Harvard Business School in one of the classes. Bricklin was referring to the variety of report generators that were in use at that time, including Business Planning Language (BPL) from International Timesharing Corporation (ITS) and Foresight from Foresight Systems. However, these earlier timesharing programs were not completely interactive, and they pre-dated personal computers. Frankston described VisiCalc as a "magic sheet of paper that can perform calculations and recalculations", which "allows the user to just solve the problem using familiar tools and concepts". The Personal Software company began selling VisiCalc in mid-1979 for under $100, after a demonstration at the fourth West Coast Computer Faire and an official launch on June 4 at the National Computer Conference. It required an Apple II with 32K of random-access memory (RAM), and supported saving files to magnetic tape cassette or to the Apple Disk II floppy disk system. 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. Bricklin and Frankston originally intended to fit the program into 16k memory, but they later realized that the program needed at least 32k. Even 32k was too small to support some features that the creators wanted to include, such as a split text/graphics screen. However, Apple eventually began shipping all Apple IIs with 48k memory following a drop in RAM prices, which enabled the developers to include more features. The initial