10. Endian Mapping
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PowerSpan II User Manual
80A1010_MA001_09
Integrated Device Technology
www.idt.com
10.4.4
True Little-endian Mode
When operating in true little-endian mode, PowerSpan II uses a data invariant scheme for mapping
PowerPC byte lanes. Data invariance preserves the relative byte significance of a structure in both PCI
and PowerPC spaces, but translates the byte addressing.
In order to access PCI device registers from the processor bus in true little-endian mode, there are
certain addressing rules which must be followed. In PowerSpan II when true little-endian mode is
selected, no address swapping takes place (refer to
). This means that the MSB on the
processor bus goes to the MSB on PCI. However, the MSB on processor bus is the low address and
MSB on PCI is the high address.
True little-endian mode cannot be used with the 4 byte read implementation in the PowerSpan II
design. The MEM_IO bit must be set to 0 when the END field is set to 11. Refer to
for a detailed explanation of the 4 byte read through the PCI Interfaces and PB
Interface.
The 4 byte read implementation can be used with the other types of endian conversion.