With the exception of "The Hitchhiker's Guide to the Galaxy" and "Shogun", the copyrights to the Infocom games are believed to be still held by Activision. "Dungeon", the mainframe precursor to the commercial Zork trilogy, is believed to be free for non-commercial use. but prohibited for commercial use. It was this copy that the popular Fortran mainframe version was based on. The C version was based on the Fortran version. and is available from The Interactive Fiction Archive as original FORTRAN source code, a Z-machine story file and as various native source ports. Many Infocom titles can be downloaded via the Internet, but only in violation of the copyright. Activision did at one point release the original trilogy for free-of-charge download as a promotion but prohibited redistribution and have since discontinued this. There are currently at least four Infocom sampler and demos available from the IF Archive as Z-machine story files which require a Z-machine interpreter to play. Interpreters are available for most computer platforms, the most widely used being the Frotz, Zip, and Nitfol interpreters.