> So, I was reading locore_mips?.S.
>
> I noticed that mips?_SystemCall saves the (caller-saved) t* regs.
>
> If the situation is the same as on alpha, registers which pass syscall
> arguments (looks like v0 and the a* regs) must be saved-restored, but
> other caller-saved regs don't have to be (which means t* and probably
> v1 and at).
>
> Any particular reason that those regs are saved?

Nothing, and it can be change as you propose.  When I created
mips?_SystemCall, I just copied part of UserGenException(). 

Tohru Nishimura