obviously, for the same type of wiring and circuits, parallel communication is faster than serial. However, serial communication wires and wiring are much simpler, and with modern technology they are fast enough that they are very popular.
    2. The new Universal Serial Bus (USB) is fast and convenient; external devices such as digital cameras or removable hard drives can be powered from it, it detects new devices on its own and installs their driver software, and up to 127 separate daisy-chained devices are supported.
    3. Many devices formerly connected by serial or parallel ports, and many removable external storage devices, are changing to USB.
    4. Currently Popular Computer Connections (Mbps = Mega Bits Per Second, MB would mean Mega Bytes)
      Standard Type Speed Other
      Printer Parallel 2 MBps Fast in its day, and still fast enough for printers
      Serial (Com) Serial To 115,000 bps  
      USB 1.0 Serial 12 Mbps Not compatible with other USB. This version is rare; it was upgraded to 1.1 almost immediately.
      USB 1.1 Serial 12 Mbps Compatible with all higher versions, but at its own speed (can seem slow connected to a large USB 2.0 storage device)
      USB 2.0 (2000) Serial 480 Mbps Compatible with USB 1.1 and any higher versions, but at its own speed.
      Firewire (IEEE 1394) Serial 400 Mbps Often faster than USB 2.0 in actual use. Common in Apple computers, also for digital camcorders. Not compatible with USB.
      Ethernet Serial 10 Mbps or 100 Mbps (can also use optical fiber). Popular for networking
      InfraRed (IrDA) Serial 4Mbps Wireless connection
      Bluetooth Serial 723 Kbps Wireless via radio, speeds are being increased