Yes, Eicar isn't detected "on execution" (i.e. when you doubleclick on it) on 64bit operating systems because it's a DOS COM file and there isn't any 16bit subsystem on 64bit Windows that would be able to execute it. So there is no execution being started, Avast isn't called to scan the file and the file isn't detected (Windows just gives you an error that the file cannot be started).
That's as expected and it's nothing to worry about. It would be detected on 32bit operating systems (because it can be started there), and it doesn't mean that Avast isn't scanning for viruses (those that actually can be started).
What if Eicar.com is run in dosbox on a 64bit Windows?
You're the curious one so why don't you try it.
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Because... I was posting at 12:26:37 AM
Now I've just tried, and Avast did not detect Eicar.com
execution in Dosbox on my 64bit Windows.
I guess such behavior is expected, is that right Igor?
EDIT:
After a while I've noticed "Virus Found ... Process PID 4292", see screenshot below.
Perhaps, Avast does not detect execution but it detects the Dosbox process using the file anyway.
And, Avast DOES detect eicar.com when I download it.
Bob, I don't care about Eicar.com as such, but I am curious because I have some old DOS and WIN 16bit programs which I might execute in virtual boxes sometimes.