Mac OS X 10.6 no longer supports PowerPC Macs
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As you may know, Mac OS X 10.6, released in 2009, no longer supports Macs based on PowerPC chip. My Mac is the last PPC chip Mac bought in late 2005.
Note from Wikipedia Mac OS X Snow Leopard, quote:
Some ways of running 10.6 Snow Leopard on certain unsupported hardware have been discovered. Users who have access to supported hardware have installed Snow Leopard on the supported machine then simply moved the hard drive to the unsupported machine. Alternatively, the Snow Leopard Installation DVD can be booted on a supported Mac, then installed on an unsupported Mac via the Firewire Target Disk Mode.
So, the decision of this must went something like this: Let's get some engineers to put in code so that old mac owner's can't upgrade. It's time for them to buy a new Mac!
See also: Switching from Mac/Unix To PC/Windows.