A well-structured program should be divided into separate modules functions in C or subroutines in assembly language. There is nothing special about this in embedded systems. If you wish to write subroutines in assembly language, you need to know how arguments are passed, local variables allocated, and results returned. This is occasionally useful for efficiency but the main reason for studying these details is to assist the debugging of C programs. It is particularly important to understand the central role of the stack.

Interrupts are a major feature of most embedded software. They are vaguely like functions that are called by hardware rather than software. The distinction sounds trivial but it makes them much harder to handle because the processor must be able to return correctly to its activity before it was interrupted. The application note MSP430 Software Coding Techniques (slaa294) describes the overall structure of a typical, interrupt-driven program for the MSP430 and describes a range of techniques to ensure that programs are robust and can easily be debugged.

The final topic in this chapter is the range of low-power modes of operation. They are described here because the MSP430 needs an interrupt to wake it from a low-power mode. In fact we see that no extra effort is usually needed to handle low-power modes in interrupts: The MSP430 automatically goes to active mode when an interrupt is requested, services the interrupt, and resumes its low-power mode afterward.