[LI] user space, kernel space and .........

[skhader@xxxxxxxxxx]

Hi there,
I am presently going thro the book " Linux Device Drivers" By Alessandro
Rubini.
There is a sentence here which is as follows:
" The chosen approach is to implement different operating modalities ( or
levels ) in the CPU itself.
The levels have different roles, and some operations are disallowed at the
lowest levels; program code can switch from one level to another only
through a
limited number of "gates". Unix systems are designed to take advantage of
this
hardware feature, but they use two such levels (while, for example, Intel
processros
have four levels). Under Unix, the kernel executes in the highest level (
also called
^^^^^^^^^^^^^^^^^^
"superuser mode "), where everything is allowed, while applications execute
in the lower
level,( the so-called "user mode"), where the processor inhibits direct
access to hardware and
unauthorized access to memory."

And can anybody elaborate why Unix uses only two levels and
why not all the four ?? And anybody here knows what are the four levels in
the Intel
processors.
Thanks
Regards
    Khader

----------------------------------   @}--------
Syed Khader Vali                  Debian 2.2.1 ( potato )
skhader@xxxxxxxxxx             Kernel 2.3.39
Live Free or Die                       http://www.cfp2000.org


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