VisiCalc was unusually easy to use and came with excellent documentation; Apple's developer documentation cited the software as an example of one with a simple user interface. Observers immediately noticed its power. Ben Rosen speculated in July 1979, that "VisiCalc could someday become the software tail that wags (and sells) the personal computer dog". For the first 12 months, it was only available for Apple II, and it became that platform's killer app. The computer was sold, John Markoff wrote, as a "VisiCalc accessory"; many bought $2000 Apples to run the $100 software — more than 25% of those sold in 1979 were reportedly for VisiCalc — even if they already owned computers. Steve Wozniak said, that small businesses, not the hobbyists he and Steve Jobs had expected, purchased 90% of Apple IIs. Apple's rival Tandy Corporation used VisiCalc on Apple IIs at their headquarters. Other software supported its Data Interchange Format (DIF) to share data. One example was the Microsoft BASIC interpreter supplied with most microcomputers that ran VisiCalc. This allowed skilled BASIC programmers to add features, such as trigonometric functions, that VisiCalc lacked.