In the preceding chapter, we discussed the system architecture the design and implementation of the PA-RISC hardware. But in order to make this useful, we need to make some decisions about how we'll use the hardware. For example, we need to agree on how procedure calls are made, how arguments are passed, and how return values are returned. This is known as the runtime architecture of the system. In this chapter, we look at these conventions, and we also take a look at some of the commonly used PA-RISC instructions.