cdrom

CD-ROM stands for compact disc read-only memory, an optical disc storage format designed to store digital data and read by a CD-ROM drive. It uses the same 120 mm disc as audio CDs, with a polycarbonate substrate, a reflective metallic layer, and a protective lacquer. Data are encoded as pits and lands and read by a laser. A standard disc holds about 650 to 700 MB of data, and many discs are capable of roughly 74 minutes of audio, depending on encoding. Data disks typically organize data in 2048-byte sectors.

File systems and formats. The most common filesystem for CD-ROM data is ISO 9660, intended for cross-platform compatibility. To accommodate longer filenames and more flexible directory structures, extensions such as Joliet and Rock Ridge are often used, especially on systems that require Unix-like permissions or Unicode filenames.

Speeds and usage. CD-ROMs are described by read speeds, written as multiples of 1x, with 1x corresponding to about 150 KB/s. Drives have advanced to higher speeds over time, improving access times and throughput. CD-ROM media are read-only memory; they are manufactured with the data encoded and cannot be rewritten. Related formats such as CD-R and CD-RW exist for write-once and rewriteable storage, but they are distinct from standard CD-ROMs.

Applications and history. CD-ROMs were widely used to distribute software, multimedia titles, games, encyclopedias, drivers, and large datasets, especially during the 1990s and early 2000s. They emerged from the same optical disc technology as audio CDs and were standardized in the Yellow Book and ISO 9660 standards. Over time, DVDs and digital downloads have supplanted many CD-ROM use cases, though the format remains important for archival storage and legacy software.