Wow, it's been a while since I updated this. So I released a new set of patches, with really limited response. This makes me sad.
I've started making a C library to make development easier. This will also exercise the compiler a bit more.
I've noticed an annoying aspect of GCC, the divmod form is invoked for each div and mod operation. Even though the processor can compute both values in a single instruction, GCC does not take advantage of this. I found a comment on the GCC development forums where this is a problem for many archetectures.
Unfortunately, I'm not eager to change the internals of the compiler to accomodate the TMS9900. Even if it would make for better code, it would likely break forward compatibility.