Stands for Expanded Memory System. System in a PC that defines the section of memory that lies above the 640Kb mark. Below the 640Kb mark the memory is called conventional memory and any program can use this. If you add more memory to your PC, you are adding expanded memory, since your PC will already have a basic 640Kb of memory fitted. If your PC has the usual 4Mb of memory fitted, the first 640Kb is conventional memory, the remainder is EMS. In order to make use of any EMS fitted, the software needs to be specially written. Luckily, Windows can use EMS memory and so any Windows program can automatically use the extra memory.
