In an IBM-compatible PC this is the term to describe the first 640Kb of memory installed. Due to a historical design feature, some software handles the first 640Kb of memory in a different way from the rest of the memory installed. If your PC has 4Mb of RAM fitted, the first 640Kb is called the base or conventional memory, the next 384Kb is called the upper memory, and the remaining 3Mb is extended memory.