Problem statement:
I just bought a system with 4GB of physical RAM in it. The BIOS posts 4GB, but Windows tells me that I have anywhere from 2.75 - 3.5GB of RAM. Where is the rest of my RAM?

Summary:
If you are running 32-bit Windows, you must live with it. You will not ever see all 4GB of RAM you've paid for.

If you are running 64-bit Windows, you may have to live with it. Depending on your motherboard's chipset, your system may support memory remapping. If so, you will be able to use all 4GB of RAM.

Detailed:
Due to an architectural decision made long ago, if you have 4GB of physical RAM installed, Windows is only able to report a portion of the physical 4GB of RAM (ranges from ~2.75GB to 3.5GB depending on the devices installed, motherboard's chipset & BIOS).

This behavior is due to "memory mapped IO reservations". Those reservations overlay the physical address space and mask out those physical addresses so that they cannot be used for working memory. This is independent of the OS running on the machine.

Significant chunks of address space below 4GB (the highest address accessible via 32-bit) get reserved for use by system hardware:

• BIOS – including ACPI and legacy video support
  
• PCI bus including bridges etc.
  
• PCI Express support will reserve at least 256MB, up to 768MB depending on graphics card installed memory
  

More detail and source: http://blogs.msdn.com/hiltonl/archive/2007...am-problem.aspx

From reports I've read, this is the same as the amount of RAM that the Graphic card has, thus if the Graphic card you are using is an SLi'ed one, with each using 1GB of RAM, be prepared to lose 2GB from the memory addressing space that windows allocated

From reports I've read, this is the same as the amount of RAM that the Graphic card has, thus if the Graphic card you are using is an SLi'ed one, with each using 1GB of RAM, be prepared to lose 2GB from the memory addressing space that windows allocated

For Windows Vista to use all 4 GB of memory on a computer that has 4 GB of memory installed, the computer must meet the following requirements:
• The chipset must support at least 8 GB of address space. Chipsets that have this capability include the following:
• Intel 975X
• Intel P965
• Intel 955X on Socket 775
• Chipsets that support AMD processors that use socket F, socket 940, socket 939, or socket AM2. These chipsets include any AMD socket and CPU combination in which the memory controller resides in the CPU.
• The CPU must support the x64 instruction set. The AMD64 CPU and the Intel EM64T CPU support this instruction set.
• The BIOS must support the memory remapping feature. The memory remapping feature allows for the segment of system memory that was previously overwritten by the Peripheral Component Interconnect (PCI) configuration space to be remapped above the 4 GB address line. This feature must be enabled in the BIOS configuration utility on the computer. View your computer product documentation for instructions that explain how to enable this feature. Many consumer-oriented computers may not support the memory remapping feature. No standard terminology is used in documentation or in BIOS configuration utilities for this feature. Therefore, you may have to read the descriptions of the various BIOS configuration settings that are available to determine whether any of the settings enable the memory remapping feature.
• An x64 (64-bit) version of Windows Vista must be used.
Contact the computer vendor to determine whether your computer meets these requirements.

Note When the physical RAM that is installed on a computer equals the address space that is supported by the chipset, the total system memory that is available to the operating system is always less than the physical RAM that is installed. For example, consider a computer that has an Intel 975X chipset that supports 8 GB of address space. If you install 8 GB of RAM, the system memory that is available to the operating system will be reduced by the PCI configuration requirements. In this scenario, PCI configuration requirements reduce the memory that is available to the operating system by an amount that is between approximately 200 MB and approximately 1 GB. The reduction depends on the configuration.

Good work. Finally a properly documented thread after so many questions and posts about this.

linkinstreet: Slight more than that. Haven't you read the MMIO part? So it's actually at least 2GB you'll be losing, says if you have GC with 2GB memory on it. Not just 2GB.

do you mean that if we installed 4GB of Ram Vista will only recognize 3.5GB only. i had that problem too then i upgraded to sp1, vista sees my ram as 4GB.

Hrmm.

My task manager seems to be reporting some weird available ram readings.

I'm currently have installed 3gb ram in 570sli mobo.

Looks at my screenie below. Is this normal?

Hrmm.

My task manager seems to be reporting some weird available ram readings.

I'm currently have installed 3gb ram in 570sli mobo.

Looks at my screenie below. Is this normal?

If you install 8 GB of RAM, the system memory that is available to the operating system will be reduced by the PCI configuration requirements. In this scenario, PCI configuration requirements reduce the memory that is available to the operating system by an amount that is between approximately 200 MB and approximately 1 GB. The reduction depends on the configuration.

Vista is memory hungry  

yo guys... i tot i heard a lot about vista 64bits having compatibility issues with programs and even worse, GAMES... it jz happens to crash half-way or doesn't work at all... jz how many of us out there is going to last a full 24-hours without gaming?

yo guys... i tot i heard a lot about vista 64bits having compatibility issues with programs and even worse, GAMES... it jz happens to crash half-way or doesn't work at all... jz how many of us out there is going to last a full 24-hours without gaming?