Free60 is the successor to the Xbox Linux Project that aims to put Linux, BSD, or Darwin on the Microsoft Xbox 360 using a software or hardware based "hack". The Xbox 360 uses hardware encryption and will not run unsigned code out of the box.
The project's goals were met in March 2007, when shortly after the announcement of a critical software vulnerability in the Xbox 360 Hypervisor a loader for Linux appeared, which allowed to run a Linux ported to the Xbox 360.
To date, there are two Linux kernel patchsets available, one for 2.6.21 and one for 2.6.24.3. Three main Linux distributions may be run on the Xbox 360; Gentoo, Debian and Ubuntu Linux, the last two of which are easily installable to the Xbox 360 hard drive by scripts provided by members of the Free60 project.
Execution Method
Since executable code on the Xbox 360 is digitally signed, and runs underneath a hypervisor, an exploit or hack is necessary in order to execute homebrew code. On the Xbox 360, the first exploit which enabled booting of unsigned code relied on a modified DVD-ROM drive firmware, a modified burned disc of the game King Kong (for Xbox 360), and the target console having either one of two vulnerable kernel revisions. Alternatively, a home-made cable may be used to dump and patch the bios with jumpers attached to the appropriate pin header on the motherboard of the console to patch the hypervisor and allows unsigned execution directly at boot, known as the "JTAG/SMC exploit", which was patched after June 2009 but replaced by "Reset Glitch Hack", in 2011, which is applicable on any kernel version on all but the latest revision of the Xbox 360 motherboard (most 360's produced from 2014 until end of production in 2015), one caveat is being a glitch and relies on timing, boot times may be unstable.
With the original King Kong exploit, the console must launch the modified game which will utilize the software vulnerability to load a small chunk of code that is included on the disc. This code may either initialize the serial port to allow upload of further code to the console, or eject the drive tray and prepare the console to load further code from a specified point on optical media that is then placed in the drive. The latter method involves the readcd binary, made specifically for this cause. The code that readcd executes in this instance is known as XeLL, short for Xenon Linux Loader. XeLL captures CPU threads and launches the Linux kernel from either network (tftp) or optical media as its only purpose, providing a flat device tree for the kernel. In the cases of released 'Live CDs', the readcd binary would typically be included on the modified game disc, which would eject the drive, and a CD containing the XeLL binary as well as the complete Linux kernel and filesystem would be inserted.
Limitations
The readcd method was initially restricted to only Hitachi branded DVD-ROM drives, but now also supports Samsung branded drives. All other commercially used drives are assumed unsupported, at this point in time the only other drives in use are manufactured by BenQ and Lite-ON. An older Kernel revision is required on the Xbox 360 itself, which may prove to be hard to find, since connecting to the Xbox Live service applies updates to the console, and many games include updates that must be applied before the game will run. This limitation is not as important as it once was, as it is now known to be possible to downgrade a Kernel greater than the last of the two exploitable Kernels by means of a timing attack. There are presently no audio drivers written to support the console's internal audio hardware, however a USB audio device can be used. Some codecs are incompatible with the current display driver, causing some videos to not play. In addition to this, framebuffer and cache issues mean that videos that do play are jumpy after the first few seconds when the cache is full.
Development
One of the main contributors to the Free60 project has developed a method of 3D graphics acceleration on the Xbox 360's GPU (codenamed Xenos) under Linux. This work has been encapsulated into an API for easier use. In order to achieve this acceleration, some data from the Xbox 360's flash needs to be uploaded to the Xenos GPU. This process may be automated to help ensure legality of any 3D graphics acceleration.