Memory size in 9.04

[Fred Roller]

On Mon, 2009-07-06 at 17:46 -0400, Александр Горлов wrote:
> Could anybody give me more technical detail whay 32-bit kernel does
> not see 4GB RAM.?
> What does mean special options for C compiler that allow to see 4GB

Here is a quick read.

http://techfiles.de/dmelanchthon/files/memory_hole.pdf

excerpt:
...
You may have noticed by now that an x86 based system with 2G or more of
memory, the BIOS and O/S may report less memory in the machine than is
actually installed. This problem occurs for 2 reasons, which are legacy
32-bit compatibility and the resource conflict caused by maintaining
legacy 32-bit compatibility.

Since the system BIOS does not know what operating system will be booted
it must setup the system in 32-bit mode. This means that memory and all
device address spaces must be mapped below 4GB. When 4GB of memory is
installed in the machine there is a resource conflict between the
physical memory and the device address space. The way most BIOS resolve
this problem is by carving a hole in memory at the top of the 4GB range.
This hole is commonly referred to as the “PCI Hole”.

The PCI hole exists below 4GB to ensure that all 32-bit software can
reach those addresses. The physical memory hidden by this hole is not
usable by the operating system software and therefore is “missing” from
the system. The size of the PCI hole is the total amount of PCI/AGP
address space consumed for all devices as configured by the BIOS.

A configuration with 4GB of memory, an FX3000 (256M video memory) and an
AGP aperture size of 256M results in about 3.0 GB of usable system
memory. Reducing the AGP aperture size in BIOS setup to 64M increases
the usable memory to about 3.3 GB. The following table shows the impact
of the PCI hole based on video card and aperture size.
...

A bit aged but informative.  Hope it helps.

-- 
Fred R.
www.fwrgallery.com

"Life is like Linux, simple.  If you are fighting it, you are doing
something wrong."