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Test
Your EQ #147— Answer
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Answer
7
Dual-mode
operation usually means that there are some instructions
that are disallowed in user mode, usually assoicated with
the interrupt mechanism and the memory management unit
(MMU). The ability to switch from user mode to supervisor
mode is carefully controlled so as to eliminate the possibility
of executing user-mode code in supervisor mode. There
are many problems with multi-user operation on a system
without this capability. In effect, everything that we
protect in modern operation systems is unprotected. This
means that:
- Memory
is unprotected. A process can read or write memory
belonging to another process. A process can also read
or write the kernel's memory.
- Timers
are unprotected. A process can change the timers arbitrarily
and interfere with the execution of other processes
or run as long as it wants.
- Devices
are unprotected. A process can arbitrarily access
devices, altering permanent storage, interfering with
other processes' use of the devices, etc.
- Special
instructions are unprotected. Any process could halt
the machine or cause other problematic operations
to execute.
Contributor: Naveen PN
Published: October-2002
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