Specifications

A–34 System Address Mapping
Subject to Change – 17 January 1997
PCI-to-Physical Memory Addressing
A.4.2 Direct-Mapped Addressing
If Wn_BASE [Wn_BASE_SG] is clear, the DMA address is direct mapped. The
translated address is generated by concatenating bits from the matching window’s
Tn_BASE with bits from the incoming PCI address (ad<31:0>). This process is
shown in Figure A–16 with n being the LSB from the Tn_BASE column of
Table A–11.
Figure A–16 Direct-Mapped Translation
The bits involved in the concatenation are defined by the window’s Wn_MASK, as
shown in Table A–11. Because memory is located in the lower 8GB of the 21164
address space, the CIA implicitly ensures that addr<39:33> are always zero.
Because Tn_BASE is simply concatenated to the PCI address, 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.
63 40
39
32
31
00
0000000000000000000
n-1
20 19
31
n-1
20
00000000
PCI
Address
Compare
Logic
W_DAC
Wn_BASE
Wn_MASK
0
33
32
n-11
n-1
10
Tn_BASE
000000000
02
XXXXX
Window
Hit
Tn_BASE Select
Physical
Memory
Address
LJ04274A.AI5
<39:33>=0
n
n-10
11111
n
n0102