Table 4-14. System Address Space Mapping Entry Fields
If any main memory address is mapped to a software visible bus, such as PCI, it must be explicitly
declared using a System Address Space Mapping entry.
In the case of a bus that is directly connected to the main system bus, system address space records
and compatibility base address modifiers must be provided as needed to fully describe the complete
set of addresses that are mapped to that bus. For example, in Figure 4-10, complete explicit
descriptions must be provided for PCI BUS 0 and PCI BUS 1 even if one of the buses is
programmed for subtractive decode.
Figure 4-10. Example System with Multiple Bus Types and Bridge Types
PCI Bus 0
Entry type 128 identifies a System Address Space
A value of 20 indicates that an entry of this type is
twenty bytes long.
The BUS ID for the bus where the system address
space is mapped. This number corresponds to the
BUS ID as defined in the base table bus entry for
System address type used to access bus
addresses must be:
0 = I/O address
1 = Memory address
2 = Prefetch address
All other numbers are reserved.
Number of addresses which are visible to the bus
PCI Bus 1
PCI Bus 2