Q. Several CPUs provide for more than two modes of operation. What are two probable uses of these multiple modes?
Answer: Although most systems merely distinguish between user and kernel modes some CPUs have supported multiple modes. Multiple modes could be utilized to provide a finer-grained security policy. For instance rather than distinguishing between just user and kernel mode you could distinguish among different types of user mode. Possibly users belonging to the same group could execute each other's code. The machine would depart into a specified mode when one of these users was running code. While the machine was in this mode a member of the group could run code belonging to anyone else in the group.
An additional possibility would be to provide different distinctions within kernel code. For instance a specific mode could allow USB device drivers to run. This would signify that USB devices could be serviced without having to switch to kernel mode thereby essentially allowing USB device drivers to run in a quasi-user/kernel mode.