The lowest 640 kilobytes of memory in an ibm pc-compatible computer running ms-dos. Other PC operating systems can usually compensate and "ignore" the fact that there is a 640K limit to base memory. This was put in place because the original cpu - the intel 8088 - could only access one megabyte of memory, and ibm wanted to reserve the upper 384KB for device drivers. The high memory area (HMA) lies above 640KB and can be accessed on MS-DOS computers that have an a20 handler.