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From Francis GALIEGUE <...@one2team.com>
Subject Re: [OT]RE: Maximum memory that can be assigned to Tomcat on windows platform
Date Thu, 01 Dec 2011 11:32:46 GMT
On Thu, Dec 1, 2011 at 12:29, Casper Wandahl Schmidt
<kalle.privat@gmail.com> wrote:
[...]
>
> That didn't quite help me understand, because how can the OS map from ie.
> 0-4GB to 4-8GB (the window is moved) when it can only use a 32bit register
> to tell the machine where to look in the psysical memory, that is where my
> knowledge ends :) So I read about PAE and found out that it uses 2 registers
> (36 bits due to some bits being used as flags) and that makes good sense,
> but how can the cpu calculate an address without overflow and send a command
> to the bus containing a 36bit address (or whatever fetches the bits from
> RAM)? That is where I'm puzzled but I guess it is because I'm not at all
> into ISA-level and below :)
>

It is the role of the MMU to do that. At any one time, it can map a
"virtual", 32-bit wide, address to a "real", 36-bit wide address. It
uses TLBs (Translation Lookaside Buffers) for that, and it is the OS'
role to have the correct TLB in place at any time.

-- 
Francis Galiegue
ONE2TEAM
Ingénieur système
Mob : +33 (0) 683 877 875
Tel : +33 (0) 178 945 552
fge@one2team.com
40 avenue Raymond Poincaré
75116 Paris

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