x86 opcode, 8B

fun:
        call    edx          ; 2B
        cmpxchg eax,    ecx  ; 3B, on 8086 use xchg and cmp instead
        jnz     fun          ; 2B
        ret                  ; 1B

input ecx(init), edx(func ecx->eax without modifying non-output var)