even though most systems only distinguish among user and kernel modes, some CPUs provide multiple modes. Multiple modes could be used to provide a finer-grained security strategy. For instance, rather than distinguishing between just client and kernel mode, you could make a distinction between different types of user mode. Possibly users belonging to the equal group could execute each other’s code. The machine would go into a specific mode when one of these users was running code. When the machine was in this mode, a member of the group could run code belonging to someone else in the group. Another option would be to provide different distinctions within kernel code. For instance, a specific mode could allow USB device drivers to run. This would mean that USB devices could be serviced without having to change to kernel mode, thereby basically allowing USB device drivers to run in a quasi-user/kernel mode.