The 640K Barrier is a problem that has always and will always plague any real-mode based Intel x86 operating system. This architecture is capable of addressing only 1 MB of RAM. The low end of this RAM is used by the processor and the BIOS, while the upper portion is stolen by ROMs and the video system. The remainder, ammounting to a measly 640K, is available for your programs. Protected mode operating systems are not subject to this flaw.