PA-RISC Linux Runtime Conventions
The PA-RISC Linux runtime is very similar to what was designed for HPUX, as described in the HP runtime documents: http://ftp.parisc-linux.org/docs/arch/pa-runtime-32-SOM.pdf and http://ftp.parisc-linux.org/docs/arch/pa64rt.pdf
RegisterConventions explains how registers are used and how they are passed to functions.
Like HPUX, PA-RISC Linux has a stack that grows up instead of down. This has some advantages (not as easy to get stack overflow exploits), but sometimes creates some porting issues.