lodsb (b for Byte) loads into al, not ax. It's lodsw (Word) which uses ax, and of course there's lodsd for eax and even lodsq for rax (64-bit only).

This will be the return value of main and the exit code of the program. Since you errored, you want to return a nonzero value.

Actually that's the non-error path; eax is set to 1 and edx to 0 as initial values for computing fib(n) in the fibloop.