
PCI Configuration
Configuration as a Target Bridge
92
EPC User
’
s Manual Revision 1.05
Copyright 1997-2000, V3 Semiconductor Inc.
The following are exceptions to the above:
Size requested for PCI-to-Local data transfer apertures (PCI_BASEx registers).
Size requested for expansion ROM transfer apertures (PCI_ROM register).
In both of these cases the PCI Specification outlines a two step interrogation process. First,
each base address register is written with FFFF.FFFFH. Then the base register is read back.
The value read back indicates the size and capabilities of the aperture (see Figure 48).
As an example, let
’
s assume that PCI-to-Local bus aperture 0 is programmed for 4
megabytes in memory space with prefetching enabled. When the system host interrogates
this register it will read back FFC0.0004H which is interpreted as:
The aperture is four megabytes in size, since the first
“
1
”
seen in the address field
(scanning up from bit 4) is at the 4M level
The aperture can be located anywhere in the 32-bit address space on a 4MB
boundary
The aperture is memory mapped
The aperture is
“
prefetchable
”
The EPC sets the size information according to the size of the aperture specified in the
PCI_MAPx register. Unused base registers in the EPC return all zeros.
Figure 48: Base Register Return Information
0
4
8
12
16
20
24
28
ADDRESS SIZE
FIRST
‘
1
’
INDICATES SIZE
PREFETCHABLE
0 =NOT PREFETCHABLE, 1=PREFETCH OK
TYPE
00 = ANYWHERE IN 32-BIT ADDRESS SPACE
01 = LOCATE BELOW 1 MB
10 = ANYWHERE IN 64-BIT ADDRESS SPACE
11 = RESERVED
MEMORY/IO
0 =MEMORY, 1=I/O
Example of Address Size Usage:
A ‘1’ here, followed by all zeros
down to bit 4 indicates an 8MB
window.