I've fixed the 16-bit multiply instructions. For some reason, when using a multiply instructions with registers as both operands, they get swapped. So instead of "A=A*B", I see "A=B*A". This is annoying, since if I constantly use one argument for all configurations in the assembly output, I could end up with "A=A*A" in some cases. Fortunately, this is easily handled. At some point, I need to find out where the swap comes from, but I'm OK with leaving this as it is for now.
Fun fact: GCC really, really doesn't want to use multiply. If one of the operands is constant, it'll prefer permutations of shifts and adds to calculate the result, even if this would be much more expensive for the TMS9900.
There's true oddness for the divide instructions. The instructions I had before work just fine, but GCC is selecting the opposite function than the one called for. For instance, if I write "A/B", I get back "A%B", and vice versa. The wierd thing is that this doesn't seem to be a bug in my code, but GCC is knowingly doing this for some reason. Research awaits.
OK, I'm losing my mind apparently. The DIV and MOD instructions are working just fine. Moving on...
That completes the list of known problems in the coverage tests. I may have missed some unsigned variants, so I'll need to double-check that. There might be a few more 32-bit instructions I could do too, but I think I'm good right now.