Some CD-ROM drives include disk cache which is usually located on the drive itself. This is same concept as cache, except instead of working with RAM, this works with your CD-ROM drive.

Like cache, disk cache is special "fast RAM" that is set aside for data that the CPU uses the most. When the CPU needs data from the CD-ROM drive, it asks the disk cache first. If the disk cache has the information, the CPU gets what it needs quicker.

The disk cache reads more information from the CD-ROM drive than the CPU needs in the hopes of having it available when the CPU needs it. Most CD-ROM drives have 256K of disk cache. The faster drives usually come with more.