All of Intel and AMD's motherboards that have the correct socket type for a 64-bit processor can operate in 64-bit mode. The socket is the part of the motherboard that connects to the processor. The motherboard may need a BIOS update to run a 64-bit processor when upgrading from a 32-bit one. If you are wondering if a motherboard you are running is 64-bit compatible it may be faster to first check if the processor you are running is 64-bit compatible. If the processor is not, you can check the motherboard's socket type.
Check the Socket Type With CPU-Z
Download, install and open CPU-Z (see References for a link).
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Click on the CPU tab.
Read the "Package" section. As an example, an AMD Phenom II processor could come up as having a "Socket AM3 (938)" connection.
Check to see if the socket type supports 64-bit processors in the Tips section.
Check Through Windows
Click on the Start menu.
Right-click on "My Computer" and select "Properties."
Click the Windows Experience Index.
Select "view and print detailed performance and system information."
Read the "System" section to determine if the computer is running in 64-bit or is 64-bit capable. A yes answer for either means the motherboard is 64-bit.
Check With a Spec Sheet
Refer to either the motherboard's features section on the spec sheet or the diagram. This information is usually included with the computer's manual.
Look for the processor connection socket type.
Compare the socket type to the list of 64-bit compatible sockets in the Tips section.
Things You'll Need
Motherboard spec sheet
The Intel supporting motherboards with sockets 478, 479, LGA 775, LGA 1156, LGA 1366 and later models are 64-bit motherboards.
AMD supporting motherboards with 754, 939, 940, AM2, AM2+, AM3, and later models are 64-bit motherboards.
Intel Motherboards that use the socket 478 connection support 32-bit and 64-bit processors. If you are running a Pentium 4 processor and Windows claims the computer is not 64-bit capable, the motherboard might still be 64-bit compatible. You can upgrade the processor to a 64-bit Pentium 4 from a 32-bit Pentium 4 on a Socket 478. In all other cases, if the processor does not support 64-bit processing the motherboard it is fit on does not either. All motherboards that have Intel sockets made before Socket 478 do not support 64-bit computing. Note that you may have to update a Socket 478 motherboard's BIOS to use it in 64-bit mode. Motherboards running Pentium 4 processors with Socket 423 are not 64-bit compatible.
AMD motherboards are more straightforward. If Windows claims the processor can't run in 64-bit mode, the motherboard is not 64-bit compatible. The AMD Athalon XP processor, which runs on the Socket 462 connection, is 32-bit, while the Athalon 64 processor uses the Socket 754 connection and is 64-bit compatible.