PC 4GB RAM limit

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?

Tim

"Tim Anderson" <timjand@no_spam_hotmail.com> wrote in message news:...
> 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?

Because *applications* have to be written to be PAE-aware,
and have to be written in 64-bit address space to make use of
anything over 4GB. This was (and still is) very much a niche
market -- very few desktop/workstation users will come close
to using 2GB, let alone 4, which is why MS originally split the
4GB address range between 2GB user space and 2GB
system space.

"Rick" <> wrote in message
news:aL_ge.6$ t...

> Because *applications* have to be written to be PAE-aware,
> and have to be written in 64-bit address space to make use of
> anything over 4GB. This was (and still is) very much a niche
> market -- very few desktop/workstation users will come close
> to using 2GB, let alone 4, which is why MS originally split the
> 4GB address range between 2GB user space and 2GB
> system space.

Rick,

Thanks for the response. I understand about that; what I'm talking about is
the hardware issue. When you boot with 4GB installed on one of these
motherboards, typically 1GB is not made available to the OS at all; it is
"consumed" before the OS boots (because of address shadowing). Even with an
AMD64 (EM64T) CPU installed.

Tim

"Tim Anderson" <timjand@no_spam_hotmail.com> wrote in message news...
> "Rick" <> wrote in message
> news:aL_ge.6$ t...
>
> > Because *applications* have to be written to be PAE-aware,
> > and have to be written in 64-bit address space to make use of
> > anything over 4GB. This was (and still is) very much a niche
> > market -- very few desktop/workstation users will come close
> > to using 2GB, let alone 4, which is why MS originally split the
> > 4GB address range between 2GB user space and 2GB
> > system space.
>
> Rick,
>
> Thanks for the response. I understand about that; what I'm talking about is
> the hardware issue. When you boot with 4GB installed on one of these
> motherboards, typically 1GB is not made available to the OS at all; it is
> "consumed" before the OS boots (because of address shadowing). Even with an
> AMD64 (EM64T) CPU installed.

The Intel D915PBL (the board referenced in your link) is a desktop
board:
ftp://download.intel.com/design/moth...l/C6859901.pdf

The board utilizes 4 GB of addressable system memory. Typically
the address space that is allocated for PCI Conventional bus add-in
cards, PCI Express configuration space, BIOS (firmware hub), and
chipset overhead resides above the top of DRAM (total system
memory). On a system that has 4 GB of system memory installed, it
is not possible to use all of the installed memory due to system
address space being allocated for other system critical functions.

These functions include the following:

BIOS/firmware hub (2 MB)
Local APIC (19 MB)
Digital Media Interface (40 MB)
Front side bus interrupts (17 MB)
PCI Express configuration space (256 MB)
MCH base address registers, internal graphics ranges, PCI Express ports (up to 512 MB)
Memory-mapped I/O that is dynamically allocated for PCI Conventional and
PCI Express add-in cards

The amount of installed memory that can be used will vary based
on add-in cards and BIOS settings. Figure 14 shows a schematic
of the system memory map. All installed system memory can be
used when there is no overlap of system addresses.

Thanks again Rick; yes, I'm familiar with that document and I've read it
before.

My question was, to quote from my original message:

"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?"

Tim

In article <aL_ge.6$> ,
Rick <> wrote:
>"Tim Anderson" <timjand@no_spam_hotmail.com> wrote in message news:...
>> 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?
>
>Because *applications* have to be written to be PAE-aware,
>and have to be written in 64-bit address space to make use of
>anything over 4GB. This was (and still is) very much a niche
>market -- very few desktop/workstation users will come close
>to using 2GB, let alone 4, which is why MS originally split the
>4GB address range between 2GB user space and 2GB
>system space.
>
>


Real (tm) operating systems, such as VMS, have done something
splitting the address space this for decades.

What OS were you running?

So what happens on a dual CPU opteron system with 8 memory slots
(and a 64 bit OS)?

I think you'll find that on a PAE mobo the CPU never sees a hardware
address higher than the 4GB limit. Hardware registers external to the
CPU are set by the PAE drivers and memory mapping is done to show
different segments of physical memory to the CPU.

What OS are you using,

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

Don't blame me. I voted for Gore.

"Al Dykes" <> wrote in message
news:d624u6$brl$...

> Real (tm) operating systems, such as VMS, have done something
> splitting the address space this for decades.
>
> What OS were you running?

This is not an OS issue, the memory is made unavailable before the OS boots.

But for the record, dual boot Win XP x64 and Suse Linux 9.3 64-bit.

> So what happens on a dual CPU opteron system with 8 memory slots
> (and a 64 bit OS)?

That's what I'm wondering, as these systems support more than 4GB RAM
successfully.

Tim

Rick writes:

> These functions include the following:
>
> BIOS/firmware hub (2 MB)
> Local APIC (19 MB)
> Digital Media Interface (40 MB)
> Front side bus interrupts (17 MB)
> PCI Express configuration space (256 MB)
> MCH base address registers, internal graphics ranges, PCI Express ports (up to 512 MB)
> Memory-mapped I/O that is dynamically allocated for PCI Conventional and
> PCI Express add-in cards

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

--
Transpose hotmail and mxsmanic in my e-mail address to reach me directly.

"Mxsmanic" <> wrote in message
news:...
> 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? Sounds like a
32-bit issue; but especially for boards that support AMD64 this is all very
unfortunate.

Tim

Tim Anderson wrote:
> "Rick" <> wrote in message
>
>> Because *applications* have to be written to be PAE-aware,
>> and have to be written in 64-bit address space to make use of
>> anything over 4GB. This was (and still is) very much a niche
>> market -- very few desktop/workstation users will come close
>> to using 2GB, let alone 4, which is why MS originally split the
>> 4GB address range between 2GB user space and 2GB
>> system space.
>
> Thanks for the response. I understand about that; what I'm talking
> about is the hardware issue. When you boot with 4GB installed on
> one of these motherboards, typically 1GB is not made available to
> the OS at all; it is "consumed" before the OS boots (because of
> address shadowing). Even with an AMD64 (EM64T) CPU installed.

Because once you assign a group of memory addresses for non-memory
(i.e. i/o) use you have to decode that range. Not only do you have
to designate a range for i/o, you also have to designate the
complementary range for actual memory. The decoders involve added
loads on the address lines, and added propagation delay. So you
want to minimize the bits used in this, and possibly strengthen
their output buffers on the CPU chip.

All of which costs time, power, and performance.

I expect the main problems are due to the added capacitance from
added fan-out. This is similar to the often encountered
difficulties when further memory modules are added to a system.

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

--
Chuck F () ()
Available for consulting/temporary embedded and systems.
<http://cbfalconer.home.att.net> USE worldnet address!