What Survives a Call

File-scope static int callee_touch = 0; is in your editor. Implement void callee(void) that increments it (proving the call happened) and int caller_keeps(int a, int b) that computes a*2 into a local, calls callee(), then returns the local + b — the local's value must survive the call, which is the caller/callee register split working as designed.