"Lynn McGuire" wrote:
My DLL is written in 700K lines of Fortran, C and C++
code. But that is all built at compile and link time.

Not necessarily. In some architectures, a "late optimization phase" is
invoked when the DLL is loaded and linked to an application. This supports
a single binary DLL that can be used on a variety of architectures.

That is what I was alluding to earlier.

However, it's a moot point. I suspect we have established that the key
factor is _PC_53 v. _PC_64 mode.

I am still awaiting the results of your trying _PC_53 mode. But I believe
my explanation of your results with _PC_64 mode most likely point _PC_53
mode as the solution.