The first megabyte of address space on the Intel x86 line of processors. Because memory is accessed using a sixteen-bit bus, 2^16 or 1,024,768 byte (one megabyte) are accessible. Cause of the 640k Barrier problem.
Actually, if it were a 16 bit bus, you'd only be able to access 64K. It's actually a 24 bit bus. Gotta love x86 Real Mode...
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