This technique is called supervisor mode; compared to more crude approaches ( such as storing the TCB in ROM, or equivalently, using the Harvard architecture ), it has the advantage of allowing the security-critical software to be upgraded in the field, although allowing secure upgrades of the trusted computing base poses bootstrap problems of its own.
32.
A PTE may also include information about whether the page has been written to ( the " dirty bit " ), when it was last used ( the " accessed bit, " for a least recently used ( LRU ) page replacement algorithm ), what kind of processes ( user mode or supervisor mode ) may read and write it, and whether it should be cached.
33.
Windows NT 3.1 featured a core kernel providing a system API, running in supervisor mode ( ring 0 in x86; referred to in Windows NT as " kernel mode " on all platforms ), and a set of user-space environments with their own APIs which included the new Win32 environment, an OS / 2 1.3 text-mode environment and a POSIX environment.
34.
In the C community, undefined behavior may be humorously referred to as " "'nasal demons "'", after a CPU might leave the behavior of some forms of an instruction undefined, but if the CPU supports memory protection then the specification will probably include a blanket rule stating that no user-accessible instruction may cause a hole in the operating system's security; so an actual CPU would be permitted to corrupt user registers in response to such an instruction, but would not be allowed to, for example, switch into supervisor mode.
