Microprocessor 4

4Subroutine

After studying the principles of basic microprocessor operation and programming in previous chapters, we present here the concept of the subroutine. Thanks to the subroutine, it is possible to implement the concepts of function and procedure of High-Level (programming) Language (HLL) such as C or PASCAL. To be able to do this, a memory called a stack is required. Its operation, as well as that of its subroutine, are studied. This concept has been derived from that of interruption, invented to accelerate I/O (for Input/Output) handling and presented in the chapter that follows for teaching purposes.

NB. The context of this study is a mono-processor unless otherwise indicated.

4.1. Stack memory

Stack memory simply means the stack as well as the LIFO (Last-In/First-Out) buffer to indicate the order of data circulation. Figure 4.1 shows the graph symbol of a stack S of a size s words.

Figure 4.1. Suggested visual representation of a stack S

The word “stack” is an analogy with a stack of plates where the last plate stacked is later the first to be picked up. In our context, the information represents the stack. The location that will be accessed is the top of the stack or TOS (Top-Of-Stack). For information, the data that precedes it takes the name NOS for Next-On-Stack. Two primitives are needed to manage the stack: stack() and destack(). The place occupied by this ...