Motorola MVME2400 Series Manuel de service télécharger pdf (Page 88)

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Programming the MVME240x

Endian Issues

The MVME240x supports both little-endian (e.g., Windows NT) and big-

endian (e.g., 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 MVME240x handles software and

hardware differences in big- and little-endian operations. For further

details on endian considerations, refer to the MVME2400-Series VME

Processor Module Programmer’s Reference Guide.

Processor/Memory Domain

The MPC750 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 Hawk MPU/PCI

bus bridge controller, SMC memory controller, as well as DRAM, Flash,

and system registers, always appear as big-endian.

Role of the Hawk ASIC

Because the PCI bus is little-endian, the PHB portion of the Hawk

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 PHB reverse-rearranges the 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.

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 PHB maintains address invariance in both little-endian and big-

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Motorola MVME2400 Series Manuel de service Page 88