An Example

Motherboard configuration

Good 256 KB 5160 motherboard with 256 KB fitted. No RAM cards fitted. Motherboard switches 3 and 4 set to four banks.
No cards fitted other than a CGA card.
Both IBM ROMS removed. Landmark/SuperSoft ROM fitted.

Weird bit

Diags test up to 640 KB.
Add the IBM floppy controller. Diags now only test as high as 256 KB.
Remove IBM floppy controller. Diags test up to 640 KB.

I could not replicate that behaviour on a 640 KB 5160 motherboard - the diags always tested up to 640 KB.