The PA-RISC/Linux Project
What is it?
PA-RISC is the proprietary HP RISC processor architecture originally designed for HP-UX. Project to run Linux on these machines started in October 1998. Linux first booted on PA-RISC on 25 June 1999. The PA-RISC Linux Kernel is now integrated into 2.6 mainline (parisc git tree). 2.4 is nonetheless way out of sync with mainline (you need to use the CVS snapshot to build it). Two linux distributions now support PA-RISC: Debian (since woody 3.0, released) and Gentoo (experimental). PA-RISC linux is not officially supported by HP.
How does it work?
Palinux describes the project for getting Linux running on the PA-RISC processor. There are two elements: The LinuxKernel which supports the hardware and the user applications. A third piece is the boot loader (also known as PALO, the equivalent of LILO) which is part of the BootingSystem.
You can find more information about the PA-RISC architecture at http://www.openpa.net/