Thanks to virtualization, running Microsoft Windows applications on Apple hardware has become a popular way for consumers to dump their PCs in favor of using a Mac. And much to the chagrin of Microsoft hardware partners, this is yet another reason for Apple's growing hardware success.
In contrast, Apple has only grudgingly allowed its own Mac OS X to run on a virtual machine, and even then they've been extremely restrictive with that policy.
If you remember the long, drawn-out discussions in the past, the policy on virtualizing Mac OS X was clear. Apple stated that the license only allowed you to install and use one copy of the Mac OS X software on a single Apple-labeled computer at a time. It was a one-license, one-machine type of end-user license agreement (EULA), which pretty much ruled the Mac OS X out as a guest operating system even though both VMware and Parallels said it was "possible" to virtualize the OS.
Fast forward to the end of 2007, when Apple eased its licensing restrictions a bit and began permitting limited virtualization of Mac OS X with the release of Mac OS X Snow Leopard Server. But that permission came with three major caveats: First, you could only run Mac OS X virtual machines on a single Apple-labeled computer. Second, you needed to acquire an individual and valid license from Apple for each of these other copies of Mac OS X running in a VM. And third, this only applied to the more expensive Snow Leopard Server version of the OS, leaving the more widely used client version out in the cold.
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