Why is that then when I use this:

mov rax,[art]
art dq ?

It compiles fine. But when I change the above register to rcx or r10;

e.g

mov rcx,[art]
art dq ?


or any other 64 bit register I get the following message "Error; Invalid size of address value". This happens under Linux and Windows. If I compile the same code in YASM, I have no problems.

Yes, a rather serious bug in the 64-bit object output. It's fixed in 1.61.1 development release (see the bottom of Download page)