"Tim Anderson" wrote in message ...
"Mxsmanic" wrote in message
...
Rick writes:

Looks like PCI Express is a staggering waste of memory space.

Yes, PCI Express is the main guilty party here.

But *why* does this address space have to be in the first 4GB?

To maintain compatibility with OS's that are limited to 4GB.
I came across this note in one of Intel's server motherboard
manuals (which supports up to 8GB):
ftp://download.intel.com/support/mot.../sp2gp2tps.pdf

Note: Memory between 4GB and 4GB minus 512MB will not
be accessible for use by the operating system and may be lost
to the user, because this area is reserved for BIOS, APIC
configuration space, PCI adapter interface, and virtual video
memory space. This means that if 4GB of memory is installed,
3.5GB of this memory is usable. The chipset should allow the
remapping of unused memory above the 4GB address, but
this memory may not be accessible to an operating system that
has a 4GB memory limit.

"Rick" wrote in message
. net...

3.5GB of this memory is usable. The chipset should allow the
remapping of unused memory above the 4GB address, but
this memory may not be accessible to an operating system that
has a 4GB memory limit.

Yes, this is helpful. Thanks. So the proper question may be this: Why don't
these 4GB boards remap this memory to make it usable by 64-bit operating
systems?

Tim

"CBFalconer" wrote in message
...

All of this deals with the physical addresses, not the virtual
addresses that the software sees.

Thanks Chuck. Can you expand on that last remark? How difficult would it be
to make this wasted RAM available to the OS? For the board deisgner, that
is. I'm not expecting any workaround for the motherboard I'm using.

Tim

Tim Anderson wrote:
"Rick" wrote in message

3.5GB of this memory is usable. The chipset should allow the
remapping of unused memory above the 4GB address, but
this memory may not be accessible to an operating system that
has a 4GB memory limit.

Yes, this is helpful. Thanks. So the proper question may be this:
Why don't these 4GB boards remap this memory to make it usable by
64-bit operating systems?

Because it is an unnecessary expense. While there may be uses for
addressing over 4GB to conveniently handle large databases etc.,
there is virtually no real need for that much actual memory.

The function can be efficiently served by virtual memory systems at
little performance cost. About the only area where I can conceive
such real memory to be useful is in high performance video
processing.

--
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http://www.caliburn.nl/topposting.html
http://www.netmeister.org/news/learn2quote.html

Tim Anderson wrote:
"CBFalconer" wrote in message

All of this deals with the physical addresses, not the virtual
addresses that the software sees.

Thanks Chuck. Can you expand on that last remark? How difficult
would it be to make this wasted RAM available to the OS? For the
board deisgner, that is. I'm not expecting any workaround for the
motherboard I'm using.

If you don't install memory in a physical area it isn't wasted.
Virtual memory systems involve remapping memory anyhow, so the
software doesn't see any gaps. There is no real problem.

--
Some informative links:
news:news.announce.newusers
http://www.geocities.com/nnqweb/
http://www.catb.org/~esr/faqs/smart-questions.html
http://www.caliburn.nl/topposting.html
http://www.netmeister.org/news/learn2quote.html

"CBFalconer" wrote in message ...
Tim Anderson wrote:
"Rick" wrote in message

3.5GB of this memory is usable. The chipset should allow the
remapping of unused memory above the 4GB address, but
this memory may not be accessible to an operating system that
has a 4GB memory limit.

Yes, this is helpful. Thanks. So the proper question may be this:
Why don't these 4GB boards remap this memory to make it usable by
64-bit operating systems?

Because it is an unnecessary expense. While there may be uses for
addressing over 4GB to conveniently handle large databases etc.,
there is virtually no real need for that much actual memory.

The function can be efficiently served by virtual memory systems at
little performance cost. About the only area where I can conceive
such real memory to be useful is in high performance video
processing.

Even for that, apps (at least if they're written properly) process
smaller chunks of ram at a time.

There are several other reasons for the 4GB limit:
o Historically, motherboards were limited by RAM VRM
---- lots of RAM sockets = lots of DIMMs = lots of watts = 30W+
---- RAM VRMs are limited - indeed BTX tackles their cooling
o Historically, memory is relatively expensive
---- PC industry focuses products around usage segments
---- multi-GB buyers tended to buy servers, not general PCs

As always, the computer industry migrates downwards.

Applications using multi-GB were once small & niche:
o dBase servers = RAM determinant
---- more RAM = more index keys, more caching
---- contrast with slow electromechanical HD rotation & seek
---- however such PCs are 1) Servers 2) 64-bit
o Photoshop production machines = RAM determinant
---- historically most pre-press is done on Apples
---- recently reinforced by the Dual G5 Apple re OS-X + 8GB+
---- Photoshop on the PC has deteriorating benefit of 2GB

Remember the focus of the IT industry:
o Create as many interfaces with buyers as possible per unit time
o Eg, CD, DVD, DVD-II speed & iteration, or mobile phone iterations

So some limits are based on guesswork "no-one will need 640KB"
and also one of creating a build-up of pressure for an obsolescence
replacement cycle as opposed to the migratory upgrade cycle.

So there are inherent assumptions in the PC architecture at work:
o Hardware both "internal" & from plug-in devices needs address space
---- efficiency has not been great here - PCI-Express has high demands
o Assumptions by the O/S come into play
---- XP may be based on NT, but it still makes relatively "old" assumptions

A lot of the assumptions are still valid somewhat:
o Most economic buyers of multi-GB = 64-bit = Dual-Opteron/Xeon64 Servers
o Many multi-GB buyers are after somewhat niche machines
---- Apple Dual G5 -- re Photoshop pre-press
---- downsized *nix box -- re displacing IBM/SUN/SGI boxes
o Eventually multi-GB buyers will migrate down to the commodity PC

It comes down to usage/economics:
o Yes CPUs can gobble memory vastly faster than a HD can move it
---- a typical RAID-10 array can manage 150MB/sec
---- a typical P4 CPU can manage 40x that on RAM to CPU transfers
o In reality very few applications run into that as a bottleneck
---- it's a tail-end of the usage distribution - few buyers, high cost, low margin
---- an error is to assume it remains that way - it never does :-)

Adding lots of memory to PCs has various electrical limitations, RAM VRM,
and various electrical dependencies required to maintain a stable I/O system.
That moves you away from mainstream buyer requirements re what they pay.
--
Dorothy Bradbury
www.dorothybradbury.co.uk for quiet NMB & Panaflo fans

In article ,
Dorothy Bradbury wrote:
There are several other reasons for the 4GB limit:
o Historically, motherboards were limited by RAM VRM
---- lots of RAM sockets = lots of DIMMs = lots of watts = 30W+
---- RAM VRMs are limited - indeed BTX tackles their cooling
o Historically, memory is relatively expensive
---- PC industry focuses products around usage segments
---- multi-GB buyers tended to buy servers, not general PCs






Are we talking virtual address space or physical memory here? I think
the latter.

Processes on a 64 bit OS see the same huge address space no matter how
much physical RAM you have. It's CPU and Mobo model implementation
specific.)

I just looked at amd.com and see that one of their Opteron chips has a
48 bit virtual address space and a 40 bit physical space.

The amount of memory you need depends on the working set of your
application mix.




--
a d y k e s @ p a n i x . c o m

Don't blame me. I voted for Gore.

I think the latter.

Yes - physical memory.

The amount of memory you need depends on the working set of your
application mix.

Which currently still makes multi-GB requirements somewhat niche.
Dbase servers, Photoshop pre-press - few boxes versus generic PCs.

Some Photoshop filters do require physical RAM to be a quite high
multiple of the image size for completion. Upgrading from 2GB to 4GB
can be frustrating in that considerably less than 4GB is realisable.

Still XP 2012 probably has some obscene memory requirements.
--
Dorothy Bradbury

Tim Anderson wrote:
The cheaper motherboards (ie. most of them) for P4 or Athlon 64
support a maximum of 4GB RAM, even if the motherboards and processors
support the AMD64 extensions that can address sqillions of GB. On the
other hand, more upmarket boards support more RAM - up to 24GB or
maybe more - for Xeon and Opteron.

If you buy one of these 4GB boards and install 4GB RAM, you don't get
the use of all of it. The top of the 4GB address space gets shadowed
by system functions such as PCI Express addressing. This is not just
a small detail - typically you lose 1GB of your 4GB. See:

http://www.itwriting.com/blog/?postid=152

I can't at the moment find a clear explanation of this. I understand
about the shadowing, but the question of course is why a modern board
can't use a higher range of addresses to make the full 4GB available
to the OS. The manufacturers mutter about "PC Architecture", but then
again they also make boards that *do* overcome this limit.

With PAE, PC processors have been able to address more than 4GB for
years. So why are we still running into this limit?

Three reasons:
1) Chipset support
2) BIOS support
3) Application support

Because we still have (or rather, had) a 32-bit virtual address space, and
there are very few apps which can use PAE. Use of addresses above the 4GB
mark requires a bit of fancy footwork from the OS, and memory above this
limit cannot be used in a general-purpose way. For example, it cannot be
shared among processes. Because of this, it has to be
allocated/deallocated/mapped using a different API than the usual malloc()
or whatever your language uses. Large database application and the like
support PAE, but your normal "consumer" application does not, and hence all
your consumer applications still have to fit into the original 4GB address
space.

On top of this, many chipsets simply do not support PAE. To support it, they
must support mapping memory banks above the 4GB line, and some only support
32-bit addresses for bank base addresses. Finally, the motherboard
manufacturer has to set up things correctly in the BIOS and due to the lack
of demand for 4GB RAM in the consumer space, I suspect it hasn't been at
the top of their list.

With a larger virtual address space (and the larger physical address space
for PCIe), most of these problems are removed. I don't know about support in
NF4 and other chipsets (manufacturers won't even tell you the most basic
interface details nowadays :( ) but given the NF4 Pro motherboards support a
large address space, I suspect it's more of a BIOS issue than anything.

--
Michael Brown
www.emboss.co.nz : OOS/RSI software and more :)
Add michael@ to emboss.co.nz ---+--- My inbox is always open