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From "Casper Wandahl Schmidt" <>
Subject RE: [OT]RE: Maximum memory that can be assigned to Tomcat on windows platform
Date Thu, 01 Dec 2011 11:29:14 GMT

-----Original Message-----
From: Ronald Klop (Mailing List) [] 
Sent: 1. december 2011 12:06
To: Tomcat Users List
Subject: Re: [OT]RE: Maximum memory that can be assigned to Tomcat on
windows platform

Op donderdag, 1 december 2011 09:39 schreef Casper Wandahl Schmidt
>  See below. I hope MS Outlook does some decent indend so my response 
> is clear -.-
>  -----Original Message-----
>  From: Christopher Schultz []
>  Sent: 30. november 2011 18:51
>  To: Tomcat Users List
>  Subject: Re: Maximum memory that can be assigned to Tomcat on windows 
> platform
>  Hash: SHA1
>  Casper,
>  On 11/30/11 3:37 AM, Casper Wandahl Schmidt wrote:
>  > Another question to ask is, why do you have 8GB memory when running  
> > 32bit? That is just stupid since 32bit cannot address more than 4GB 
> of  > memory no matter what you do. Any sysadmin should know that right?
>  That's per process. All reasonably recent 32-bit OSs can address way more
than 4GiB internally.
>  For example:
> s.85%29.aspx#memory_limits
>  This is generally done through PAE
>  ( which allows
32-bit OSs to access more than 4GiB at the kernel level, though each process
is still limited to 4GiB.
>  Aha so I learned something new today :) I'm still puzzled as to how a 
> 32 bit CPU can compute and fetch a memory cell with address above 4GB 
> since it cannot hold this large value. Anyway that is just too much 
> low-level computer science for me, all I ever had was a seven week 
> course on architecture and networking (a single week out of the seven) 
> :)
>  -Casper
>  Running a machine with more than 4GiB in 32-bit mode isn't stupid at all
IMO. If you have relatively small processes, there's no need for the
overhead of 64-bit even if you have 16GiB or more.
>  - -chris
 I have an analogy for you.
 If you look out of your window you only see a small part of the world. If
you move your window you wil see another part of the world.
 This is what the OS does with PAE. It moves the window on your RAM
frequently. That is why a 32 bits application only sees max. 4GB. That is
the size of its window.


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 :)


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