[OpenBIOS] Why protected mode?

Maciej W. Rozycki macro at ds2.pg.gda.pl
Thu Jul 15 22:39:38 CEST 1999

On Thu, 15 Jul 1999, [iso-8859-1] Matthias Wächter wrote:

> Don't say that: There are some Xeon motherboards around (well, most of
> them are sold in complete supported systems) supporting 16GB of memory and
> more. Since you need more than 32 bits to address this space, they had to
> design work-arounds like segmentation (I don't know anything more about
> it). Even worse: The switching is done externally, by the chipset...

 You haven't read Intel processor specifications, have you?

 Since PPro ia32 processors support the so called Physical Address
Extensions which for current processors allow to address up to 64 GB of
memory.  The limitation comes from the limited number of processor's
address lines which currently support 36-bit addressing.

 Using Physical Address Extensions the CPU may address memory above 4 GB
using C-mode paging which needs a special setup of page tables; in general
after setting the PAE bit of CR4, paging becomes three-level deep and CR3
becomes PDBR (Page Directory Base Register).  Page directory and table
entries are now 64-bit.  You have a choice of 4 kB and 2 MB page sizes in
this mode.  See Intel documentation for details. 

 Actually A-step Pentiums also included this feature but probably due to
bugs in the implementation the feature has been removed for B-step and
later chips.  A-step chips were never available to the market. 

 Since PII Xeon, some further extensions were added -- see appropriate

+  Maciej W. Rozycki, Technical University of Gdansk, Poland   +
+        e-mail: macro at ds2.pg.gda.pl, PGP key available        +

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