User`s guide

If SGEN is cleared, the DMA address is direct mapped. The translated address
is generated by concatenating bits from the matching window translated base
register with bits from the incoming PCI address. The PCI mask register
determines which bits of the translated base register and PCI address are used
to generate the translated address as shown in Table 4–9.
Note that the unused bits of the translated base register must be cleared
for correct operation. Because system memory is located in the lower half of
the CPU address space, sysBus<33> is always zero. Bits sysBus<32:5> are
obtained from the translated base register.
Table 4–9 PCI Target Address Translation—Direct Mapped
PCI_MASK<31:20> Translated Base <32:5>
0000 0000 0000 T_BASE<32:20>:PCI ad<19:5>
0000 0000 0001 T_BASE<32:21>:PCI ad<20:5>
0000 0000 0011 T_BASE<32:22>:PCI ad<21:5>
0000 0000 0111 T_BASE<32:23>:PCI ad<22:5>
0000 0000 1111 T_BASE<32:24>:PCI ad<23:5>
0000 0001 1111 T_BASE<32:25>:PCI ad<24:5>
0000 0011 1111 T_BASE<32:26>:PCI ad<25:5>
0000 0111 1111 T_BASE<32:27>:PCI ad<26:5>
0000 1111 1111 T_BASE<32:28>:PCI ad<27:5>
0001 1111 1111 T_BASE<32:29>:PCI ad<28:5>
0011 1111 1111 T_BASE<32:30>:PCI ad<29:5>
0111 1111 1111 T_BASE<32:31>:PCI ad<30:5>
1111 1111 1111 T_BASE<32>:PCI ad<31:5>
If the SGEN bit is set, the translated address is generated by a table lookup.
The incoming PCI address indexes a table stored in system memory. The
table is referred to as a scatter-gather (SG) map. The translated base register
specifies the starting address of the SG map. Bits of the incoming PCI address
are used as an offset from the base of the map. The map entry provides the
physical address of the page.
4–22 System Address Mapping