When starting the program R12 ... R15 are all zeroes.
When I am in WNDPROC -- R13 is same as RDX (message identifier).
But I was thinking that R13 is saved by Windows x64 API.
I am sure that R13 is saved by a usual API call, like GetDlgItem.
But here we are looking at a CALLBACK function.
R12, R14, R15 are all zeroes, just like they have been at the Entry Point.
Weird, but OK.

When Windows calls your callback function it is its copy of the registers that your see, not your copy.

Just like when you can set r12-r15 to anything of your choosing and then call an API, Windows doesn't expect you to call with the original values that r12-r15 had when the program was started.

I see.
Thanks, that sounds good.

That is not part of any spec. It appears to be a hack to avoid leaking secrets or something.

I wouldn't recommend relying on that behaviour..

Sorry to hijack this thread to ask a simple question: Is Handle value in Windows 64-bit API function also 64-bit?

Yes.

Thank you!