of the PC platform allows players to edit or modify their games and distribute the results over the Internet as "mods". A healthy mod community greatly increases a game's longevity and the most popular mods have driven purchases of their parent game to record heights. It is common for professional developers to release the tools they use to create their games (and sometimes even source code) in order to encourage modding, but if a game is popular enough mods generally arise even without official support. Mods can compete with official downloadable content however, or even outright redistribute it, and their ability to extend the lifespan of a game can work against its developers' plans for regular sequels. As game technology has become more complex, it has also become harder to distribute development tools to the public. Modding has a different connotation on consoles which are typically restricted much more heavily. As publicly released development tools are rare, console mods usually refer to hardware alterations designed to remove restrictions. Dominant software Although the PC platform is almost completely decentralized at a hardware level, there are two dominant software forces: the Microsoft Windows operating system and the Steam distribution service. Microsoft introduced an operating environment named Windows on November 20, 1985, as an add-on to DOS in response to the growing interest in graphical user interfaces (GUIs). Microsoft Windows came to dominate the world's personal computer market with over 90% market share, overtaking Mac OS, which had been introduced in 1984. Valve does not release any sales figures on its Steam service, instead it only provides the data to companies with games on Steam, which they cannot release without permission due to signing a non-disclosure agreement with Valve. However, Stardock, the previous owner of competing platform Impulse, estimated that, as of 2009, Steam had a 70% share of the digital distribution market for video games. In early 2011, Forbes reported that Steam sales constituted 50–70% of the $4 billion market for downloaded PC games and that Steam offered game producers gross margins of 70% of purchase price, compared with 30% at retail. In 2011, Steam served over 780 petabytes of information, double what it had delivered in 2010. Digital distribution services PC games are sold predominantly through the Internet, with buyers downloading their new purchase directly to their computer. This approach allows smaller independent developers to compete with large publisher-backed games and