Vmebus Domain - Motorola MVME2301 Installation And Use Manual

Programming Considerations
PCI and Ethernet
Ethernet is byte-stream-oriented; the byte having the lowest
address in memory is the first one to be transferred regardless of the
endian mode. Since the Raven maintains address invariance in both
little-endian and big-endian mode, no endian issues should arise
for Ethernet data. Big-endian software must still take the byte-
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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 processorÕs 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 processorÕs domain.
In big-endian mode, byte-swapping is performed first by the
Universe ASIC and then by the Raven. 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-
rearranging effect of the Raven into account.
For further details on endian considerations, refer to the
MVME2300-Series VME Processor Module ProgrammerÕs Reference
Guide.
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