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I'm doing some logging and noticed that on 64-bit platforms I can use the /64bit switch to run 64-bit dxdiag. I assume there must be some reason for doing this, namely additional functionality in the 64-bit version (or incorrect/insufficient functionality when using the 32-bit version on x64). What exactly are the differences?

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One is a 64-bit process the other is a 32-bit process. So the only differences would be in how a 64-bit Windows installation would treat a 32-bit process. – Ramhound Jun 28 '12 at 14:06
up vote 3 down vote accepted

From personal experience, dxdiag can also be used to determine if there is a problem with the 32 or 64 bit portions of the drivers. For example, running dxdiag in 32 bit might show no acceleration for DirectDraw or D3D; yet running DXDiag64 might show everything as fine.

This is useful when trying to figure out video issues, particularly since most games execute in x86 (32 bit) mode only.

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No, there is nothing "extra" when running DXDiag in 64-bit mode, it simply runs the native 64-bit executable files instead of the 32-bit files. The 64-bit ones will not need to be emulated or done in compatibility mode and will be able to make the maximum usage of the system, so they (should) be able to run faster and more reliably.

(That said, it would be possible that in 32-bit mode, it has to skip some tests that would only be possible in 64-bit mode, similar to how a game can have "extra" content in the form of special graphical effects that are only available with a fancier video-card that supports them. However, that doesn't really apply to DXDiag; speed and stability are really the only "extras" you get.)

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besides the driver info, the 32-bit dxdiag has a video memory limited to 4095, while the 64-bit dxdiag has a not easily reachable limit (I couldn't find it).

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Yes there is "extra" info, all drivers show their driver date. In a laptop that has dual GFX adapters only the primary would show it's driver daye and the secondary that is buried further down wouldn't have a driver date.

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