Calling Conventions
Where arguments go, who saves what, and why the ABI is a contract between compilers, not a language guarantee.
The invisible agreement
When you call a function, both sides agree — without any C syntax saying so — where each argument travels (registers first, then stack), which register returns the value, and which registers the callee must restore. That agreement is the calling convention, the core of the platform ABI.
Caller-saved versus callee-saved
Registers the callee may freely scratch are caller-saved: if the caller still needs the value, it must spill it before the call. Registers the callee promises to preserve are callee-saved: the callee spills and restores them. This division is why adding a call can change register allocation in the caller — visible in module 13's disassembly.
Variadic functions break the pattern on purpose
printf-style functions take a runtime-shaped argument list. <stdarg.h> (va_start/va_arg/va_end) is the standard's portable window onto whatever the convention actually did — and the reason printf is a special beast for every ABI ever designed.