* it seems that qemu initialize mstatus with all 0s, which is wrong

Xv6

See merge request projectn/nemu!24

* do not call page_translate() in M mode

* With RVC, an instruction may cross page boundary. To fix this, we
  should fetch twice.

* machine level interrupts can not be delegated

* They are really the subset of mstatus and mie. We should refactor the
  access of CSR to support this.

Reset at 0

See merge request projectn/nemu!23