According to this article:
https://www.lifewire.com/windows-memory-diagnostic-review-2626154
The Windows Memory Diagnostic Tool for Windows 7 only scans the first 4GB of RAM for errors. This Super User question regarding Windows 10 (which is NOT a duplicate of this question) repeats that claim, but provides no source for it:
Does Windows Memory Diagnostics Tool scan RAM over 4GB?
The comments for that question debate whether there really is a 4GB limit for the Windows 7 tool.
This article makes no mention of a 4GB limit:
So, will the Windows Memory Diagnostic Tool that is included with Windows 7 check all available RAM or only the first 4GB?
How can you tell?
EDIT: Super User super helper Moab and I have been discussing the XML results of the WMDT in the comments, so I am posting the results here:
<Event xmlns="http://schemas.microsoft.com/win/2004/08/events/event">
- <System>
<Provider Name="Microsoft-Windows-MemoryDiagnostics-Results" Guid="[GUID]" />
<EventID>1201</EventID>
<Version>0</Version>
<Level>4</Level>
<Task>0</Task>
<Opcode>0</Opcode>
<Keywords>0x8000000000000000</Keywords>
<TimeCreated SystemTime="[TIME]" />
<EventRecordID>718360</EventRecordID>
<Correlation />
<Execution ProcessID="4880" ThreadID="4885" />
<Channel>System</Channel>
<Computer>[NAME]</Computer>
<Security UserID="[ID]" />
</System>
- <UserData>
- <Results xmlns:auto-ns2="http://schemas.microsoft.com/win/2004/08/events" xmlns="http://manifests.microsoft.com/win/2005/08/windows/Reliability/Postboot/Events">
<CompletionType>Pass</CompletionType>
</Results>
</UserData>
</Event>
