CD-ROM Sectors & Modes

Data stored on a CD-ROM disc are divided into sectors which are equivalent to the audio frames for a CD audio disc. At normal playback speed 75 sectors are read every second. For double speed CD-ROM drives this increases to 150 sectors per second and so on. Seek times, while the disc rotates to the required starting position, will also reduce as speeds increase.

Because CDs were designed primarily for audio, their use for computer data requires the addition of header data and error correction codes which are included in every sector. There are two different types of sectors defined in the CD-ROM specification, mode 1 and mode 2 (the latter being used for CD-ROM XA discs).

Mode 1 Sectors

Mode 1 sectors are intended for the storage of computer data. They are comprised of:

Header which consists of Minutes, Seconds, Sectors (frames) and Mode (= 1).
2048 bytes of user data.
Error detection and correction codes to give the CD- ROM data extra protection.
While CD audio players are able to conceal errors that are not correctable which may result from a scratched disc, CD-ROM
data (especially application code) needs to be error free.
Mode 1 sectors are the simplest type and are used for most CD-ROM based formats which follow Yellow Book guidelines.