How Does My Computer Read a CD of a Video Game?
When you put a video game CD in your computer, it uses a number of different processes to extract the information from the disc. However, to fully understand how your computer's disc drive reads the information on a CD, you have to first understand the components of a computer's drive, and the properties of the disc itself.
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The Disc
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The CD is made of a very thin layer of clear polycarbonate plastic. The disc is impressed with thousands of microscopic pits (called "bumps") that are arranged into a spiral ring all around the disc, which resemble the tracks on a phonograph record. The size and frequency of the bumps is what carries the disc's information in a binary code, where every bump is encoded as 0 and smooth portions of the disc as 1.
The Drive Motor
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The computer's drive motor spins the disc. Spinning the disc allows the laser to read the information along the disc's spiral without having to track in multiple directions.
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Tracking Mechanism
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The tracking mechanism controls the position of the laser. It slowly moves the laser away from the center of the disc so that it can stay aligned with the disc's spiral while it spins.
Laser
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Your computer uses a laser to extract the information from the disc. The small laser, which is housed underneath the disc, shines off the bumps on the CD. The goal is to reflect the laser off the bumps, and allow the pickup assembly to interpret the binary information encoded on the disc.
Laser Pickup Assembly
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The reflection of the laser changes when it hits a bump on the disc. This is how the information is transferred from the disc. The laser pickup assembly monitors the reflection of the laser off the disc, and identifies the information carried by the bumps. It transmits that signal into bits, which your computer can use to form bytes of information.
Application Program Interface
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Video game discs require more information than can be put on a normal CD. To solve this problem your computer uses a process called the Application Program Interface, or API, to extract information that is not written directly on the disc. Your computer's API contains information that is common to video games and other programs. Video games use a system of "standardized requests" to extract predefined information from the API, instead of having to store all of the information directly on the disc.
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References
- Photo Credit cd on cd image by Stephen Kirkby from Fotolia.com
