Recursive Fibonacci in ARM

Fibonacci Forces Two Recursive Calls

fib(n) needs both fib(n-1) and fib(n-2) unless you memoize. In assembly each bl clobbers x0 and overwrites lr, so you must spill live values to callee-saved registers or the stack. iOS and Android crash reports show these frames as repeated fib symbols when students forget to save lr.

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• Base case n <= 1 returns n in x0
• x19-x28 are callee-saved scratch if you need them across bl
• RET jumps to the address in lr

For this exercise, you will implement recursive fib without loops. This task asks you to save lr before every recursive bl, because leaf-vs-non-leaf distinction is the first thing arm64 unwinders check when a stack trace looks truncated.

Keep the relevant man page, ABI doc, or Rust reference chapter open while you work. When your output disagrees with the reference implementation on the same machine, the mismatch is usually an alignment rule, an off-by-one terminator, or a register slot you misread in GDB. Skim the official documentation for the tool or ABI named in the exercise; the prose changes, but register roles, syscall numbers, and ownership rules stay stable across releases.