Shadow RAM is a copy of Basic Input/Output Operating System (BIOS) routines from read-only memory (ROM) into a particular area of random access memory (RAM) so that they can be accessed more rapidly. Access in shadow RAM is typically in the 60-100 nanosecond range while ROM access is in the 125-250 ns range. In some operating systems like DOS, certain BIOS routines are not only used during the boot or start-up of the system, but also during normal operation, particularly to drive the video display terminal. In Windows and OS/2, thus, these routines are not used and the use of shadow RAM is not essential. In some systems, the user can turn the use of shadow RAM off or on.