The CD-ROM, an abbreviation for “Compact Disk Read-Only-Memory”, is an optical data storage medium using the same physical format as the audio compact discs. Digital information is encoded at near-microscopic size, allowing a large amount of information to be stored. CDs record binary data as tiny pits (or non-pits) pressed into the lower surface of the plastic disc; a semiconductor laser beam in the player reads these. Most CDs can not be written with a laser, but CD-R discs have coloured dyes that can be “burned” (written to) once, and CD-RW (rewritable) discs contain phase-change material that can be written and overwritten several times.The standard CD-ROM can hold approximately 650-700 megabytes of data, although data compression technology allows larger capacities. The yellow-book standard for the CD-ROM was first established in 1985 by Sony and Philips.