Memory efficient way to iterate on a big range

I want to compare two really big data sets byte-by-byte in a specific range.

So for example I'd have one 0x80000000 bytes long file and another 0xffffffff bytes long file. And lets say I'd like to compare these two files in a specific range byte-by-byte from 0x1000 to 0x7200000.

If it was a smaller range I'd probably go for until, but now since it's a much bigger range until would be pretty inefficient memory wise.

How would one implement such basic operation in a functional and memory efficient way?