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Windows computer with 4GB RAM

Basically, there are two switches for Boot.ini to address the scenarios in
which 4 GB or more RAM is installed on a Windows XP computer. They are /PAE
and /3GB. You may added these two switches to the entry in the Boot.ini

When the /3GB switch is used with Windows XP Professional, the user-mode
memory space is now limited to 3 GB. As the normal applications, such as
Adobe Photoshop, are running in the user mode, so you are recommended to
add this /3GB switch to the boot.ini file.

PAE is an Intel-provided memory address extension that enables support of
greater than 4 GB of physical memory for most 32-bit (IA-32) Intel Pentium
Pro and later platforms. PAE is supported only on 32-bit versions of the
Windows operating system. In PAE mode, data can be present at a physical
address that is larger than 4 GB.

However, in Windows XP SP 2, the System Properties dialog box may report
approximately 3.12 GB of RAM, and the System Information tool may report
that the total physical memory is approximately 2,770.00 MB. This is
because that to reduce driver compatibility issues, Windows XP Service Pack
2 includes hardware abstraction layer (HAL) changes that mimic the 32-bit
HAL DMA behavior. The modified HAL grants unlimited map registers when the
computer is running in PAE mode. Additionally, the kernel memory manager
ignores any physical address that is greater than 4 GB. Any system RAM that
is greater than the 4 GB barrier would be made unaddressable by Windows and
be unusable in the system. I

More information
A description of the 4 GB RAM Tuning feature and the Physical Address
Extension switch

The amount of RAM reported by the System Properties dialog box and the
System Information tool is less than you expect after you install Windows
XP Service Pack 2

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