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1.13 Direct-Mapped Addressing
The target address is translated by direct mapping or scatter-gather mapping as deter-
mined by the Wx_BASE_SG (scatter-gather) bit of the window's PCI base register.
If the Wx_BASE_SG bit is clear, the DMA address is direct mapped, and the trans-
lated address is generated by concatenating bits from the matching window's trans-
lated base register (T_BASE) with bits from the incoming PCI address. The bits
involved in the concatenation are defined by the window mask register as shown in
Table A–13. The unused bits of the translated base register (also in Table A–13) must
be cleared (that is, the hardware performs an AND-OR operation to accomplish the
concatenation). Because memory is located in the lower 8GB of the 21164 address
space, the 21174 ensures (implicitly) that address bits <39:33> are always zero.
Because the translated base is simply concatenated to the PCI address, then the direct
mapping is to a naturally aligned memory region. For example, a 4MB direct-
mapped window will map to any 4MB region in main memory that falls on a 4MB
boundary (for instance, it is not possible to map a 4MB region to the main memory
region 1MB–5MB).
Table A–13 lists direct-mapped PCI target address translations.
Table A–13 Direct-Mapped PCI Target Address Translation
W_MASK<31:20>
0000 0000 0000
0000 0000 0001
0000 0000 0011
0000 0000 0111
0000 0000 1111
0000 0001 1111
0000 0011 1111
0000 0111 1111
0000 1111 1111
0001 1111 1111
0011 1111 1111
Size of Window
Translated Address <32:2>
1MB
Translated Base<33:20> : ad<19:2>
2MB
Translated Base<33:21> : ad<20:2>
4MB
Translated Base<33:22> : ad<21:2>
8MB
Translated Base<33:23> : ad<22:2>
16MB
Translated Base<33:24> : ad<23:2>
32MB
Translated Base<33:25> : ad<24:2>
64MB
Translated Base<33:26> : ad<25:2>
128MB
Translated Base<33:27> : ad<26:2>
256MB
Translated Base<33:28> : ad<27:2>
512MB
Translated Base<33:29> : ad<28:2>
1GB
Translated Base<33:30> : ad<29:2>
Direct-Mapped Addressing
(Sheet 1 of 2)
System Address Space
A–37
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