 Internally, PCR is largely built up of custom applications on top of a VxWorks kernel.A benefit of the software is that it is completely modular and can load components to Passport control processors (CPs) and function processors (FPs) on an as-needed basis.FPs (also known as line cards) each run their own instance of the Operating System, and as such can be rebooted without the need to take the entire switch out of service, for example due to a software failure.