GD-ROM (an abbreviation of "giga disk read-only memory") is the proprietary optical disc format used by the Sega Dreamcast. It is similar to the standard CD-ROM except that the pits on the disc are packed more closely together, resulting in a higher storage capacity: around 1.2 gigabytes, which is almost double the storage capacity of a typical CD-ROM. The format was developed for Sega by Yamaha.
GD-ROM was also made available as an upgrade for the Dreamcast's arcade cousin, the Sega NAOMI, providing alternate media to its cartridge-based software. It is also used for the Sega Chihiro system board and can also be used by the Sega/Nintendo/Namco Triforce system board.
Regions of a GD-ROM
There are 3 data areas on a GD-ROM disc. The first is in conventional CD format, and usually contains an audio track with a warning that the disc is for use on a Dreamcast, not an ordinary CD player. This audio track often uses the game's character's voices in a lighthearted or humorous message (for example, Skies of Arcadia gives the message "We can't save the world from a CD player, so put us back in the Dreamcast so we can do our job!"). 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 (for example, Sonic Adventure contains images of Sonic characters to use on the desktop). There then follows a separator track which contains no data except for the text Produced by or under license from SEGA Enterprises LTD Trademark SEGA. The final (outer) section of the disc contains the game data itself in a higher density format. This section is 112 minutes long, with a data size of 1.2 GiB.
A normal CD-reader will not read beyond the first track because, according to the CD table of contents (TOC), 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.
The most popular way to access data from GD-ROMs, however, is to use the Dreamcast itself as a drive, and copy the data to a computer by means of a "coder's cable" or a Dreamcast Broadband Adapter.
The GD-ROM in the Dreamcast works in CAV (Constant Angular Velocity) mode, different from a common CD-ROM drive, which spins the disc in CLV (Constant Linear Velocity) mode. 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.
The NetBSD project has developed a GDRom driver for netBSD. A port of that driver for Linux exists, though due to licencing issues and the poor compatibility of that driver with Linux kernel interfaces, a new Linux driver is under development.