* 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.

* they can be used to enable/disable DiffTest during run time

* see src/monitor/diff-test/ref.c for interfaces
* qemu-based codes are moved to tools/qemu-diff/