External ISA bus slots

QNX Neutrino doesn't require the external ISA bus to be mapped into the usual x86 0x00000-to-0xFFFFF address range. This simplifies the hardware design, eliminating issues such as shadow RAM and the requirement to move a portion of the RAM (usually 0xA0000 through 0xFFFFF) to some other location.

But if your hardware needs to run with a standard BIOS and to support BIOS extensions, then this optimization can't be implemented, because the BIOS expects extensions at 0xA0000 through 0xEFFFF (typically).