CD-ROM

A CD-ROM (/ˌsiːdiːˈrɒm/, compact disc read-only memory) is a pre-pressed optical compact disc that contains data. Computers can read—but not write to or erase—CD-ROMs, i.e. it is a type of read-only memory. A CD-ROM (/ˌsiːdiːˈrɒm/, compact disc read-only memory) is a pre-pressed optical compact disc that contains data. Computers can read—but not write to or erase—CD-ROMs, i.e. it is a type of read-only memory. During the 1990s, CD-ROMs were popularly used to distribute software and data for computers and fourth generation video game consoles. Some CDs, called enhanced 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 ISO 9660 format PC CD-ROMs). The CD-ROM format was developed by Japanese company Denon in 1982. It was an extension of Compact Disc Digital Audio, and adapted the format to hold any form of digital data, with a storage capacity of 553 MiB. CD-ROM was then introduced by Denon and Sony at a Japanese computer show in 1984. The Yellow Book is the technical standard that defines the format of CD-ROMs. One of a set of color-bound books that contain the technical specifications for all CD formats, the Yellow Book, standardized by Sony and Philips in 1983, specifies a format for discs with a maximum capacity of 650 MiB. CD-ROMs 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 is 120 mm in diameter, though the smaller Mini CD standard with an 80 mm diameter, as well as shaped compact discs in numerous non-standard sizes and molds (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 is converted into binary data. Several formats are used for data stored on compact discs, known as the Rainbow Books. The Yellow Book, published in 1988, defines the specifications for CD-ROMs, standardized in 1989 as the ISO/IEC 10149 / ECMA-130 standard. The CD-ROM standard builds on top of the original Red Book CD-DA standard for CD audio. Other standards, such as the White Book for Video CDs, further define formats based on the CD-ROM specifications. The Yellow Book itself is not freely available, but the standards with the corresponding content can be downloaded for free from ISO or ECMA. There are several standards that define how to structure data files on a CD-ROM. ISO 9660 defines the standard file system for a CD-ROM. ISO 13490 is an improvement on this standard which adds support for non-sequential write-once and re-writeable discs such as CD-R and CD-RW, as well as multiple sessions. The ISO 13346 standard was designed to address most of the shortcomings of ISO 9660, and a subset of it evolved into the UDF format, which was adopted for DVDs. The bootable CD specification was issued in January 1995, to make a CD emulate a hard disk or floppy disk, and is called El Torito. Data stored on CD-ROMs follows the standard CD data encoding techniques described in the Red Book specification (originally defined for audio CD only). This includes cross-interleaved Reed–Solomon coding (CIRC), eight-to-fourteen modulation (EFM), and the use of pits and lands for coding the bits into the physical surface of the CD. The structures used to group data on a CD-ROM are also derived from the Red Book. Like audio CDs (CD-DA), a CD-ROM sector contains 2,352 bytes of user data, composed of 98 frames, each consisting of 33-bytes (24 bytes for the user data, 8 bytes for error correction, and 1 byte for the subcode). Unlike audio CDs, the data stored in these sectors corresponds to any type of digital data, not audio samples encoded according to the audio CD specification. To structure, address and protect this data, the CD-ROM standard further defines two sector modes, Mode 1 and Mode 2, which describe two different layouts for the data inside a sector. A track (a group of sectors) inside a CD-ROM only contains sectors in the same mode, but if multiple tracks are present in a CD-ROM, each track can have its sectors in a different mode from the rest of the tracks. They can also coexist with audio CD tracks as well, which is the case of mixed mode CDs.