To be more specific, COM files were loaded into a single segment (64k) - this is where the limitation comes from. At the time of DOS, memory access was unrestr

To be more specific, COM files were loaded into a single segment (64k) - this is where the limitation comes from. At the time of DOS, memory access was unrestricted, so, in a basic form, one had direct access to the entire base memory (640k). DOS viruses for example frequently resided just below the top of it, and/or manipulated the interrupt descriptor table (located instead at the very beginning).