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Your computer supports a maximum of 4 GB of memory when you use four 1-GB DIMMs. Current operating systems, such as Microsoft® Windows® 2000 and
Windows XP, can only use a maximum of 4 GB of address space; however, the amount of memory available to the operating system is slightly less than 4 GB.
Certain components within the computer require address space in the 4-GB range. Any address space reserved for these components cannot be used by
computer memory.
 The following components require memory address space:
l  System ROM
l  APIC(s)
l  Integrated PCI devices, such as network connectors
l  PCI and AGP cards
l  The AGP aperture size specified in system setup
 At start-up, the BIOS identifies the components that require address space. The BIOS dynamically calculates the amount of reserved address space required.
The BIOS then subtracts the reserved address space from 4 GB to determine the amount of usable space.
l  If the total installed computer memory is less than the usable space, all installed computer memory is available for use only by the operating system.
l  If the total installed computer memory is equal to or greater than the usable address space, a small portion of installed memory is available for use by
the operating system.