Apple's Mountain Lion clocks 3 million downloads in four days

That makes it the fastest launch of an Apple operating system ever, the company says. It released Mountain Lion Wednesday.

Apple charges $20 for the software. That pays for downloads for all of a buyer's personal computers.

Apple also provides the OS for free to buyers who bought a Mac on or after June 11.

Mountain Lion brings features from the iPhone and iPad to the Mac. The enhancements include tight integration with Apple's online storage service, iCloud, and a "Notification Center" that shows incoming mail, calendar reminders and other events.