How quickly we forget how we got to where we are...
Remember the 640MB limit and the 1MB limit. It's the same thing in a new
package.
Large areas of the memory between three and four gigabytes are cordoned off
for system devices in exactly the same way that chunks of the Upper Memory
Area were purloined in the old days. The processor (and other system
components) can talk with some devices by reading and writing memory
addresses up above 3Gb.
Windows users can see all of the reserved memory areas on their PC in Device
Manager, using the View -> Resources By Connection option. This is what that
view looks like for the PC I'm using now.
The addresses are all in hexadecimal, which makes it less than instantly
obvious to the untutored viewer which reserved areas are tiny (and usually
old) and which large (and all relatively new). Spend the time to figure it
out, though, and you can see the old backwards-compatibility stuff and the
new 3Gb-barrier stuff, plain as day.
Hex addresses A0000 to BFFFF, for instance, are still assigned to the video
card (a GeForce 7800 GT, in this case). That's addresses 655360 to 786431 in
decimal, 640 kilobytes to 768k. This is the old 128 kilobyte reservation for
the monochrome, CGA and EGA graphics buffers, still there in case you find
your old Leisure Suit Larry disk and want to see if it works.
In the above picture, though, you'll see a much more considerable
reservation from C0000000 to CFFFFFFF, that's also assigned to my video card.
That's 268,435,456 bytes, equal to the 256 megabytes of memory on the card,
and it's the chunk of memory addresses that system devices use when they want
to access the card's memory.
If I had a video card with 512Mb or 768Mb of memory on it, it'd take up even
more space in the 3Gb-to-4Gb memory map.
And if I were still using an AGP graphics card, there'd be another block of
memory reserved for the AGP aperture, used when devices on other buses in the
computer want to talk to a graphics card on the AGP bus. I've got a PCIe
graphics card, though, which sits on the same bus as all of the other stuff
and so doesn't need an aperture.
(If you've got a computer with one of those cheap graphics adapters that
uses system memory instead of having RAM of its own, it will of course eat
some of your RAM no matter how much you've got installed.)
Power users with a hankerin' for dual graphics cards may be experiencing
something of a sinking feeling, at this juncture. Yes, the 256Mb reserved for
my little old graphics card means exactly what you think it means: Those two
768Mb graphics cards you can totally justify buying will eat one point five
gigabytes of your 32-bit memory map all by themselves, cutting you down to a
2.5Gb ceiling before you even take the other reservations into account.
This also explains why 1Gb graphics cards haven't hit the consumer market
yet. Nobody yet needs anything like that much memory on one card for any
desktop computer purpose, but some people would still be very happy to pay
for such a card just for the pose value. It'd eat the whole of the fourth
gigabyte of their system memory, though. And then they'd probably demand
their money back.
(This fact has apparently not stopped certain unscrupulous companies,
coughDellcough, from allowing people to buy a computer with WinXP, 4Gb of
RAM, and a pair of Nvidia's oddball 1Gb GeForce 7950 GX2 cards. Result:
56.25% of the installed memory absent without leave. You might as well have
only bought 2Gb.)
As with the Upper Memory Area problem, the 3-4Gb space must have seemed
stratospherically far away when people first started buying $15,000 80386
PCs. But here we are running into it, and the result is not pretty.
That was then, this is now. 64-bit CPUs are widely available, and 64-bit
OSes are starting to trickle into the mainstream market. The nightmare will,
with any luck, soon be over.
A 64-bit PC running a 64-bit OS has a truly vast basic memory address space.
The 4Gb 32-bit address space was 4096 times the size of the 1Mb 20-bit space,
but the 64-bit address space is 4,294,967,296 times the size of the 32-bit
one.
By default, an all-64-bit PC will still have the standard big holes in its
memory from three to four gigabytes. This is the lowest-hassle way to deal
with the problem - just install more than 4Gb of memory, and live with the
fact that your 8Gb PC with a 768Mb graphics card only actually has
seven-point-not-much gigabytes of visible RAM.
One advantage of this is that you can still boot a 32-bit OS, if you want
to. Another is that this vanilla configuration is most likely to actually
work. Better memory configurations aren't necessarily properly supported by
hardware and OSes yet.
If you don't care about these factors, though, there are two ways to get the
lost memory back.
Some 64-bit motherboards these days give you an option for "memory hole
remapping". That moves the fourth-gigabyte MMIO memory holes higher into the
64-bit address space, probably way above the maximum RAM you can physically
install.
Many other 64-bit boards, though, are even smarter, and can leave the memory
holes where they are and remap (at least some of) the physical RAM out from
under the holes and up past 4Gb. This process is often entertainingly
referred to as "memory hoisting", and it used to be the preserve of server
motherboards. It's been showing up in more and more desktop mobos, though.
And on some of them, the memory-hoisting BIOS setting even works, and doesn't
horribly crash the system as soon as something tries to use the remapped RAM.
You may only be able to "hoist" the last 512Mb of the 4Gb address space, but
that's better than nothing. If it works.
I should add a note about the /3GB, /4GT and /PAE Windows boot.ini switches,
too, because they often come up when people are talking about 4Gb-plus
Windows PCs.
They are all useless to you. You do not want them.
/3GB and /4GT are config settings for different versions of Windows that
tell the operating system to change the partitioning of the 4Gb 32-bit
address space so that applications can use 3Gb and the OS kernel only 1Gb, as
opposed to the standard 2Gb-each arrangement. They don't help at all with the
3Gb barrier, and most applications don't even notice them, so desktop users
lose kernel memory space (and system performance) for no actual gain at all.
The /PAE boot.ini switch, on NT-descended Windows flavours, activates the
Physical Address Extension mode that's existed in every PC CPU since the
Pentium Pro. That mode cranks the address space up to 64 gigabytes (two to
the power of 36), and the computer can then give a 4Gb addressing block
within that space - or even more, with extra tricks - to each of several
applications.
PAE's no good to the everyday 3Gb-problem-afflicted user, though, for two
reasons.
First, it presents 64-bit addresses to drivers, and thus causes exactly the
same compatibility problems as a proper 64-bit operating system, except
worse, because now you need PAE-aware drivers for 32-bit Windows, instead of
just plain 64-bit drivers for a 64-bit OS. From a normal user's point of
view, PAE gives you the incompatibility of a 64-bit operating system when
you're still running a 32-bit OS.
For this reason, Microsoft changed the behavior of the /PAE option in all
versions of WinXP as of Service Pack 2. They fixed the endless driver
problems by, essentially, making /PAE in XP not do anything. All versions of
WinXP - even the 64-bit versions - now have a hard 4Gb addressing limit, no
matter what hardware you use them on and what configuration you choose.
This isn't a big problem, of course, since XP is not meant to be a server
operating system. But it's still mystifying to people who try the /PAE flag
and can't figure out why it doesn't work.
Oh, and just in case you for some reason still wanted to try PAE: It eats
CPU time, too.
(If you've got 4Gb of RAM, by the way, the Vista installer may not work
anyway. You can work around that problem, if you have it, by pulling some of
the RAM while you install Vista, then putting it back. Apparently, Microsoft
limits the maximum available memory in 32-bit Vista to 3.12Gb anyway, so it's
hardly worth the trouble of buying more
64 bit enabled 