"Compact Disc-Interactive" format began in 1984 (two years after the launch of Compact Disc) and it was first publicly announced by Philips and Sony – two of the largest electronics companies of the time – at Microsoft's CD-ROM Conference in Seattle in March 1986. Microsoft's CEO Bill Gates had no idea beforehand that the format was under development. The Green Book, formally known as the "CD-i Full Functional Specification", defined the format for interactive, multimedia compact discs designed for CD-i players. The Green Book specification also defines a whole hardware set built around the Motorola 68000 microprocessor family, and an operating system called CD-RTOS based on OS-9, a product of Microware. The standard was originally not freely available and had to be licensed from Philips. However, the 1994 version of the standard was eventually made available free by Philips. CD-i discs conform to the Red Book specification of audio CDs (CD-DA). Tracks on a CD-i's program area can be CD-DA tracks or CD-i tracks, but the first track must always be a CD-i track, and all CD-i tracks must be grouped together at the beginning of the area. CD-i tracks are structured according to the CD-ROM XA specification (using either Mode 2 Form 1 or Mode 2 Form 2 modes), and have different classes depending on their contents ("data", "video", "audio", "empty" and "message"). "Message" sectors contain audio data to warn users of CD players that the track they are trying to listen to is a CD-i track and not a CD-DA track. The CD-i specification also specifies a file system similar to (but not compatible with) ISO 9660 to be used on CD-i tracks, as well as certain specific files that are required to be present in a CD-i compatible disc. Compared to the Yellow Book (specification for CD-ROM), the Green Book CD-i standard solves synchronisation problems by interleaving audio and video information on a single track. The format quickly gained interest from large manufacturers, and received backing from many particularly Matsushita. Although a joint effort, Philips eventually took over the majority of CD-i development at the expense of Sony. Philips invested many millions in developing titles and players based on the CD-i specification. Initially branded "CD-I", the name was changed in 1991 to "CD-i" with a lowercase i. The CD-i Ready format is a type of bridge format, also designed by Philips, that defines discs compatible with CD