Infocom's games' sales benefited significantly from the portability offered by running on top of a virtual machine. "InfoWorld" wrote in 1984 that "the company always sells games for computers you don't normally think of as game machines, such as the DEC Rainbow or the Texas Instruments Professional Computer. This is one of the key reasons for the continued success of old titles such as Zork." Dornbrook estimated that year that of the 1.8 million home computers in America, one half million homes had Infocom games ("all, if you count the pirated games"). Computer companies sent prototypes of new systems to encourage Infocom to port Z-machine to them; the virtual machine supported more than 20 different systems, including orphaned computers for which Infocom games were among the only commercial products. The company produced the only third-party games available for the Macintosh at launch, and Berlyn promised that all 13 of its games would be available for the Atari ST within one month of its release.