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ROM drive
Read Only Memory Drive. CD-ROM (an initialism of “compact disc read-only memory”) is a pre-pressed compact disc that contains data accessible to, but not writable by, a computer. While the compact disc format was originally designed for music storage and playback, the 1985 “Yellow Book” standard developed by Sony and Philips adapted the format to hold any form of binary data.
CD-ROMs are popularly used to distribute computer software, including games and multimedia applications, though any data can be stored (up to the capacity limit of a disc). Some CDs hold both computer data and audio with the latter capable of being played on a CD player, while data (such as software or digital video) is only usable on a computer (such as PC CD-ROMs). These are called enhanced CDs.
Although many people use lowercase letters in this acronym, proper presentation is in all capital letters with a hyphen between CD and ROM. It was also suggested by some, especially soon after the technology was first released, that CD-ROM was an acronym for “Compact Disc read-only-media”, or that it was a more “correct” definition. This was not the intention of the original team who developed the CD-ROM, and common acceptance of the “memory” definition is now almost universal. This is probably in no small part due to the widespread use of other “ROM” acronyms such as Flash-ROMs and EEPROMs where “memory” is usually the correct term.
CD-ROM discs are identical in appearance to audio CDs, and data are stored and retrieved in a very similar manner (only differing from audio CDs in the standards used to store the data). Discs are made from a 1.2 mm thick disc of polycarbonate plastic, with a thin layer of aluminium to make a reflective surface. The most common size of CD-ROM disc is 120 mm in diameter, though the smaller Mini CD standard with an 80 mm diameter, as well as numerous non-standard sizes and shapes (e.g. business card-sized media) are also available. Data is stored on the disc as a series of microscopic indentations. A laser is shone onto the reflective surface of the disc to read the pattern of pits and lands (“pits”, with the gaps between them referred to as “lands”). Because the depth of the pits is approximately one-quarter to one-sixth of the wavelength of the laser light used to read the disc, the reflected beam’s phase is shifted in relation to the incoming beam, causing destructive interference and reducing the reflected beam’s intensity. This pattern of changing intensity of the reflected beam is converted into binary data.
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