
Hawk PCI Host Bridge & Multi-Processor Interrupt Controller
MVME5100 Single Board Computer Programmer’s Reference (6806800H17B)
99
Address modification happens to the originating address regardless of whether the transaction
originates from the PCI bus or the PPC bus. The three low order address bits are exclusive-ORed
with a three-bit value that depends on the length of the operand, as shown in the following
table.
Since this method has some difficulties dealing with unaligned PCIoriginated transfers, the PPC
master of the PHB will break up all unaligned PCI transfers into multiple aligned transfers into
multiple aligned transfers on the PPC bus.
2.3.4.3
PHB Registers
The PHB registers are not sensitive to changes in Big-Endian and Little- Endian mode. With
respect to the PPC bus (but not always the address internal to the processor), the PPC registers
are always represented in Big- Endian mode. This means that the processor’s internal view of
the PPC registers appears different depending on which mode the processor operates.
With respect to the PCI bus, the configuration registers are always represented in Little-Endian
mode.
Table 2-16 Address Modification for Little Endian Transfers
Data
Length
(bytes)
Address
Modification
1
XOR with 111
2
XOR with 110
4
XOR with 100
8 no
change
The only legal data lengths supported in Little-Endian mode are 1, 2, 4, or 8-byte aligned
transfers.
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