Well, for one thing, we aren't sure (or at least I'm not sure) that the Op's application uses files in increments of 16 bits. Maybe this is download to an 8-bit uP? The OP's spec appears to be a bizarre thing where only a "window" of the binary file is check-summed. I've never seen anything like that before.

I am not at all sure that performance is an issue here at all! I think the goal should be clear, understandable code and then work on performance optimization later.

I just made a post to try to explain a simple way to do what needs to be done.
If that doesn't work, I at least feel like I tried.

BTW: increasing the multiple of 4K bytes may not help all that much in terms of execution performance. This depends upon the O/S and file system AND how many other disk intensive processes are running at the same time. Depending upon the OS and file system, these 4K units can wind up being scattered around over the disk surface. Your processes is not calculating if it is waiting for the disk to finish some very large request. Sometimes there is an optimal "quanta" of info to request. I think that discussion is well beyond the scope of the original question.

In reply to Re^4: Accessing individual bytes of a binary file as numerical values by Marshall
in thread Accessing individual bytes of a binary file as numerical values by Chris01234

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