Need some semi-machine-independent representation for thread state. Much of it is machine independent, but is of limited use to the debugger. However, there's thread state stuff that should be okay --
scheduling state (MI)
saved errno value (MI)
stack:
entire stack segment
currently-used part (valid only when thread is not running? Need a flush call)
debugger info:
stack info
saved registers (need call to flush stack and registers in some std place) (MD but GC might be able to use them in a MI way)