A calculation like “the factorial of a number” may be used several times in a large program. Subroutines allow this kind of functionality to be abstracted into a unit. It’s a benefit for code reuse and maintainability. Even though PASM is just an assembly language for a virtual processor, it has a number of features to support high-level subroutine calls. PIR offers a smoother interface to those features.

PIR provides several different sets of syntax for subroutine calls. This is a language designed to implement other languages, and every language does subroutine calls a little differently. What’s needed is a set of building blocks and tools, not a single prepackaged solution.

Parrot-Calling Conventions

As we mentioned in Chapter 9, Parrot defines a set of calling conventions for externally visible subroutines. In these calls, the caller is responsible for preserving its own registers, and arguments and return values are passed in a predefined set of Parrot registers. The calling conventions use the Continuation Passing Style to pass control to subroutines and back again.

The fact that the Parrot-calling conventions are clearly defined also makes it possible to provide some higher-level syntax for it. Manually setting up all the registers for each subroutine call isn’t just tedious, it’s also prone to bugs introduced by typos. PIR’s simplest subroutine call syntax looks much like a high-level language. This example calls the subroutine _fact with two arguments and ...

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