Expanded memory specification is a specification which defines a method to access system memory above 1MB of RAM on PC-XT and AT computers. The specification is called EMS Memory, LIM Memory and Expanded Memory. The EMS is not part of main memory. The EMS memory is arranged in the block of 16KB each, to access this memory; one blow of EMS is copied into the window in the main memory. After processing it is copied back to the EMS memory.
In 80386 and latter processors, copying the content of the slow BIOS ROM into faster system RAM is known as Shadow Memory. It is also known as Shadow RAM.
The shadow memory provides an improvement in the computer speed by providing fast response to the ROM request. Access in shadow RAM is typically in the 60-100 N-s range where as ROM access is in the 125-250 N-s range.