Why not make it simple and just have the CPU put bits on the control bus to say that the address on the main bus is a geographic one for PCI configuration? Well, most CPU's are not capable of doing this so the PCI host bridge gets to do it instead.
Range Check (ISA Testing for IO Address Conflicts)
On the ISA bus, there's a method built into each PnP card for checking that there are no other cards that use the same I/O address. If two or more cards use the same IO address, neither card is likely to work right (if at all). Good PnP software should assign bus-resources so as to avoid this conflict, but even in this case a legacy card might be lurking somewhere with the same address.
The test works by a card putting a known test number in its own IO registers. Then the PnP software reads it and verifies that what it reads is the same as the known test number. If not, something is wrong (such as another card with the same address). It repeats the same test with another test number. Since it actually checks the range of IO addresses assigned to the card, it's called a "range check". It could be better called an address-conflict test. If there is an address conflict you get an error message.
Communicating Directly via Memory
Traditionally, most I/O devices used only I/O memory to communicate with the CPU. The device driver, running on the CPU would read and write data to/from the I/O address space and main memory. Unfortunately, this requires two steps. For example, 1. read data from a device (in IO address space) and temporarily store in in the CPU; 2. write this data to main memory. A faster way would be for the device itself to put the data directly into main memory. One way to do this is by using ISA DMA Channels or PCI bus mastering. Another way is for the physical device to actually contain some main memory (at high addresses so as not to conflict with main memory chip addresses). This way the device reads and writes directly to it's self-contained main memory without having to bother with DMA or bus mastering. Such a device may also use IO addresses.
