{{Rewrite, date=June 2022 XPostFacto is an

open source Open source is source code that is made freely available for possible modification and redistribution. Products include permission to use the source code, design documents, or content of the product. The open-source model is a decentralized sof ...

utility that enables the installation of

PowerPC PowerPC (with the backronym Performance Optimization With Enhanced RISC – Performance Computing, sometimes abbreviated as PPC) is a reduced instruction set computer (RISC) instruction set architecture (ISA) created by the 1991 Apple– IBM� ...

versions of

Mac OS X macOS (; previously OS X and originally Mac OS X) is a Unix operating system developed and marketed by Apple Inc. since 2001. It is the primary operating system for Apple's Mac (computer), Mac computers. Within the market of ...

up to Mac OS X v10.4 (Tiger), and Darwin on some PowerPC-based

Apple Macintosh The Mac (known as Macintosh until 1999) is a family of personal computers designed and marketed by Apple Inc. Macs are known for their ease of use and minimalist designs, and are popular among students, creative professionals, and software en ...

systems that are not officially supported for them by Apple. XPostFacto - often referred to as "XPF" - runs under

Mac OS 9 Mac OS 9 is the ninth major release of Apple's classic Mac OS operating system which was succeeded by Mac OS X (renamed to OS X in 2011 and macOS in 2016) in 2001. Introduced on October 23, 1999, it was promoted by Apple as "The Best Internet O ...

and allows an unmodified Mac OS X installation disc to be launched on machines 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 System 7, codenamed "Big Bang", and also known as Mac OS 7, is a graphical user interface-based operating system for Macintosh computers and is part of the classic Mac OS series of operating systems. It was introduced on May 13, 1991, by Apple Co ...

Mac OS 8 Mac OS 8 is an operating system that was released by Apple Computer on July 26, 1997. It includes the largest overhaul of the classic Mac OS experience since the release of System 7, approximately six years before. It places a greater emphasis on ...

, albeit sometimes with incomplete functionality. For example, it can be used to install Mac OS X v10.2 on a PowerPC 603 and 604 Mac like the 604 equipped

Power Macintosh The Power Macintosh, later Power Mac, is a family of personal computers designed, manufactured, and sold by Apple Computer as the core of the Macintosh brand from March 1994 until August 2006. Described by ''MacWorld'' as "the most important t ...

8600 or 9600 (Though 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). It also allows more recent versions of Mac OS X to be installed on older G3 Macs which can only officially run earlier versions of OS X - for example, it allows Mac OS X v10.3 and

Mac OS X v10.4 Mac OS X Tiger (version 10.4) is the 5th major release of macOS, Apple's desktop and server operating system for Mac computers. Tiger was released to the public on April 29, 2005 for US$129.95 as the successor to Mac OS X 10.3 Panther. Some ...

to run successfully on

Beige G3 The Power Macintosh G3 (also sold with additional software as the Macintosh Server G3) is a series of personal computers designed, manufactured, and sold by Apple Computer from November 1997 to August 1999. It represented Apple's first step tow ...

s, 10.2 having been the last version supported by Apple. It can even facilitate otherwise-awkward installations on supported machines; for instance, it allows

, which shipped on

DVD The DVD (common abbreviation for Digital Video Disc or Digital Versatile Disc) is a digital optical disc data storage format. It was invented and developed in 1995 and first released on November 1, 1996, in Japan. The medium can store any kin ...

media, to be installed onto a

Blue and White G3 The Power Macintosh G3 (also sold with additional software as the Macintosh Server G3) is a series of personal computers designed, manufactured, and sold by Apple Computer from November 1997 to August 1999. It represented Apple's first step tow ...

with only an external, non-bootable DVD drive. Likewise, Mac OS X 10.4 Tiger ended support for machines that lacked built-in

FireWire IEEE 1394 is an interface standard for a serial bus for high-speed communications and isochronous real-time data transfer. It was developed in the late 1980s and early 1990s by Apple in cooperation with a number of companies, primarily Sony ...

ports, these being the original

iMac G3 The iMac G3, originally released as the iMac, is a series of Macintosh personal computers sold by Apple Computer from 1998 to 2003. The iMac was the first major new product release for Apple under Steve Jobs, Apple's interim CEO and cofounder ...

s and

iBook G3 iBook is a line of laptop computers designed, manufactured, and sold by Apple Computer from 1999 to 2006. The line targeted entry-level, consumer and education markets, with lower specifications and prices than the PowerBook, Apple's higher-en ...

s, but XPostFacto allows the use of 10.4 on them. The name is a

pun A pun, also known as paronomasia, is a form of word play that exploits multiple meanings of a term, or of similar-sounding words, for an intended humorous or rhetorical effect. These ambiguities can arise from the intentional use of homophonic ...

on '' ex post facto'', a

Latin Latin (, or , ) is a classical language belonging to the Italic branch of the Indo-European languages. Latin was originally a dialect spoken in the lower Tiber area (then known as Latium) around present-day Rome, but through the power of the ...

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. This software engineering feat by developer Ryan Rempel is made possible by the publication of the source code for Darwin, the open-source foundation of Mac OS X. XPF's functionality is achieved via three different routes. The program provides a replacement

boot loader A bootloader, also spelled as boot loader or called boot manager and bootstrap loader, is a computer program that is responsible for booting a computer. When a computer is turned off, its softwareincluding operating systems, application code, an ...

for Macs with

OldWorld Old World ROM computers are the Macintosh (Mac) models that use a Macintosh Toolbox read-only memory (ROM) chip, usually in a socket (but soldered to the motherboard in some models). All Macs prior to the iMac, the iBook, the Blue and White Power ...

firmware In computing, firmware is a specific class of computer software that provides the low-level control for a device's specific hardware. Firmware, such as the BIOS of a personal computer, may contain basic functions of a device, and may provide h ...

, provides a set of

kernel Kernel may refer to: Computing * Kernel (operating system), the central component of most operating systems * Kernel (image processing), a matrix used for image convolution * Compute kernel, in GPGPU programming * Kernel method, in machine learni ...

modules to allow the Mac OS X kernel to support various pieces of system hardware which are unsupported in the retail release, and includes a mechanism to transparently boot the kernel off a supported medium - termed a "helper drive", such as the internal hard disk - even when installing Mac OS X to or from a non-bootable volume, such as a

USB Universal Serial Bus (USB) is an industry standard that establishes specifications for cables, connectors and protocols for connection, communication and power supply ( interfacing) between computers, peripherals and other computers. A broad ...

optical drive In computing, an optical disc drive is a disc drive that uses laser light or electromagnetic waves within or near the visible light spectrum as part of the process of reading or writing data to or from optical discs. Some drives can only ...

or a

hard disk A hard disk drive (HDD), hard disk, hard drive, or fixed disk is an electro-mechanical data storage device that stores and retrieves digital data using magnetic storage with one or more rigid rapidly rotating platters coated with magneti ...

which is unsupported by the kernel or system firmware. XPF does not support all models of

. Only certain PCI-based models with

Open Firmware Open Firmware is a standard defining the interfaces of a computer firmware system, formerly endorsed by the Institute of Electrical and Electronics Engineers (IEEE). It originated at Sun Microsystems, where it was known as OpenBoot, and has bee ...

will work. It does not support

NuBus NuBus (pron. 'New Bus') is a 32-bit parallel computer bus, originally developed at MIT and standardized in 1987 as a part of the NuMachine workstation project. The first complete implementation of the NuBus was done by Western Digital fo ...

-based PowerMacs. Once Mac OS X has been installed, XPF also runs under that

operating system An operating system (OS) is system software that manages computer hardware, software resources, and provides common daemon (computing), services for computer programs. Time-sharing operating systems scheduler (computing), schedule tasks for ef ...

. On unsupported machines, XPF 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 like Mac OS 9. Although versions of XPF were available before the release of Mac OS X v10.2, v10.3 and v10.4, no version was available at the time of the release of v10.5, which XPF 4 does not support. XPostFacto is not required to run v10.5 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 with LeopardAssist, and once installed Leopard will run without problems or requiring XPostFacto. On Macs that have only PCI graphics though, such as the Beige G3, the version of XPostFacto intended for use with Mac 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 G3s), although it cannot be used to actually install the operating system as it does not recognise the 10.5 DVD (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).

External links

Official XPostFacto site
Free system software