Complex sentence: However, even with video RAM, the ROM BIOS and I/O ports for expansion cards, much of this 384 KB of address space was unused and as the 640 KB memory restriction grew tighter, techniques were found to fill the empty areas with RAM.
Split into simpler sentences: However, even with video RAM, the ROM BIOS and I/O ports for expansion cards, much of this 384 kiB of address space was unused. And as the 640 kiB memory restriction became ever more of an obstacle, techniques were found to fill the empty areas with RAM.