Shadow memory is the transfer of the content from read-only memory to operating memory. The shadow gives the substantial performance improvements as a request is not addressed to the slow read-only device memory but to its pattern located in the fast operating memory. The simplest example is BIOS parameters which are stored in the non-volatile memory but after switching on a computer they are transferred to an operating memory thus optimizing the configuration of a system.
The shadow is performed in the memory area from 640 Kbyte to 1 MB that is in line with a physical address “a0000h-fffffh”. The shadow mechanism is not very important in modern operating systems – 32-bit system (Windows95 and higher) do not operate with the shadow device memory but directly with the drivers booted in operating memory.
Nevertheless, you may specify the necessity to shadow some parts of system memory such as network or video cards and other in old computers that use the intellectual devices. Such procedure is especially useful for 16-bit systems – MS DOS and Windows 3x.
All BIOS parameters for shadow memory are located in the sections “Chipset Features Setup”, “Advanced Chipset Features” and “Advanced”.
The shading of operating memory for system BIOS is executed using the parameter “System BIOS Shadow” (System Shadow, System ROM Shadow F000, 64K). This parameter may have two values: Enabled and Disabled.