Data Transfer Apertures
PCI-to-Local Bus Apertures
Copyright 1997-2000, V3 Semiconductor Inc.
EPC User
’
s Manual Revision 1.05
21
4.1.2
Selecting PCI Memory or I/O Space Mapping
The PCI-to-Local apertures may be mapped into PCI memory space or PCI I/O space. The
IO bit in the PCI_BASEx registers controls this mapping. When IO=1, the aperture will only
"capture" transfers on the PCI bus that are to I/O space and fall within the bounds set by the
address base and size. Similarly, when IO=0, the corresponding PCI-to-Local aperture will
only respond to memory space transfers on the PCI bus.
4.1.3
PCI-to-Local Address Translation
PCI-to-Local address translation is controlled via the MAP_ADDR field in the PCI_MAPx
register. When an access is bridged from PCI-to-Local, the upper address bits of the PCI
address are alwaysreplaced with the significant bits in the MAP_ADDR field. Which bits are
considered "significant" is controlled by the size of the aperture. For example, with a 1
megabyte aperture size, the EPC will create the local bus address by replacing the A[31:20]
of the PCI address with the entire MAP_ADR field. With larger apertures, less of upper
address bits are replaced. For example with a 2 megabyte aperture, only PCI address bits
A[31:21] will be replaced. Address remapping is "disabled" by simply setting the MAP_ADR
and ADR_BASE fields for a particular aperture to the same value.
4.1.4
Byte Order Conversion
The PCI-to-Local bus apertures also control the conversion of data byte ordering as data
flows across the bridge. Writes from PCI space to local space are converted before entry
into the write FIFO, reads from local space destined for PCI space are converted on their
way from FIFO to the PCI bus (see Figure 5). Three modes of endian conversion are
supported as is shown in Table 3. When byte order conversion is enabled, the byte enable
signals are automatically corrected for the bridge transfer.
Table 2: PCI Address Remapping By Aperture Size
a
a. Some combinations are used for special DOS Compatibility apertures.
Aperture Size
PCI Address Bits Replaced by MAP_ADR Bits
1 meg
A[31:20]
2 meg
A[31:21]
4 meg
A[31:22]
8 meg
A[31:23]
16 meg
A[31:24]
32 meg
A[31:25]
64 meg
A[31:26]
128 meg
A[31:27]
256 meg
A[31:28]