* Improve abstraction in vmem.h and limit memory benchmarking to first 2GB.
The third GB may get used for remapping memory regions that are only
accessed during startup, so it's not safe to use it for the memory
speed tests.
* Fix calculation of end limit for locating memory benchmark workspace.
* Document vmem.h.
* Use window number, not current start address, to detect first window.
* Increase the program low-load range from 1MB to 4MB and make more robust.
If the BIOS has reserved some parts of low memory, there may not be
enough contiguous space left to load the program there (issue #49).
So increase the low-load range to include the first 3MB of high
memory. Also guard against the program being initially loaded
straddling the new boundary.
Co-authored-by: Martin Whitaker <memtest@martin-whitaker.me.uk>
