Programming Considerations
endian mode, no endian issues should arise for Ethernet data. Big-endian software must still take the byte-swapping effect into account when accessing the registers of the PCI/Ethernet device, however.
Role of the Universe ASIC
Because the PCI bus is little-endian while the VMEbus is big-endian, the Universe PCI/VME bus bridge ASIC performs byte swapping in both directions (from PCI to VMEbus and from VMEbus to PCI) to maintain address invariance, regardless of the mode of operation in the processor’s domain.
VMEbus Domain
The VMEbus is inherently big-endian. All devices connected directly to the VMEbus must operate in big-endian mode, regardless of the mode of operation in the processor’s domain.
In big-endian mode, byte-swapping is performed first by the Universe ASIC and then by the PHB. The result is transparent to big-endian software (a desirable effect).
In little-endian mode, however, software must take the byte-swapping effect of the Universe ASIC and the address reverse-rearrangingeffect of the PHB into account.
For further details on endian considerations, refer to the MVME2400- Series VME Processor Module Programmer’s Reference Guide .
