19. Portability

Linux is a portable operating system that supports a wide range of computer architectures. Portability refers to how easily—if at all—code can move from one system architecture to another. We all know that Linux is portable because it has already been ported to various systems. But this portability did not occur overnight—it required many important design decisions along the way. Consequently, it is now easy (relatively speaking) to bring Linux up on a new system. This chapter discusses how to write portable code—the issues you need to keep in mind when writing core kernel code or device drivers.

Some operating systems are designed with portability as a primary feature. As little code as possible is machine specific. Assembly is kept to a minimum and interfaces and features are sufficiently general and abstract that they work on a wide range of architectures. The obvious benefit is the relative ease with which a new architecture can be supported. In some cases, highly portable and simple operating systems can be moved to a new architecture with just a few hundred lines of unique code. The downside is that architecture-specific features are not supported and code cannot be hand-tuned for a specific machine. With this design choice, optimal code is traded for portable code. Some examples of highly portable operating systems are Minix, NetBSD, and many research systems.