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The PCI bus connects the Uni-N IC to the boot ROM, the KeyLargo I/O controller, and the PCI slots. The PCI bus is a 33-MHz, 64-bit bus for the highest possible PCI card performance.
The Uni-N 1.5 IC used in the Power Mac G4 computer supports a new PCI feature called Write Combining. This feature allows sequential write transactions involving the Memory Write or Memory Write and Invalidate commands to be combined into a single PCI transaction. The memory write transactions being combined must be to sequential, ascending, and non-overlapping PCI addresses. Placing an eieio or sync command between the write commands will prevent any write combining.