* other scripts can report an error when NEMU does not return
  successfully

* now different instances can connect to QEMU via different ports

* If we put mainargs at a device, x86 may directly use it to perform
  computation, such as cmp (%eax), 0, where %eax points to the device.
  But the monitor will skip every instruction accessing device, which
  makes REF skip generating correct EFLAGS.
* Putting mainargs at RAM solves this issue.

* the LSB of funct7 may be "1" due to the shift amount can be >= 32

* the src should be first sign-extended

* Now we can run PAL over Nanos-lite, but it gets stuck while battling.
  It seems mul/div instructions are wrong.