4GB RAM limit?

Erik Trulsson ertr1013 at student.uu.se
Tue Jul 1 17:06:49 PDT 2003


On Tue, Jul 01, 2003 at 04:46:55PM -0700, Joshua Oreman wrote:
> On Tue, Jul 01, 2003 at 06:30:13PM +0000 or thereabouts, george donnelly wrote:
> > hi
> > 
> > I've got a 4.7 freebsd machine and after upgrading from 4 to 6GB of RAM, top
> > does not recogmize the other 2 GB (only shows 4) and actually there was a
> > message that said "ignoring 2 GB".
> > 
> > Can anyone shed light on this or suggest where i can look to understand
> > this?
> 
> AFAIK, on i386 the maximum addressable memory space is 4096 MB.
> Here's why:
> C pointer = 4 bytes = maximum value 4294967296

Yes and no.  i386 addresses are indeed 32 bits which means that a
process can only address 4GB worth of memory.  On the Pentium Pro and
later x86 CPUs there some special tricks that can be used to have a
physical address space of 36 bits (64 GB memory), even though a single
process still only can use 4 GB.
FreeBSD 4.x and earlier does not have any support for those tricks so
it cannot use more than 4 GB of RAM.
FreeBSD 5.1 and later does have support for PAE (Physical Address
Extension) so it can handle up to 64 GB RAM (while a single process
is still constrained to a 32-bit address space.)




-- 
<Insert your favourite quote here.>
Erik Trulsson
ertr1013 at student.uu.se


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