What is the effect of the /3GB switch on physical memory?

[Guest]

I know this topic has been touched on several times before, but i was looking
for some fresh information.

I work in tech support for an Autodesk reseller. One of the products we
support is a RAM thirsty application called Revit. In one of our recent
support cases, Autodesk suggested that we have a client upgrade to 4GB of ram
and enable the 3GB Switch. i have been doing research into the 3GB switch,
and have been unable to determine what the effect of the 3GB switch is on
PHYSICAL memory, all the documentation i have found details what the 3GB
switch does for VIRTUAL memory.

So my question is two fold,

1. How does 32 Bit Windows handle 4GB of ram
2. How does the 3GB switch effect the physical ram on a computer

 

[John Butler]

Jeff
1 Windows XP 32-bit uses some of 4 GB RAM below the line and only makes 3 GB
available for applications
2 The switch has no effect if you have only 4 GB RAM in 32-bit Windows
3 If you are using a 32 bit OS, the last few hundred mb of the address space
are reserved for hardware stuff like DMA. (Check this article
technet.microsoft.com/en-us/library/bb457155.aspx )
..

In my experience Revit does not need more than 3GB of RAM. See below for
published requirements:

a.. Intel® Pentium® 4 processor, 2.8 GHz, or equivalent AMD Athlon®
processor
b.. Microsoft® Windows® XP Professional (SP2 or later)
c.. 2 GB RAM
d.. 1 GB free disk space
e.. 1280 x 1024 monitor and display adapter capable of 24-bit colour
f.. Two-button mouse with scroll wheel
What sort of RAM have you got to use?

 

[Bob I]

I know this topic has been touched on several times before, but i was looking
for some fresh information.

I work in tech support for an Autodesk reseller. One of the products we
support is a RAM thirsty application called Revit. In one of our recent
support cases, Autodesk suggested that we have a client upgrade to 4GB of ram
and enable the 3GB Switch. i have been doing research into the 3GB switch,
and have been unable to determine what the effect of the 3GB switch is on
PHYSICAL memory, all the documentation i have found details what the 3GB
switch does for VIRTUAL memory.

So my question is two fold,

1. How does 32 Bit Windows handle 4GB of ram
2. How does the 3GB switch effect the physical ram on a computer

1. There is only 4 GB of address space and hardware addressing has to
come out of that so you don't have access to the RAM for those
addresses. That typically means that most of the 4th GB of RAM is
unaddressable.
2. Shifts the user program RAM from 2 GB to 3 and kernel RAM from 2 GB
to 1.

 

[jorgen]

So my question is two fold,

1. How does 32 Bit Windows handle 4GB of ram

As you've read elsewhere, they can handle around 3GB, plus/minus some
megabytes.

2. How does the 3GB switch effect the physical ram on a computer

It has no effect on physical memory. It only changes how the virtual
address space is split between user and kernel

 

[Guest]

First off, thanks for responding so quickly to my questions!

The reason i posted this thread is that i disagree with Autodesk's solution
for my client; that they need a 4th GB of ram and have to enable the 3GB
switch.

So enabling the 3GB switch will free up more Virtual ram space for
applications regardless of the amount of physical ram in a machine...

Is there a general rundown on how applications divide themselves between
Physical and Virtual RAM?

Which would be more effective,
adding a 4th gig of ram or enabling the 3GB switch?

The user is getting an error stating that the application has run out of
memory.

 

[John John]

I know this topic has been touched on several times before, but i was looking
for some fresh information.

I work in tech support for an Autodesk reseller. One of the products we
support is a RAM thirsty application called Revit. In one of our recent
support cases, Autodesk suggested that we have a client upgrade to 4GB of ram
and enable the 3GB Switch. i have been doing research into the 3GB switch,
and have been unable to determine what the effect of the 3GB switch is on
PHYSICAL memory, all the documentation i have found details what the 3GB
switch does for VIRTUAL memory.

So my question is two fold,

1. How does 32 Bit Windows handle 4GB of ram

Being that you posted in an XP group I will keep the information here as
it applies to 32-bit Windows XP. With 32-bit Server 2003 or Vista using
Physical Addressing Extensions (PAE) the way this is handled is different.

32-bit Windows can address a total of 4GB of memory, it cannot address
any memory above the 4GB barrier. Memory doesn't mean RAM specifically,
it means memory for everything in the computer. Video cards, PCI
devices and others also need memory addresses, they too must use
addresses below the 4GB barrier. If you add the requirements for all
the devices plus the requirements for the installed RAM, and if the
requirements exceeds 4GB, addressing space will be made available to
other devices before the RAM so there will not be enough addresses
remaining to fully supply the RAM addressing requirements. For example,
high end workstations using CAD/CAM software are usually equipped with
good video cards, lets say that the video card has 512MB of memory, the
address space requirements for the installed RAM and the video card is
4.5GB, Windows will take 512MB of address space at the top of the memory
range and reserve it for the video card, now there is only 3.5GB of
addressing left so you can only use 3.5 of the 4GB of installed RAM.
When you look at the reported memory figure in the System Properties you
will not see all of the installed RAM reported, you will see 3.5GB, you
will not be able to use the full 4GB of installed RAM. Keep in mind
that there is more than just the video card that needs reserved memory
addresses so the amount of usable RAM will vary, for most XP computers
it usually is anything between 2.75 to 3.5GB.

2. How does the 3GB switch effect the physical ram on a computer

It affect absolutely nothing with the "physical" RAM. It affects the
way Windows allocates "Virtual Memory Addresses". Once again, 32-bit
Windows can allocate a total of 4GB of "Virtual Address Space" (VAS is
not the same thing as discussed in #1 above). This virtual address
space is divided equally between user process and the operating system.
Each user process can make use of (its own) 2GB of Virtual Address
Space and the operating system can make use of the other 2GB. Using the
/3gb switch changes the way this is allocated. When using the switch
the operating system is limited to 1GB of virtual address space and user
processes can use 3GB. In order to use the increased address space the
applications must be LargeAddressAware, they must be written to use more
than the normal 2GB of virtual address space.

Hope that makes things a bit easier to understand.

John

 

[Tim Slattery]

I know this topic has been touched on several times before, but i was looking
for some fresh information.

I work in tech support for an Autodesk reseller. One of the products we
support is a RAM thirsty application called Revit. In one of our recent
support cases, Autodesk suggested that we have a client upgrade to 4GB of ram
and enable the 3GB Switch. i have been doing research into the 3GB switch,
and have been unable to determine what the effect of the 3GB switch is on
PHYSICAL memory, all the documentation i have found details what the 3GB
switch does for VIRTUAL memory.

The /3GB switch has NO effect on physical RAM. It tells the OS to
divide the 4GB virtual space given to each process into 3GB for the
program and 1GB for the OS. Normally it's divided in half: 2GB for
each side. Very few programs need the /3GB switch, it should not be
used except in very rare cases. This is VIRTUAL address space, not
physical memory.

So my question is two fold,

1. How does 32 Bit Windows handle 4GB of ram

See http://members.cox.net/slatteryt/RAM.html

The hardware provides a 4GB address space. This must be used to
address the BIOS, video memory, and other things presented by your PCI
or PCIx cards in addition to the physical RAM. The upshot is that you
cannot actually use 4GB of physical RAM in a 32-bit machine.

2. How does the 3GB switch effect the physical ram on a computer

As I said above, it does not affect physical RAM in any way.