Hi,
> Something at ring-0 (the kernel) can execute priviledged
> processor instructions which, for example, manipulate the processor memory
> management tables, io ports, and other things like that. When a user
> program call the kernel to do something, the internal kernel code switches
> to ring-0.
While the Intel processors provide IMHO four operating levels, UNIX
takes advantage of this feature by using two of them, with the kernel
executing in the highest level (supervisor mode) and applications
executing in the lowest one (user mode) with some operations, e.g.
on memory, are disallowed. Hence, an application can switch to kernel
mode operating in behalf of itself.
ciao,
peterw
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