Operating Instructions
PCI and SCSI
SCSI 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 ASIC maintains address invariance in both little-endian and big-endian modes, no endian issues should arise for SCSI data. Big-endian software must still take the byte-swapping effect into account when accessing the registers of the PCI/SCSI device, however.
PCI and Ethernet
Ethernet is also 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-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 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-rearrangingeffect of the Raven into account.
