First thing to keep in mind is that I'm talking about physical
addresses, not OS-specific virtual addresses. Second thing to keep in
mind is that I'm talking about *address space*, not *RAM*. There is a
big different here. An address represents a location where data can be
stored or retrieved. That location does _not_ have to be RAM. It could
be a register on a APIC chip, or a memory array on a PCI card, or a
location in a local RAM chip.
PCI (AGP is really just like PCI from this perspective) specifically
allows the CPU to access registers and memory arrays on the cards as if
they were local addresses, that's the point of the MEMIO registers and
Base Address Registers. When the CPU does a load or store of an address
that falls into these address ranges, the request doesn't go to RAM, it
goes to the PCI bus and is serviced by the appropriate card there.
Local RAM doesn't get involved at all.
PCI doesn't actually care much which addresses are used, but by
convention the PC platform puts them at the top of the 32-bit address
space. But, what happens when you have so much RAM that the RAM could
service those very high addresses? For many years that wasn't an issue
because it wasn't possible or practical to put that much RAM into a PC.
But now it is, so it's up to the memory controller and host bridge to
figure out what to do. Many systems cause that high RAM to simply be
ignored, resulting in the loss of effective RAM (as you saw in your
case). More complex systems will take the RAM that would occupy that
3.5-4GB address space and re-map it into the 4.0-4.5 address space. The
RAM doesn't care because it's just an array of storage cells, it's up to
the memory controller to associate addresses with those storage cells.
Of course, that only works if you're using a 64-bit (or 32bit PAE
enabled) OS that can deal with physical addresses larger than 32 bits.
Intel Xeon systems typically do the remapping trick, so when you boot
FreeBSD i386+PAE or amd64 on them, they might show 4.5GB of RAM when
there reall is only 4GB (this is a limitation of how we compute RAM and
is purely cosmetic, but should be fixed). Many AMD Opteron systems do
not do the remapping and result in you loosing effective RAM. The
difference between Intel and AMD is because AMD puts the memory
controller into the CPU instead of in the PCI host bridge, so it's much
harder to have the two work together to do the remapping. I believe
that there are some Opteron systems that can do this, though.
A junior doc writer task would be for someone to collect all of the
email responses that I give on this topic (I seem to get at least one
query a month) and turn it into an FAQ for the FreeBSD doc set.
I have 4 GB of ram installed in my computer, but the BIOS and OS is only reporting 3.2-3.7GB of available ram.
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