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Can I use one CPU in a dual processor motherboard?
29

Oct, 2019

One question we get asked at least one time each week is whether or dual socket motherboards meant for dual processors can work with one CPU. A common driver behind this question is because dual processor motherboards allow for easy expansion at a later date. Add a second CPU, heatsink/ fan and additional memory to a system without replacing any internal components or adding a second machine. The answer to the question “Can I use one CPU in a dual processor motherboard?” is yes… with some caveats. Let’s explore why.

The vast majority of motherboards “just work” if you install a CPU into the lowest numbered CPU slot. This is usually CPU0 if there is CPU0 and CPU1 or CPU1 if the board is tagged CPU1 and CPU2. This is true for both Intel and AMD motherboards in general and when in doubt, most motherboard manuals will instruct a user as to which processor slot can be populated by a single CPU and a dual CPU motherboard. Here is an example from an ATX X79DUAL motherboard that was a fairly popular dual processor Intel C600 series Xeon motherboard.
In the Intel socket LGA1366 days, and with the AMD C32 and G34 platforms, both the northbridge and southbridge were found on the motherboard so the CPUs generally connected through the northbridge to get to an expansion card bus. In these systems, the main impact of only having one CPU (other than the obvious loss in CPU performance without the second CPU) is that one could not populate the memory banks for the empty processor slot.
SZMZ X79DUAL Layout
SZMZ X79DUAL Layout
Today’s architectures add another level of complexity. With the dual Intel Xeon E5 LGA2011 platform as an example, the PCIe expansion slots are directly tied to a CPU. On the picture above, one can see each PCIe slot tied to a specific CPU. When a CPU socket is not populated, the PCIe lanes do not terminate. This has a major impact on the functionality of the motherboard. For example, if the second CPU socket is not filled in a dual socket motherboard oftentimes onboard devices such as Ethernet and SAS controllers may not work. Furthermore, PCIe lanes may not work. This will render the slots useless until the additional CPU(s) are added. Here is an associated diagram for the SZMZ X79DUAL:
SZMZ X79DUAL Block Diagram
SZMZ X79DUAL Block Diagram
Here we can see that if CPU1 was missing, the onboard Intel X540 10GbE LAN and LSI SAS2208 controller do not have a PCIe but to hang off of. Conversely, with no CPU2, one would not have access to several PCIe 3.0 x8 slots. Luckily, most motherboard manufacturers state in their manuals which socket to populate.

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