On the Rust side, it is expecting the handle in r0.

```text
00003860 <neotron_os::program::api_write>:
    3860:       b5f0            push    {r4, r5, r6, r7, lr}
    3862:       af03            add     r7, sp, #12
    3864:       b083            sub     sp, #12
    3866:       9202            str     r2, [sp, #8]
    3868:       9101            str     r1, [sp, #4]
    386a:       2105            movs    r1, #5
    386c:       020c            lsls    r4, r1, #8
    386e:       b2c5            uxtb    r5, r0
    3870:       f009 f965       bl      cb3e <neotron_api::file::Handle::new_stdout>
    3874:       b2c0            uxtb    r0, r0
    3876:       42a8            cmp     r0, r5

```