How does the Subtractive Decode feature of the PIIX4E work?
The greatest misunderstanding of the PIIX4E’s Subtractive Decode feature is that it will capture ALL unclaimed PCI cycles and forward them to the ISA bus. This is not true. The PIIX4E will only forward those cycles that fall within the ISA range of memory. The ISA bus only supports a total of 24 address lines, limiting memory addresses to 00FFFFFFh, or 16MB. Memory address 01000000h or above are only supported by the PCI bus. The PIIX4E will never Subtractive Decode an address above 16MB.
