#9 Modules, Recursion, ARM Application Procedure Call Standard (AAPCS)

Summary

In this lesson, Miro Samek explores advanced concepts in C programming, specifically focusing on functions. Key topics include splitting code into multiple files, creating recursive functions, and implementing the ARM Procedure Call Standard (AAPCS). The tutorial walks through moving a function into a separate file, avoiding repetition with header files, and using recursion to calculate factorials. It also highlights stack management during function calls and introduces function calling conventions in ARM architecture. Practical tips on managing resources in embedded systems, such as avoiding deep recursion, round out this in-depth lesson on C functions and modularization.

Takeaways

• 😀 Functions in C allow you to split your program into separate files, improving modularity and reusability.
• 😀 The ARM Procedure Call Standard (APCS) outlines conventions for calling and returning from functions in ARM architecture.
• 😀 Moving the delay function to a separate file (delay.c) helps modularize the code and avoid cluttering the main file.
• 😀 The DRY (Don't Repeat Yourself) principle can be applied by placing function prototypes in a header file (delay.h) to avoid repetition.
• 😀 Header files should include protection against multiple inclusions using #ifndef and #define preprocessor directives.
• 😀 Recursive functions, like calculating the factorial, call themselves and utilize the stack to store intermediate values during execution.
• 😀 When a function returns a value, it can be assigned to a variable or used in an expression, and if unused, it can be explicitly cast to void.
• 😀 Recursive functions in embedded systems can be memory-intensive due to stack usage, so iterative or alternative approaches may be better for saving RAM.
• 😀 The ARM calling convention defines how registers are used to pass arguments and return values, and how registers must be preserved.
• 😀 When working with recursion, the link register (LR) must be saved, and arguments may be moved between registers to ensure correct operation across calls.

Q & A

What is the main purpose of using functions in C, as discussed in the script?

The main purpose of using functions in C is to split a program into separate files, allowing for better organization and reusability of code, thus avoiding repetition and making the program more manageable.

What is the first step in the lesson's exercise related to moving the delay function to a new file?

The first step is to create a new file and then cut-and-paste the delay function into this new file. After that, save the file as delay.c in the project directory.

Why is it problematic to include the function prototype in both the main.c and delay.c files?

Including the function prototype in both files violates the DRY (Don't Repeat Yourself) principle, which can lead to inconsistencies if one prototype is modified but the other is not.

How can you avoid repeating the function prototype across different files?

To avoid repetition, the prototype should be placed in a separate header file (delay.h), which can then be included in both the main.c and delay.c files.

What is the purpose of the '#ifndef' and '#endif' directives in the header file?

These preprocessor directives provide protection against multiple inclusions of the same header file. They ensure that the content of the header file is only included once during the compilation process.

What does the script suggest about the efficiency of organizing a program into multiple files?

Organizing a program into multiple files speeds up the compilation process since only the modified files need to be recompiled, improving both development efficiency and program structure.

What is the significance of the recursive approach to calculating the factorial in this lesson?

The recursive approach is significant because it demonstrates how a function can call itself. This is used in the script to visualize the stack growth and illustrate the working of the stack during recursive function calls.

What role does the Link Register (LR) play in the recursive factorial function?

The Link Register (LR) holds the return address of the function. In the case of recursive functions like factorial, the LR is saved and restored to ensure that the correct return address is used after each recursive call.

What is the purpose of using the 'volatile' keyword for the variable 'x' in the factorial example?

The 'volatile' keyword prevents the compiler from optimizing away the variable 'x', ensuring that its value is not altered unexpectedly and that the result of the factorial function is correctly stored.

What is the ARM Procedure Call Standard (AAPCS), and why is it important in the context of this lesson?

The ARM Procedure Call Standard (AAPCS) defines how functions should call each other and manage registers in ARM-based systems. It is important because it ensures a consistent calling convention, such as how arguments are passed and how return values are handled, which is crucial for function interoperability and correct execution in embedded systems.