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 [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/