5.3 Non-von Neumann Architectures 131 Let’s look for a moment at the acronym CD. CD, of course, stands for compact disk—you probably have a collection of them with recorded music. A CD drive uses a laser to read information stored optically on a plastic disk. Rather than having concentric tracks, there is one track that spirals from the inside out. Like other disks, the track is broken into sectors. Unlike magnetic disks where the tracks near the center are more densely packed, a CD has the data evenly packed over the whole disk, thus more information is stored in the track on the outer edges and read in a single revolution. In order to make the transfer rate consistent throughout the disk, the rotation speed varies depending on the position of the laser beam. The other letters attached to CD refer to various properties of the disk, such as formatting, and whether or not the information on them can be changed. CD-DA is the format used in audio recordings: CD-DA stands for C ompact D
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