Aaeon SBC-558, Manual

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Chapter 3 Award BIOS Setup 61
Shadowing copies firmware from ROM into system RAM, where
the CPU can read it through the 16-bit or 32-bit DRAM bus.
Firmware not shadowed must be read by the system through the 8-
bit X-bus. Shadowing improves the performance of the system
BIOS and similar ROM firmware for expansion peripherals, but it
also reduces the amount of high memory (640 KB to 1 MB) avail-
able for loading device drivers, etc.
Enable shadowing into each section of memory separately. Many
system designers hardwire shadowing of the system BIOS and
eliminate a System BIOS Shadow option.
Video BIOS shadows into memory area C8000-DFFFF. The remain-
ing areas shown on the BIOS Features Setup screen may be
occupied by other expansion card firmware. If an expansion
peripheral in your system contains ROM-based firmware, you need
to know the address range the ROM occupies to shadow it into the
correct area of RAM.