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The GD-ROM (Gigabyte Disc Read Only Memory) is the Optical disc used by the Sega Dreamcast. GD-ROMs has the capacity of holding 1 GB of data, rather than 650-700 MB of a ordinary CD-ROM. Developed by Yamaha, the GD-ROM is a proprietary format which works by packing the pits on the disc closer together to store more data. Along with increased storage, the GD-ROMs provide a level of copy protection as it cannot be reproduced using a CD burner.
Regions of a GD-ROM
 A diagram of a GD-ROM disc, showing its distinct regions. There are three distinct regions when you look at the data (reflective) side of a GD-ROM disc.
- The low-density inner track (dark gray) is in the standard CD format, and contains up to 35 MB (4 minutes) of data. In most cases, this contains an audio track with a warning that the disc is for use on a Dreamcast, not an ordinary CD player. The CD section also contains a data segment, readable in PCs. Although most discs include only text files identifying the game, its copyright and bibliography, some contain bonus material for home computer users.
- The separator track (black) does not contain data and acts as a border. In this ring, the following text is stored.
- Produced by or under license from SEGA Enterprises LTD
- Trademark SEGA
- The outer track (light gray) contains about 1 GB (112 minutes) of data written in a high density format which cannot be accessed by normal CD-readers. This section contains the game data.
A normal CD-reader will not read beyond the first inner track because, according to the Table of contents (TOC) it reads, there is no data there. With modified firmware that looks for a second TOC in the high-density region it is possible to read data from the high-density region even on a normal CD-reader. One can also utilize a "swap-trick" by first letting the CD-reader read the TOC from a normal CD with a large track and then swapping that disc with a GD-ROM in a way that avoids alerting the CD-reader that a new disc has been inserted. It is then possible to read as much data from the high-density region as indicated by the TOC from the first disc.
Sega achieved the higher density by decreasing the speed of the disc to half and by letting the standard CD-ROM components read at the normal rate thus nearly doubling the disc's data density. This method allowed Sega to use cheaper off-the-shelf components when building the Dreamcast.
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