With a total of 4GB (gigabytes) of memory installed in the computer, the amount of available memory (memory that is available to the operating system and to applications) will typically be less than 4GB. The amount of available memory will vary from model to model, depending upon the combination of several factors, including whether the CPU, system controller chip set, BIOS, and OS are of the 32 bit or the 64 bit variety.  

If the computer has a 32 bit CPU, a chipset which is limited to 32 bit memory addressing, a 32 bit BIOS, or a 32 bit OS, then most of the 4th gigabyte of the computer's memory address space will necessarily be pre-allocated for PCI and graphics use, limiting the computer's total available memory to roughly 3GB.

If all of these components -- CPU, chipset, BIOS and OS -- fully support 64 bit memory addressing, then the computer's available memory can be greater -- nearly 4GB for some models. Computer manufacturers have limited control over the design of these components, which are produced by other manufacturers. As of August 2007, some of the chipsets used in Toshiba notebook computers -- including some AMD chipsets and the Intel 965 chipset -- support 64 bit memory addressing.

BIOS versions for the models listed above offer varying degrees of support for 64 bit memory addressing. For some models, relatively newer versions of the BIOS may increase somewhat the amount of available memory. Toshiba recommends periodically checking the Service and Support website at http://pcsupport.toshiba.com for newer versions of the BIOS for your computer.

Even without complete support for 64 bit memory addressing (by the CPU, BIOS, chipset and OS), the computer's performance may be significantly improved by expanding its memory to the maximum capacity of 4GB.

For more information, see the Microsoft Knowledge Base article on this topic: The system memory that is reported in the System Information dialog box in Windows Vista is less than you expect if 4 GB of RAM is installed .

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