Motorola MVME2600 manual Endian Issues, Processor/Memory Domain, PCI Domain

	Programming Considerations
Endian Issues	2

The MVME2603/2604 supports both little-endian (for example, Windows NT) and big-endian (for example, AIX) software. The PowerPC processor and the VMEbus are inherently big-endian, while the PCI bus is inherently little-endian. The following sections summarize how the MVME2603/2604 handles software and hardware differences in big- and little-endian operations. For further details on endian considerations, refer to the MVME2600 Series Single Board Computer Programmer’s Reference Guide, listed in Appendix D, Related Documentation.

Processor/Memory Domain

The MPC603/604 processor can operate in both big-endian and little-endian mode. However, it always treats the external processor/memory bus as big-endian by performing address rearrangement and reordering when running in little-endian mode. The MPC registers in the Raven MPU/PCI bus bridge controller ASIC and the Falcon memory controller chip set, as well as DRAM, ROM/Flash, and system registers, always appear as big-endian.

Role of the Raven ASIC

Because the PCI bus is little-endian, the Raven performs byte swapping in both directions (from PCI to memory and from the processor to PCI) to maintain address invariance while programmed to operate in big-endian mode with the processor and the memory subsystem.

In little-endian mode, the Raven reverse-rearrangesthe address for PCI- bound accesses and rearranges the address for memory-bound accesses (from PCI). In this case, no byte swapping is done.

PCI Domain

The PCI bus is inherently little-endian. All devices connected directly to the PCI bus operate in little-endian mode, regardless of the mode of operation in the processor’s domain.

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