An Application Binary Interface defines how compiled binaries interact, ensuring functions can call each other across compilers and languages. For ARM Cortex-M systems, the AAPCS governs calling conventions, data layout, and stack behavior. A recent technical breakdown examines this by compiling C functions with different argument counts and inspecting the generated assembly, revealing the ABI's concrete rules versus compiler implementation details.

The investigation shows that for up to four integer arguments, the ABI mandates they be passed in registers r0-r3. Adding a fifth argument forces the fifth parameter onto the stack, a hard boundary visible in optimized assembly. Data alignment is also proven: a char followed by an int results in three bytes of padding, ensuring the int starts on a 4-byte boundary as required.

Observing assembly output with tools like `nm -S` separates ABI law from compiler artifact. Function size shrinks under optimization, but the ABI-mandated register usage and stack layout remain constant. This practical verification demystifies low-level code, making it predictable. Understanding these rules is crucial for embedded developers working with precompiled libraries or mixed-language projects.