Recursive Factorial in Assembly

Recursion Is Just Nested Stack Frames

factorial(n) calls itself with n-1 until n <= 1. Each call allocates a fresh frame: saved rbp, return address, locals, saved registers. Deep recursion overflows the stack; tail-call optimization turns the pattern into a loop at -O2, which Compiler Explorer will show you later.

nasmCopy
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• edi holds n; result returns in eax
• Save rdi across the inner call because it is caller-saved
• Base case must return 1 without another call

Why You Preserve Callee-Saved Registers

If you touch rbx or r12-r15, save and restore them in the prologue/epilogue.

For this exercise, you will implement recursive factorial in NASM with proper frames. This task asks you to multiply the returned eax by the saved n, because botching the stack turn is how "recursive assembly works once" demos fail on the second call.

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.