All high level languages have the concept of a subroutine (sometimes called procedure, function, or method). A subroutine is a logical division of the code that may be regarded as a self-contained operation. For example, the sine function is usually implemented as a subroutine. It can be regarded as an operation that can be used as needed in a program.
This chapter looks at a simple implementation in assembly language of this idea. The simple implementation is not adequate for the full power of subroutines (as implemented in high level languages), but is a good starting point. It corresponds to the type of subroutines implemented in the early days of programming.
The two chapters after this one discuss subroutine call methods that are similar to those of modern languages.
(Review: ) What instructions unconditionally transfer control from one address to another?