PCI Legacy Mode - Why does it use subtractive decoding?
On most modern Intel boards, they have a feature called 'PCI Legacy Mode' that allows users to add old PCI cards. The datasheets say -
"PCI functionality is not supported on new generation of PCH requiring methods such as using PCIe*-to-PCI bridges to enable external PCI I/O devices. To be able to use PCIeto-PCI bridges and attached legacy PCI devices, the PCH provides PCI Legacy Mode. PCI Legacy Mode allows both the PCI Express* root port and PCIe-to-PCI bridge look like subtractive PCI-to-PCI bridges. This allows the PCI Express root port to subtractively decode and forward legacy cycles to the bridge, and the PCIe-to-PCI bridge continues forwarding legacy cycles to downstream PCI devices."
Can someone just explain what the purpose of using subtractive coding is on both the bridges, what function does this provide?
What memory or I/O ranages does it allow to be picked up that wouldn't otherwise be possible with positive decoding?