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21555 Non-Transparent PCI-to-PCI Bridge User Manual 37
Address Decoding
This new base address, also called the translated base address, references a new location in the
secondary bus address map. The offset is not affected. The process is similar for transactions
forwarded from the secondary bus to the primary bus.
Each memory address range using direct offset address translation has its own translated base. The translated base
addresses are programmable in registers corresponding to each BAR. These registers are mapped both in
device-specific configuration space and in CSR space. The number of bits of the translated base address
corresponds to the number of writable bits in the respective BAR. Likewise, the number of bits of the offset also
varies and depends on the size of the BAR. Figure 6 shows an example of address translation of downstream
memory transactions. Again, upstream transactions are treated similarly.
4.3.3 Lookup Table Based Address Translation
As mentioned in Section 4.3.2, Upstream Memory 2 address translation is treated differently than the other ranges.
The 21555 uses a page size based lookup table to perform address translation for transactions falling into this range.
A lookup table provides a flexible way of translating secondary bus local addresses to primary bus system
addresses.
Figure 5. Direct Offset Address Translation
Figure 6. Downstream Address Translation Example
A7463-01
Translated Base
Translated Address
Offset
Base
Original Address
Offset
A7464-01
Base + Offset
Translated
Base + Offset
Primary Address Map Secondary Address Map