I'll post this here in case anyone else is searching for this issue
Since my computer updated to Windows 10, build number 1607 (also known as the "Anniversary Update", because it was a feature release timed to coincide with a year since the first release of Windows 10), I've been getting an error when I try to run any command in a command prompt window:
[0x7FFCF5BB70E3] ANOMALY: use of REX.w is meaningless (default operand size is 64)
The first part, in square brackets, is a hexadecimal number (as seen by the "0x" with which it begins). I do not know what that refers to. It is always the same on my computer. I believe it is unique to me - I've seen other people report this error online, and they gave a different hex number. I also searched on Google for 0x7FFCF5BB70E3 and found nothing.
It seems this is a conflict between Trend Micro's anti-virus protection and Windows 10 build 1607. Reading other webpages that discuss this, it seems Trend Micro have solved this, but have not yet issued a program update (through the built-in updates mechanism) to address. Why they've not released their fix is beyond me.
However, apparently, if you ask their support department, they'll give you the hotfix you need.
Sure enough, one short, very helpful and efficient chat session later, I have a link to their website which tells you where to download the hotfix and how to apply it. It's not hard. Visit https://esupport.trendmicro.com/en-us/home/pages/technical-support/11154... for more information. (That link no longer works - see comment below for more details.