A main memory is realized by all internal semiconductor memories. The main memory is the primary memory in which the data is stored that is accessed by the central processing unit via the system bus. The main memory is composed of the working memory and the read-only memory.
The working memory is a RAM, the read-only memory is a ROM whose values can only be read and not changed. The read-only memory stores parts of the operating system that are needed when the computer is booted. The read-only memories are written with system-relevant data by the manufacturers.
The differentiation of the different memory areas of a main memory is causally connected with the DOS operating system, since the operating system could manage originally only 1 MBmemory capacity. From this area, the first 640 kilobytes( KB) were designated as conventional working memory, the memory area above this of 384 KB as Upper Memory Area( UMA). The area above 1 megabyte ( MB) is called the Expanded Memory Specification( EMS).