Actually, that was pretty doable too. Later, my father bought D40, which was a 5.25 inch disk unit produced by Czech company. The D40 had its own 16kB ROM which

Actually, that was pretty doable too. Later, my father bought D40, which was a 5.25 inch disk unit produced by Czech company. The D40 had its own 16kB ROM which got mapped instead of the standard Spectrum ROM when NMI was invoked (the D40 ROM contained code to handle the disks). So it was totally possible for an external device to remap original ROM with something else, even RAM.