The part of a two-part memory address that remains constant and provides a reference point from which the location of a byte of data can be calculated. A base address is accompanied by an offset value that is added to the base to determine the exact location（the absolute address）of the information. the concept is similar to a street address system. For example, 2010 main street plus an offset（10from the beginning of the block ）. Base address are known as segment address in IBM pcs and compatibles; data in these computers is identified by its position as a relative offset the start of the segment.