What is the PDC?
The Processor Dependent Code (aka "PDC") is a firmware executable that lives in the machine's PROM, and, among other things, handles basic initialization during boot up and provides a BootConsoleHandler (BCH). It could be considered as a sort of BIOS for the PA-RISC architecture.
When the machines starts, it loads the PDC that then performs various hardware checks, and finally shows the user a limited interface (the BCH) through which information can be collected about the hardware, and some settings can be changed (such as Paths, Fast Boot, Fan Choice, etc). For more details about how to use that PDC interface, one can look at the corresponding section of the PA-RISC/Linux Boot HOWTO.
Aside these boot-time facilities, the PDC is also responsible for providing a set of low-level routines that can be accessed by the operating system of the running machine.
Linux on PA-RISC uses these routines for various tasks, including device discovery, chassis log reporting and stable storage accessing. Some of the kernel hooks to access the PDC are documented here.