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XPostFacto (formerly Unsupported UtilityX) is a freeware utility created by Ryan Rempel that allows Mac OS X to be used on PowerPC-based Mac models which cannot boot Mac OS X unaided. This allows Mac OS X to be installed on certain Mac models which could otherwise only run System 7, Mac OS 8 or Mac OS 9, albeit sometimes with incomplete functionality.[1] It can be used to install later versions of Mac OS X on machines that Apple has declared unsupported, such as the early iMacs, beige Power Macintosh G3, the PowerBook G3 "Wallstreet" and "Lombard". XPostFacto can also be used to install Mac OS X on pre-G3 Macs like the 603 and 604 (though only up to 10.2 as Mac OS X 10.3 and newer will not run at all on a 603 or 604 processors unless the machines are upgraded to a G3 or G4 processor). [1]

Once Mac OS X has been installed, XPostFacto also runs under that operating system. On unsupported machines, XPostFacto should be used instead of Apple's "Startup Disk" control panel or system preference pane to select whether the Mac should boot into Mac OS X or an older version.

The XPostFacto name is a pun on ex post facto, a Latin phrase meaning "after the fact", commonly used in legal matters to refer to retroactive actions applying a later state of affairs (such as legislation) to earlier situations. In this context, it refers to the installation of software which did not exist yet at the time the hardware was manufactured: it retroactively "applies" Mac OS X to pre-existing hardware. Online retailer Other World Computing is a major supporter and contributor to XPostFacto. Its fourth major revision added support for installing Mac OS X 10.4 "Tiger." [2]

Although versions of XPostFacto were available before the release of Mac OS X 10.2, 10.3, and 10.4, no version was available at the time of the release of Mac OS X 10.5 "Leopard", which XPostFacto 4 does not support. XPostFacto is not required to run 10.5 on unsupported Macs as long as they have a G4 processor and AGP graphics, the official requirement of an 867 MHz G4 processor only needs to be bypassed at the time of installation and once installed Leopard will run without problems or requiring XPostFacto. On Macs that have only PCI graphics though, such as the Beige Power Macintosh G3, the version of XPostFacto intended for use with OS X 10.4 needs to be used in order to boot 10.5 successfully (provided that it has been upgraded to a G4 processor as Leopard will not run at all on a G3), although it cannot be used to actually install the operating system as it does not recognize the 10.5 DVD-ROM (another machine would have to be used for the installation and then either have the hard drive transferred into the unsupported machine or put the unsupported machine into Target Disk Mode to have the unsupported OS installed to it). [2]


  1. Unsupported UtilityX renamed XpOSfacto, updated by Peter Cohen, Macworld. 2002-01-24.
  2. XPostFacto 4.0 Adds Tiger to More Legacy Macs by Glenn Fleishman, TidBITS. 2005-09-19.

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